Kaiser Book

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Experimental Investigation on the Formation of Carbon-Bearing Molecules inthe Interstellar Medium via NeutralNeutral Reactions

Ralf I. Kaiser*

Department of Chemistry, University of York, YO10 5DD, U.K.Received August 15, 2001

ContentsI. Introduction 1309

A. Interstellar Medium 1310B. Extraterrestrial Environments 1311

1. Diffuse Clouds 13132. Translucent Clouds 13133. Dense Clouds 13134. Star-Forming Regions, Young Stellar

Objects, and Hot Molecular Cores1313

5. Circumstellar Envelopes 13156. Planetary Nebulae 1316

C. Formation of Molecules 1316

1. Solid State 13172. Gas Phase 1319

II. Key Reactants in the Interstellar Medium 1321A. Atomic Carbon, C(3Pj), Dicarbon, C2(X1g+),

and Tricarbon, C3(X1g+)1321

B. Cyano Radicals, CN(X2+) 1321C. Ethinyl Radicals, C2H(X2+) 1321D. Phenyl Radicals, C6H5(X2A) 1322

III. Kinetics 1322A. Room-Temperature Kinetic Studies 1322B. Low-Temperature Kinetic Studies 1324

IV. Dynamics: Crossed Molecular Beam Studies 1326A. Principle 1326B. Experiment 1327

1. Fixed Sources with Rotatable QuadrupoleMass Spectrometer

1327

2. Rotatable Sources with Laser-InducedFluorescence Detection

1329

C. Results 13301. Reactions of C(3Pj) 13302. Reactions of C2(X1g+/a3u) and

C3(X1g+)1340

3. Reactions of CN(X2+) 13414. Reactions of C2D(X2+) 13455. Reactions of C6H5(X2A) 1348

V. Implications for Solar System Chemistry 1349

VI. Implications for Combustion Processes andChemical Vapor Deposition

1349

VII. Summary and Outlook 1350VIII. Acknowledgments 1351

IX. References 1351

I. IntroductionThe physical a nd chemical processes lea ding t o the

formation of molecules in the interstellar medium

(ISM)sth e vast void s b etw e en th e sta rssfascin ate dscientists since the first detection of CH, CH +, a n dC N r a d i ca l s i n e xt r a t e r r es t r i a l en v ir on m en t s 60year s a go. Although more tha n one-ha lf of a centur yh a s p a s s ed b y a n d 121 s pe ci es f r om m ol ecu la rhydrogen (H 2) t o pol ya t o mi cs s uch a s t h e s ug a rglycolaldehyde (HOCH 2CHO), benzene (C 6H 6), cy-a nopenta a cetylene (HC 11N), a nd possibly the a minoa cid glycine (H 2NC H 2COOH) have been identified sofar , th e e n ig m a Ho w are th e se m o le cu le s actu al lyformed under the harsh conditions in the interstellarmedium? is still under debate. 1

This r eview focuses on t he new ly emergin g field ofastrochemistry and anthologizes the latest trends inlaboratory experiments on the formation of carbon-b ear in g m olecu les in th e in te rstel lar m e d iu m vian e u tral-neutr a l react ions. To int roduce th is topic toth e g e n e ral ch e m ical co m m u n ity an d n o vice s, th efirst sections provide a n overview of t he chemica lcomposition (atoms versus molecules; neutrals versusion s; g as p h ase ve rsu s sol id st at e ) an d th e p h ysicalproperties (temperatures and number densities) ofvarious interstellar environments (sections I .A andI.B). This provides the crucial background to under-sta nd t he basic molecula r processes a nd prerequisitesof how molecules might be synthesized in the stronglydiverse regions of the interstellar medium (section* E-mail: [email protected].

Ralf I. Kaiser was born on May 24, 1966, in Unna, Germany. He receivedhis Ph.D. degree in Chemistry from the University of Munster (Germany)and Nuclear Research Center (Julich). He did postdoctoral work on theformation of astrophysical molecules in the interstellar medium at UCBerkeley (Department of Chemistry). From 1997 to 2000 he received afellowship from the German Research Council (DFG) to perform hisHabilitation at the Department of Physics (University of Chemnitz,Germany) and Institute of Atomic and Molecular Sciences (AcademiaSinica, Taiwan). His research interests include chemical reaction dynamics(gas phase and solid state), planetary chemistry, and laboratory studiesrelevant to astrochemistry.

1309Chem. Rev. 2002, 102, 13091358

10.1021/cr970004v CCC: $39.75 2002 American Chemical SocietyPublished on Web 04/11/2002


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I . C ). S i n ce ca r b on i s t h e f ou r t h m os t a b u n d a n te le m e n t in th e u n ive rse an d th e b asis o f a l l l i fe aswe know it, an underst a nding of elementa ry chemica lreactions involving carbon-bearing species is of par-ticula r importa nce to expose the chemica l processingof interst ella r ma tt er. Selected reaction classes w hichare o f p aram o u n t s ig n if ican ce to k e y qu e stio n s inast ro ch e m istry an d ast ro biolog y are e xam in ed insections II .A-D . Th e su b se qu en t section s re vie w

modern experimenta l techniques to unta ngle the rat econ sta n ts (k in e tics ; se ction I I I ), in te rm ed iate s in -volved, products, a nd t he rea ction m echa nisms (dy-n a m i cs ; s ect i on I V) of t h e se i m por t a n t n eu t r a l -neutral reactions. The last sections summarize thesef in d in g s, e valu ate th e b e n e f i ts an d l im itat io n s o fcurrently operating experimental setups critically,an d e m ph asiz e fu tu re re search d irection s to st u d yimportant classes of neutral-neutral rea ctions in theinterstellar medium. Finally, implications for solarsystem sciences and terrestrial stages such as com-bustion processes and chemical vapor deposition areaddressed.

A. Interstellar MediumThe IS M conta ins a bout 10% of the ma ss of our

ga laxy a nd consists of ga s (99%) and s ubmicrometer-size d g rain p art icle s (1%) w ith ave rag e d n u m b erd e n sit ie s o f 1 H ato m cm -3 a n d 1 0-11 g rain s cm -3,respectively.2-5 These data translate to pressures ofab o u t 10-18 m b a r a t 1 0 K , w h i c h i s b e y o n d a n yultrahigh vacuum achieved in terrestrial laboratoriesso fa r. The chemica l composition of t he int erstellarmedium is domina ted by neutr a l hyd rogen (93.38%)a nd h elium (6.49%), w herea s biogenic element s oxy-gen, carbon, and nitrogen contribute 0.11%(O:C:N 7:3:1).6 The third-row elements neon, silicon,

ma gnesium , a nd su lfur a re less copious (0.002%) a ndh ave re lat ive a b u n d an ces of 8 :3:3:2; a l l re m a in in gelement s fur nish only 0.02%.

This elementary classification is well-reflected inthe molecular composition of the interstellar medium.Molecules, radicals, and ions are ubiquitous in ex-traterrestrial environments and have been detectedin e xtrao rd inar y d iversi ty r an g in g fro m sm a ll m ol-e cu les su ch as h yd rog en (H 2) to astrobiologicallyimport an t species such as the simplest suga r glyco-laldehyde (HOC H 2CH O) an d possibly t he a mino acidglycine (H 2NC H 2COO H). Ta ble 1 compiles a ll speciesidentified in the interstellar medium so far , ma ny ofth e m t h e rm ally u n sta b le an d e xtre m e ly re active in

Table 1. Classification of Neutral Species and IonsDetected in the Interstellar Medium via MicrowaveSpectroscopy Unless Noted Otherwisea

a I R , i nfr ar e d ; UV, ul t r avi ol e t ; VI S , vi si b l e ; ? , t e nt at i veidentification; *, observat ion only in carbon-rich circumst ellarenvelopes; #, observation only in carbon-rich planetary nebulae.

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te rre strial lab ora to rie s.7 The majority of these mol-ecules were detected by radio telescopes observingth e ir ro tat ion al tra n si t ion s in e m ission ; t o a m in orextent, infrared (IR), visible (VIS), and ultraviolet(UV ) a stro n om y foste red th e ir id en ti f icat ion . D i-at omic molecules w ith second- a nd third-row ele-m e n ts a re p art icu lar ly p revale n t ; in p art icu lar , car-bon (C 2, CN, CO, C S) and silicon (SiC, SiN, SiO, SiS)b ea r i n g s p eci es h a v e t o b e n a m e d. C P a n d P N a r e

th e only phosphorus-cont a ining m olecules identifiedso far ; N O, N S , a n d S O re p rese n t th e so le extra te r-restria l radicals carrying a toms of the fif th a nd sixthperiod.

Ha lid e s a n d p seu d oh alid es re pre sen t a se con dimportant class of molecules. Quite surprisingly, twoh alo g e n h yd rid e s HF an d HCl to g e th e r with th re ealkali carry ing species NaCl, KC l, and NaC N presenta significant component of the int erstellar medium;in pa rt icular , t he open-shell species MgC N, MgNC ,AlF, AlCl, a nd S iCN denote crucial tr acers of metalsan d semi-meta ls bound in m olecular form.

Di-, tri- , and tetravalent hydrides methane (CH 4),s i lan e (S iH 4), am m o n ia (N H 3), w a t e r (H 2O ) , a n d

hydr ogen s ulfide (H 2S) are very importa nt species asthey implicate t he parent molecules of CH, C H 2, C H 3,N H , N H 2, O H , a n d S H r a d i ca l s . N ot e t h a t n ei t h erphosphine (PH 3) nor silicon-bearing radicals SiH 3,S iH 2, and SiH have been identified in the interstellarmedium; however, phosphine, its higher homologuear sine (AsH 3) , an d g e rm an e (G e H 4) d e sig n at e su b -s t a n t i a l t r a c e c o n s t i t u e n t s i n t h e a t m o s p h e r e s o fJ u p ite r a n d S a tu rn . M e th an esthe simplest, closed-sh ell a n d fu lly sat u rat e d h yd ro car b on m o le cu lesl ea d s u s t o ol ef in es a n d a l k y n es . H e r e, e t h y le ne(C2H 4), a cetylene (C2H 2), methylacetylene (CH 3CCH),an d methy ldiacetylene (CH 3CCCCH) contribute sig-nifica ntly to the cosmic carbon budget. Very recently ,dia cety lene (C 4H 2), tr ia cety lene (C 6H 2), and benzene(C 6H 6) were detected as well. The search for allene(H 2C C C H 2)sa st ructura l isomer of methyla cetylenesha s been unsuccessful so far .

T h e se h yd ro carb o n s stan d in stro n g co n trast tohydrogen-deficient, l inea r carbon chain molecules.Here, hydrogen-termina ted car bon clusters from t heethinyl radical (C 2H) to octat etra ynyl (C 8H), the bar ecarbon clusters C 2, C 3, a n d C 5, cummulene carbenes(H 2CCC, H 2CCCC, H 2CC CC CC ) together w ith oxygen(C 2O, C 3O, C 5O)-, sulfur (C 2S , C 3S)-, and silicon (C 4-S i)-te rm in ate d carb o n ch ain s a re ve ry a b u n d an t inspace. Likewise, cyanopolyacetylenes (H(CC)nCN),

together with their radicals ((CC)n-CN ), a n d m e th -ylcyanopolyacetylenes (CH 3-(CC)nCN ) p re sen t im -porta nt molecules which a re t o some extent consid-er e d a s p r ecu r s or s t o a m i n o a c id s . Th es e l in ea rspecies expose a beau tifu l cont ra st to cyclic moleculessilicon dica rbide (SiC 2), s i licon tr icarb id e (S iC 3),tr icar b on h yd rid e (c-C 3H ), cyclopropenylidene (c-C 3H 2), and ethylene oxide (C 2H 4O).

The latter connects to complex, organic moleculesob serve d in th e in te rste l lar m e d iu m . P ar t icu lar at -tent ion ha s been devoted t o alcohols (metha nol (CH 3-OH), etha nol (C 2H 5OH ), a nd viny l a lcohol (C 2H 4O)),8

aldehydes (formaldehyde (H 2CO), aceta ldehyde (CH 3-CHO)), acids (formic acid (HCOOH), acetic acid (CH 3-

COOH)), an d th e ir fu lly oxid iz ed p rod u ct carb ondioxide (CO 2). F u r t h e r , f or m i c a c i d m et h y l es t e r(HCOOCH 3), acetone (CH 3C O C H 3), dimethyl ether(CH 3OC H 3), ketene (H 2CCO), propynal (HCCCHO),and the formyl radical (HCO) are present in detect-a ble qua ntit ies. Two of these species possess int er-stellar sulfur counterpar ts, na mely, thioforma ldehyde(H 2CS ) a n d th iom e th an o l (CH 3SH).

B e sid es cyan op olyacetyle n es an d th e ir rad icals ,

ra ther sa tur at ed, nitrogen-carry ing molecules are ofcrucial importance to the interstellar nitrogen bud-get. Here, methyl- and ethylcyanide (CH 3CN , C 2H 5-CN ) to g e th e r with th e CH 2C N a n d C H C N r a d i c a l sh ave to b e ad d re sse d . L ik e wise , h yd ro g e n cyan id e(HCN) and the isocyanide isomer (HNC), their re-duced forms H 2C N , H 2C N H , a n d C H 3NH 2, plus H 2-NCN a re worth mentioning. Vinylcya nide (C 2H 3C N)completes the homologous row from the bare, nitrogen-te rm in ate d clu ste r CCCN via cyan o ace tyle n e (HC-CCN ) t o e th ylcya n id e (C 2H 5CN). As the molecularcomplexity rises, molecules with four different typesof a t o m s h a v e b ee n d et e ct e d i n t h e i n t er s t el la rm e d i u m i n s m a l l q u a n t i t i e s ; t h e s e a r e H C O N H 2,

H N C O , a n d H N C S .Str uctura l isomerssmolecules wit h t he sa me chemi-

cal formula but distinct connectivities of atomssa n dmolecular ions received special fascination. As therelat ive abunda nces of isomers should depend str onglyon t h e p hy s ica l a n d ch em i ca l con d it i on s i n t h einterstellar medium, isomers a ct a s tr a cers t o eluci-da te tempera ture- an d density-dependent forma tionroutes to extraterrestrial molecules. Species of thegross formula C 2H 4O, C 2H 4O2, a n d H C 3N representth e o n ly case s wh e re th re e iso m e rs h ave b e e n o b -se rve d , n a m e ly, e th ylen e oxide , a ceta ld eh yd e , a n dvin yl alcoh ol (syste m 1), ace tic a cid , form ic acidmethylester, a nd glycolaldehyde (syst em 2), a s w ell

as cyan oace tylen e , isocya n oace tylen e , a n d th e car-bene structure HNC CC (system 3). Six isomer pairsof cyclic (c) an d linear (l) C3H , c/l-C 3H 2, H CN/HN C,CH 3CN/CH 3NC, MgC N/MgNC, a nd H CO +/H OC + haveb ee n a s s i gn ed a s w e ll . I on s , e sp eci a l ly H 3+, a r eth o u gh t to b e im p orta n t in g red ien ts to d rive a r ichchemistry in those extraterrestrial environments inwh ich n e u tral p art icle s are sp arse . N o tice th at , o nave rag e , 97% of al l m olecules ar e n e u tral wh e re asonly 3%a re positively cha rged; so far, no a nion ha sb ee n id en ti f ie d u n am b ig u ou sly in th e in te rstel larmedium.

B. Extraterrestrial Environments

Th e m a j or i t y of t h e v ol um e of t h e i n t er s t el la rm e d iu m is ve ry h o t (T > 10 000 K) and does notcon t a i n a n y m ol ecu le s a t a l l . H e n ce , i n t er s t el la rmolecules, radicals, a nd ions detected so far a re notdistributed homogeneously but are confined to dis-tinct environments. These can be categorized thor-oughly into six classes based on their size (large-scalestr uctures versus point sources), physical par a meters(density, a verage tra nslational tempera ture), and thechemical cha ra cteristics (Ta ble 2 a nd Figure 1).9-23

The interstellar medium is composed primarily ofthr ee types of la rge-scale structur es wh ich a re oftencalled clouds. These a re diffuse clouds, tr a nslucent

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clouds (semitransparent clouds), and dense clouds(cold clouds; molecular clouds).24 The nomenclatureis b ase d o n an in cre asin g ly d e n se r m e d iu m risin gfrom 101 to104 atoms cm -3 as th e t e m pe ratu re d ro pss im u lt a n eou s ly f r om 120 t o a b ou t 10 K . As t h ed e n sity r ise s fu rth e r to 109 a t o m s cm -3, w e m o v etowa rd st a r-forming regions with hot molecula r coresa nd young st ella r objects (YSO) wh ich resemble sta rsi n t h e ir v er y e a r ly p ha s e of l if e.25-27 This cla sspresents the link betw een la rge-scale structures a ndpointlike sources such a s circumstellar envelopes(CS E) an d planeta ry n ebula e (P Ne). The lat ter clas-

si ficat ion is in l in e with an in cre asin g ly a g in g star .As st a rs evolve and reach t he end of their lives, theyd ev el op w i n d s w h i ch r et u r n p a r t of t h e ir s t el la rmatter back into the interstellar medium (low-masss t a r s w i t h m a s s e s M < 1 Mo (Mo ) sola r ma ss)) orth ey end th eir life violently in a supernova explosion(m a s s i ve s t a r s ; M >1-10 Mo).28-32 These ejectacontain elements heavier than helium; a fraction ofthese ejecta is incorporated into solid matter (dustg r a i n s); t h e r e s t r e ma i n s i n t h e g a s p ha s e . Th efollowing sections (sections I .B .1-6 ) e x a m i n e t h ephysica l a nd chemica l char a cteristics of each region.

Table 2. Overview of Physical Parameters in Important Interstellar Environments

region m olecules densit y, cm -3 temp, K

diffuse clouds simple molecules H 2, C H +, C H , C N, C 2,O H , C O , H C O +, H C N

101-102 100-120

t ra nslucen t clouds simple molecules H 2, C H +, C H , C N, C 2,O H , C O , H C O +, H C N, C 3

102-103 50-100

dense clouds (molecularclouds) (cold clouds )

carbon-rich, linea r a nd cyclic moleculeswith u p to 13 atoms

102-104 10-15

h ot m olecu la r cor es 1. s at ur a ted m olecu les C H 3OH, C 2H 5OH ,C 2H 5C N, C H 3C O C H 3, C H 4, NH 3, H 2O

106-109 100-300

2. no ca rbon-rich linea r m olecules3. vibrat ionally excited HC CC N/C 2H 3C N4. large deuterium fra ctiona tion

ci r cu m s t el la r e nv el op es ca r b on r i ch : ca r b on cl us t er s a n dhydrogen-terminated clusters

va ria ble 10-4500

oxygen rich: sma ll oxygen bea ring speciespla net a ry nebula e ca rbon r ich: di- a nd t r ia cet y lene, benzen e va ria ble 200-3000

Figure1. Cycling of mat ter in th e interstellar medium: diffuse clouds, tran slucent clouds, molecular clouds, core forma tion,high- and low-mass star-forming regions, supernovae, circumstellar envelopes, and planetary nebulae.



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This sets the st age to discuss the ongoing chemistryin t he interst ella r medium (section I .C).

1. Diffuse Clouds

Diffuse clouds are tenuous concentrations of inter-stellar ma tt er with t ypica l number densities of 101-102 at om s cm -3 an d ave rag e tran slat io n al te m p e ra-tures of th e gas of 100-120 K .33 They contain up toa few th o u san d solar m asse s. Oph (Ophiuchi), an d

P er (Persei) represent typical exa mples of diffuseclouds. These structures are called diffuse becauset h e i n t er s t e ll a r u lt r a v iol et r a d i a t i on e a s il y p en -e trate s. UV p h o to n s p lay a s ig n if ican t ro le in th echemistry of diffuse clouds as they ensure photodis-sociat ion of molecules. Fur ther, a toms a nd m oleculeswith io n iz at io n p o te n tials ( I P) le ss th an o f ato m ichydrogen (13.59 eV) can be photoionized. This limitre p re se n ts th e h ig h e st e n e rg y avai lab le to p h o to -dissocia te a nd photoionize; shorter wa velengths a reabsorbed by interstellar h ydrogen a toms a nd, hence,a r e con f in ed t o r e gi on s a r o u nd b r ig h t es t s t a r s . 34

Therefore, most of the atomic carbon C(3P j) (11.26eV), silicon Si(3P j) (8.15 eV), s ulfur S(3P j) (10.36 eV),

an d m ag n e siu m M g (1S 0) (7.64 eV) is expected to beionized; nitrogen N(4S 3/2) (14.53 eV) a nd oxygen O(3P j)(13.62 eV) should exist in their a tomic form. So fa rth e d ia to m ics H 2, C H , C H +, C S , C N , C 2, O H , H C l ,a nd C O ha ve been detected una mbiguously. The onlytr iat omic species identified are HC N, HNC , C2H , a n dH C O+.35 Very r ecent ly, C 3 ha s been observed towa rdO p h a n d P e r .36 These m olecules a ccommodat estrong ca rbon-hydrogen, carbon-carbon, and car bon-nitrogen ba ckbones. F ormaldehyde (H 2C O ) a n d t h earomatic species cyclopropenylidene (c-C 3H 2) h a v ebeen detected a s w ell. Although la rge molecules a rea l s o t h o u gh t t o b e p r es en t i n d if fu s e c lou d s a spolycyclic a romat ic hydrocarbon (P AH)-like st ruc-

t u r es , t h e l a t t er a r e n ot f or m ed i n s i t u b u t a r einjected int o the interst ellar medium from w inds ofdying, carbon-rich stars or indirectly via erosion ofcarbonaceous solid-sta te ma tt er (section II .D ).

2. Translucent Clouds

Tr a n s l u cen t cl ou d s a r e s u gg es t e d t o f or m t h ebridge between diffuse a nd dense structur es. Theseclouds ar e defined by modera te num ber densities of102-103 at om s cm -3 and relatively low kinetic tem-p e ratu re s o f 50-100 K com p are d to d i ffu se stru c-tures. 37 The molecular composition of tra nslucentcl ou d s s u ch a s C y g O B 2 N o. 12, H D 29647, H D

147889, a nd C a s A is well-reflected by sma ll diat om-ics H 2, C H , C H +, C S , C N , C 2, OH, an d CO to g e th e rwit h larger species HCN, HNC , C2H , H C O+, an d H 2-CO. The t ricar bon molecule C 3 wa s te n ta t ively id en -tified tow a rd H D 147889. Further, th e abunda nce ofin te rstel lar CH 2 is significantly enhanced in a coldtra nslucent cloud compar ed t o low-density regions.So far, no signat ures of early st a rs, so-called pre ma insequ e n ce st ar s , h ave b ee n fo u n d in th e se e n viro n -ments.

3. Dense Clouds

Dense clouds are formed from low-density cloudsan d ch aracte rize d b y typ ical n u m b er d e n sit ies of

102-104 ato m s cm -3. D u e t o t h e ir l ow k in et i c g a stemperat ures of only 10-15 K,38 these dense struc-tures a re a lso referred to a s cold clouds. Whereas ind if fu s e a n d t r a n s l u ce nt cl ou d s t h e f or m a t i on ofmolecules is dominated by photochemistry and pho-to io n iz at io n , in te rste l lar d u st p art icle sssubmicro-meter-sized silicate- a nd carbonaceous-based gra innucleisinsid e dense clouds sh ield complex moleculesfrom the destr uctive short w a velength r a diat ion field.

Therefore, the flux of the interstellar UV radiationfield drops from 108 photons cm -2 s -1 to a re sid u alf lu x of on l y 103 photons cm-2 s -1. Th e l a t t e r i sdicta ted by t he cosmic ray part iclesmostly energeticprotons and helium nuclei-driven ionization of mo-lecular hydrogen followed by a recombina tion of a nelectron from the cosmic rad iat ion field wit h H 2+ t oform excited hydrogen. These excited states decayr a d i a t i v e l y a n d e m i t p h o t o n s i n t h e U V a n d V I Srange of the electromagnetic spectrum. Note, how-ever, t ha t compared t o the inner regions of molecularclouds, the outer rims are more diffuse (102 a t o m scm -3) and hence bombarded by a significantly higherphoton flux. Due t o the efficient extinction of UV a nd

V I S l ig h t b y d u st , d e n se clo u d s b lo ck e n tire ly th elight of stars, which lie behind them. Therefore, denseclou d s a re a lso k n own as d ark clou d s b e cau se t h e yof t en a p pe a r on i m a g es a s b la c k p a t c hes . Th es eclouds contain an unprecedented variety of neutralmolecules and hence are dubbed molecular clouds.Th e ch em istry of th e Tau ru s M olecular Clou d 1(TMC-1) has been studied extensively. Its composi-tion is domina ted by molecula r hydrogen (H 2); onlytra ce am o u n ts o f at om ic h yd rog en of a b ou t 1 a to mcm -3 exist. In strong contrast to diffuse clouds, thefractional ionization in dense clouds is only 10 -6-10-8. This is w ell-reflected in a significan t d ensity ofn e u tral carb o n ato m s C(3P j) as d e te cte d to wa rd t h e

molecula r clouds Orion A,39 TMC-1, 40,41 L134N, a ndIC 514642 w ith ra tios of 0.05-0.2 compared t o carbonmonoxide (CO). The la tt er furn ishes t he second mostab u n d a n t m olecu le with fract ion al ab u n d an ce s f of8 10-5 com p are d to h yd rog en , fol lowe d b y th ehydroxyl radical (OH; f) 10-7). Dense clouds containfurther a rich variety of hydrogen-deficient carbonchains cyanopolyynes, cummulene carbenes, methy-l a t ed m ol ecu les , a s w e ll a s H -, N -, O -, a n d S -termina ted ca rbon clusters (Table 1). In part icular,the la rge fra ctiona l a bunda nces of th e cyclic moleculec-C3H 2 (fe 2 10-8) and c-C 3H (fe 10-9) should behighlighted. Cya nopenta a cety lene (HC 10CN) presentsthe largest single molecule identified so far in TMC-

1.

4. Star-Forming Regions, Young Stellar Objects, and HotMolecular Cores

D en s e cl ou ds a r e i n a p recol la p se ph a s e a n dco n tain th e b asic in g re d ie n ts to fo rm m assive an dlow-m a ss st ars . 43 Dense clouds represent thereforethe format ion sites of sta rs such as our Sun. Whereasspectr a l lines observed t owa rd t he dense cloud TMC-1show lit tle evidence of cloud rota tion or collapse, t heth e ory of s ta r form a tion r e qu ire s th a t th e core o f aquiescent molecular cloud collapses under its owng ravity . T h e se co re s h ave m asse s o f typ ical ly 104

solar m asse s a n d a re d e n se r th a n t h e ou te r re g ion s



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of the molecular clouds. The very first step towardthe core collapse is thought to be the condensationof ga s-phase species onto th e cold ma ntles of gra insinside dense clouds.44 This a ggrega tion cont inues forabout 105 year s, unt il the force of gra vity overcomest h e r es i st a n c e p r ov id ed b y t h e g a s p re ss u r e a n dma gnetic pressure. As the core collapses, it fra gmentsinto clumps of about 10-50 solar masses. Note thatthe Orion Molecular Cloud (OMC) has two of these

clumps in the direction of the Kleinman -Low (KL)Nebulasthe hot core and the compact ridge. 45

Once a clump has broken free from the other partsof the cloud core, it has its own gravity. The dynamiccollapse proceeds, a nd a ngular momentum turns thecl um p i n t o a r ot a t i n g d i sk : a pr ot os t a r a n d a naccretion disk are formed onto which material fromthe surrounding ma tt er falls down. The infalling gasreleases its kinetic energy in the form of heat, henceth e te m p e ratu re an d p re ssu re in th e ce n te r o f th eprotosta r increase. As its temperat ure a pproaches afew t housand K elvin, it becomes a n infra red source,which is indicative of a young stellar object (YSO).The whole process might take up to 107 ye ars an d is

accompanied by a significant change of the physica lparameters. The density increases from 104 ato m scm -3 in co ld clo u d s u p to 109 a t o m s c m -3 i n t h ecircumstellar disk still surrounding the young stellarobject. Tempera tur es ra nge from 10 K in the collaps-in g e n ve lo p e to a fe w th o u san d K e lvin in th e g ass h ock ed b y t h e i mpa c t . A ft er t h e n e w s t a r h a sform e d , i ts ra d iat io n h e ats th e su rro u n din g m at te ran d molecules sublime from the r emaining icy gra inmantles back into the gas phase. The infall of massis s to pp ed wh e n th e rm on u clear fu sion (h yd rog enburning) begins. This phase is accompanied by strongs t e l l a r w i n d , u s u a l l y m o r e o r l e s s p a r a l l e l t o t h erota t ion a xis. S in ce th ey o ccu r a t th e n o rth e rn an dsouthern poles of the young stars perpendicular tothe a ccretion disk, they a re often dubbed a s bipolaroutflows. These outflows drive st rong sh ock w a vesto the surr ounding ga s, wh ich can be heated t o 1000K or more; t his presents a second source to sublimemolecules from the gra in ma ntles into the ga s phase.T h is e arly p h ase o f th e l i fe o f th e star is cal le d TTauri phase.46 T Taur i sta rs such a s t he Tra peziumClu ste r in th e Orio n N e b u la are always e m b e d d e din the clouds of gas from which they were born andcan lose up to one-ha lf of its m a ss before becoming am ain se qu e n ce star . T h is u l t im ate m ass lo ss le ad sto th e cleaning sta ge wh ere the envelope is dispersed.

Once the ma in sequence is rea ched, no ma ssive lossof ma tt er occurs. The ma ss of the sta r is fixed as longa s t h e h y d r og en b u r ni n g i n i t s ce nt e r con t i n ue s.Young main sequence stars are often hot (OB stars)and influence the molecular abundances by photo-dissocia tion a nd ionizat ion-forming a reas of ionizedhydrogen, so-called HII regions.47

The energetic environment surrounding a protostaris called the hot molecular core (HMC) containings e v e r a l t e n s o f s o l a r m a s s e s . H M C s a r e f o u n d i nr eg ion s of m a s s i ve s t a r f or m a t i on s u ch a s t h e FOphiuchi complex wit h 300 young stella r objects. 48,49

The thermal ra diation from the centra l sta r heat s thegrain particles and sublimes the ice mantles progres-

sively.50,51 This phase is characterized by densitiesup t o 108 atoms cm -3 a nd kinetic temperat ures of gas-phase molecules of 100-300 K as fo u n d in th e h o tcore Sgr B2(N), Cepheus A, Orion A, and W51. 52,53 I tis apparent that this thermal sublimation should leadto a distinct molecular composition of hot molecularcore s com p are d to d e n se clou d s. Th is h as b ee nconfirmed. The molecular inventory of hot cores isdominat ed by sat ura ted molecules (CH 3OH, C 2H 5OH ,

C H 3OC H 3, C 2H 5CN, CH 3COCH 3, C H 4, H 2O, and NH 3)wh ich a re enriched by a factor of 103-105 comparedto quiescent molecula r clouds.54-58 These environ-m e n ts con ta in fu rth e r a g re at varie ty of com p le xepoxides, aldehydes, ketones, and acids, for instancethe cyclic molecule ethylene oxide (C 2H 4O),59-61 a c-eta ldehyde (CH 3CHO), formic acid (HCOOH), acetica cid (CH 3COOH ), glycolaldehyde (HOC H 2CH O), car-bon dioxide (CO2), and possibly glycine (H 2NC H 2-COOH).62-67 Th e on ly u n satu ra te d m olecu les d e -tected so far are vibrationally excited cyanoacetylene(HC CCN) towa rd G 10.47+0.0368,69 a nd vinyl cyanide(C 2H 3CN) in S gr B 2(N).70 With the exception of theethinyl ra dica l (C 2H), no hydr ogen-termina ted carbonch ain s C nH (n ) 3-8) ha ve been found in hot cores.

Since current gas-phase processes cannot repro-d u ce t h e o b serve d n u m b e r d e n sit ies of sat u rat e dmolecules, these species are suggested to be synthe-sized on interstellar gra ins a t 10 K in the molecula rcloud stage and then released into the gas phase bysublimation in hot molecular cores. This hypothesishas been supported very recently employing formal-dehyde (H 2CO) and deutera ted molecules as t ra cers.D e pe nd in g on w h e t h er t h e n u cl ea r s pi n of b ot hprotons is para llel or a ntipar a llel, forma ldehyde ca ne xi st i n a n or t h o (o) o r p a r a (p ) for m . S i n ce t h econ v er s ion f r om on e t o t h e ot h e r s pi n s t a t e b y

collisiona l pr ocesses is str ictly forbidden, o/p a bun-dances yield information on the molecular formationtemperature. 71-73 At elevat ed tempera tur es of 300 Kas found in the hot molecular core L1498, the ratioof the sta tist ica l weight of o-H 2C O/p-H 2CO of 3 wouldbe expected, if forma ldehyde is synth esized in t he ga sphase. However, the actually observed ratios in hotcores L723 and L1228, w hich ha ve ty pica l t empera -tures of 300 K, of 1.5-2.1 suggest a thermalizationa n d h en ce f or m a t i on of f or m a l d eh y d e a t a l ow e rkinetic t empera ture. This finding st rongly suggestsa synthesis of formaldehyde on 10 K grains, followedby a sublima tion into the ga s phase during the HMC

stag e .

74

Second, a significant deuterium enrichmenta s f ou n d i n H D O , N H 2D , H D C O , a n d D 2CO wh ichexceeds t he cosmic D/H ra tio of 10-5 b y a facto r o f102-103 supports the hypothesis that molecules inhot cores sublime from grains. 75 Since t he process ofD fractionation depends on small zero-point vibra-tional energy differences, the observed fractionationin HMCs must have occurred at T < 20 K but not inthe h ot ga s of the molecular cores.76,77 An explosivedesorption of molecules from grains, which is trig-gered by ra dica l recombina tion on gra ins a t 10 K orupon heat ing to T > 30 K, presents a nonequilibriumscenar io to replenish molecules from th e gra ins ba ckin to th e g as p h ase .78



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As Y S Os pr es en t b r ig h t a n d n a t u r a l i nf ra r e dsources, they allow a detection of molecules in theirenvelopes a long the line-of-sight to terrestr ial orspace born telescopes. The identification of gaseoushydrogen cyanide (HCN), acetylene (C 2H 2), carbondioxide (CO2) , a n d m e t h a n e ( C H 4) i n t h e i n f r a r e dre gim e e m p loyin g th e I n frare d S p ace Ob se rvat ory(ISO)79,80 clearly demonstrates the unique power ofth is te ch n iqu e.81 M os t i m por t a n t , t h e m ol ecu la r

composition of ices can be unra veled once the YS Oforms in the infra red regime utilizing the IS O; reca llthat molecules frozen on grains show no rotationals pe ct r u m a n d h en ce ca n n ot b e p r ob ed v ia r a d i otelescopes. W33A and NG C7538:IRS 9 denote th e beststu d ied p roto stel lar an d h ig h -m a ss yo un g ste llarobjects, respectively.82-85 S o far , o n ly e ig h t s im p lemolecules have been identified unambiguously in icesto ward p ro to ste l lar an d YS Os. 86,87 This is based onthe relat ively insensitive IR detection method com-pared to microwave spectroscopy. The identificationof m in or sp ecie s of con cen tra t ion s less th a n 0.01relative to wa ter rema ins a tremendous challenge forthe future. However, once those species are released

into the ga s pha se, ra dio telescopes ca n observe thesemoleculessif they bear a perma nent dipole moment.The water molecule (H 2O) is b y far th e d o m in at in gice component, whereas carbon monoxide (CO; 0.08-0.15) a nd ca rbon dioxide (CO 2; 0.12) a re a bout 1 orderof m a g n i t u de l es s a b u n d a n t .88-93 M eth a n e (CH 4;0.004-0.019), a mm onia (NH 3; 0.15), formaldehyde(H2CO; 0.06), a nd car bonylsulfide (COS , 0.018) presentonly minor components. The a bunda nce of meth a nol(CH 3OH) depends strongly on the history of the ice(unpr ocessed vers us ph otolyzed/bomba rded ices) a ndcan vary significantly (0.03-0.3) from its averagedvalue of 0.18. 94-98 Th e assig n m e n t of form ic a cid(H C O O H ; 0. 07), t h e H C O O - a nion (0.008), a nd

ace tald e hyd e (CH 3C H O ; 0. 1) m u s t b e r e ga r d e d a ste n tat ive .99,100 Furt her, the 4.62 m b an d wh ich wassu g g este d to corre lat e with ch ar g ed p art icle p ro -ce ss ed i ce s h a s b ee n a t t r i bu t ed t o O C N- (0.04);however, this a ssignment remains t o be investiga tedin depth. Note that molecules in ices around YSOsare not distributed homogeneously but highly frac-t ion at e d . I n cold ou te r e n ve lop es of YS O, w h e retypical t empera tures of 10-20 K reside, ices conta ina significant fraction of nonpolar carbon monoxide(CO) a nd carbon d ioxide (CO 2) (apolar ices). As wemove closer t o the inner w a rm envelope, the temper-a t u r e r is es t o 20-90 K a n d m ol ecu les s uch a sm e th an e (CH 4) a n d carb on m on oxid e (CO) a re t oo

v ol a t i le t o e xi s t i n i ce s; t h e r ef or e , p ol a r i ces ofm e th an o l an d w at e r are ve ry ab u n d an t . M ost l ik ely,this fr actionat ion could be the r esult of a successivesu b lim atio n o f ap o lar ice co n sti tu e n ts with r isin gtemperature.

5. Circumstellar Envelopes

Once the contra ction of a protosta r is f inished, thes t a r e vol v es on l y s l ow l y b u r ni n g h y d r og en i n t ohelium.101,102 Our S un presents a typical example ofthese main sequence stars. The continuing nuclearfu sion le ad s t o s tru ctu ra l ch an g e s of th e sta r a s t h erad iu s an d th e lu m in o sity in cre ase ste ad ily . On cehydrogen is exhausted in the central core, the evolu-

tion accelera tes ra pidly. H ydrogen is now fused in ashell surrounding th e inert helium core. In t urn, t hecore increases its density and temperature via con-traction until helium ignites. In the center, heliumfusion via metast able 8B e t o 12C a n d 16O is the centra lenergy source. The cycle of core and shell burningwith in cre asin g d e n sit ie s an d te m p e ratu re s in th ecore con tin u e u n ti l th e m ost s ta b le 56F e n u cle i a reformed. Elements hea vier tha n iron ca nnot be formed

by nuclear fusion as these processes are endoergic.T h e f in al fate o f th e star d e p e n d s o n i ts m ass an dlead s to a wh ite d w ar f , n eu tro n sta r , o r b lack h ole.

D u r i n g t h e h e l i u m f u s i o n , t h e s t a r e x p a n d s i t ssh e ll . Un d e r ce rtain circu m stan ce s, a re d g ian t isformed, w here the car bon-oxygen core is sur roundedby a h elium-burning shell, a helium buffer layer, a nda hydrogen-burning shell. These stars are becomingin cre asin g ly u n sta b le an d leave th e m a in se qu en ceto become an asymptotic giant branch (AGB) star. 103,104

R e c a l l t h a t e a c h l o w a n d i n t e r m e d i a t e m a s s s t a rhaving 1-5 solar masses goes in its evolution throught h e AG B s t a g e. At t h is s t a g e of i t s l if e, t h e s t a rreturns ma tt er in steady w inds back into interstellar

space. Since these ejecta contain elements heaviert h a n h y d r og en , t h e ch em i ca l com p os i t ion of t h einterstellar m edium is enriched in these heavy a toms.N ew sta rs a re form e d from th is m a te rial , an d t h e irr et u r n w i l l e n r i ch t h e i n t er s t el la r m ed iu m e ve nfurther. These late-type AGB stars lose significanta m ou n t s o f t h ei r m a s s on ce t h ey l ef t t h e m a i nsequence and are surrounded by expanding matter,which is often called the circumstellar shell or thecircumstellar envelope (CSE).105-107 A G B s t a r s a r eknow t o be surrounded by expanding shells. Circum-stellar envelopes consist of gas-phase molecules andsubmicrometer-sized grain particles. Therefore, thesestars m u st b e re g ard e d as a so u rce o f in te rste l largra in ma teria l: once dust is formed, it is subjectedt o r a d ia t i on pr es su r e f r om t h e cen t r a l s t a r a n daccelerated outward from the circumstellar envelopei n t o t h e i n t er s t el la r m ed iu m . D e pen d in g on t h eoxyg en -to -carb on rat io of th e ou t-f lowin g m a tte r ,AG B sta rs can be divided into three cla sses: M-, C-,a n d S -t y pe s t a r s . S -t y pe s t a r s a r e d ef in ed b y acar bon-to-oxygen ra t ios C /O ) 1. M-ty pe sta rs d epictC /O < 1 , a n d a l l t h e c a r b o n i s l o c k e d i n c a r b o nmonoxide (CO). The IR spectra of t hese sta rs showfeatures of amorphous silicate grainssmost probablyolivine-like matter (Fe,Mg)2S iO 4. As expected froma n oxygen-rich environment, M-type st a rs fa vor t he

production of silica tes a nd t he ga s-phase chemistryis dicta ted by simple oxygen-carry ing m olecules. InC- typ e stars , th e si tu atio n is re ve rse d an d d e f in e dby C /O > 1. Therefore, th ese sta rs ar e called carbonsta rs. All oxygen resides in carbon m onoxide (CO);C-type stars display a prominent feature of siliconcarbide (SiC).

The carbon star IRC+10216 is th e brightest carbon-rich object in the infrared sky.108 I t h a s an e xte n d edenvelope in which more than 60 species have beenobserved. This object is pa rticularly carbon rich, a sm an y carb o n clu ste rs C n (n ) 2, 3, 5), hydrogen-deficient carbon chains C nH (n ) 2-8), cyanopolyynes(H C 2nC N (n ) 1-4)), their r a dicals C 2nC N (n ) 1-2),



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cummulenes C nH 2 (n ) 3, 4, 6), and hydrocarbons(CH 4, C 2H 2, C 2H 4) have been detected in its circum-ste llar e n velop e. N ote th a t th e m olecules ar ou n dsta rs ar e n e arly arr an g e d in o n ion -l ik e sh e ll s t ru c-tu re s a ro un d th e cen tra l s t ar . Th e id e n ti ficat ion ofm e th an e , a ce tylen e , e th ylen e , to g eth e r with si lan e(SiH 4) m ak e s th is s tar e sp e cial ly valu ab le . T h e semolecules ha ve no perma nent dipole moment a nd a ren ot d e tecta b le w ith rad io sp ectroscop y; th e re fore ,

powerful IR background sources together with cut-t in g -e d ge I R te lescop es su ch as I S O ar e cru cial .I RC +10216 depicts furt her a n un precedented va rietyof metal-bearing species such as NaCl, AlCl, AlF,K C l , N a C N , M g NC , a n d M g NC , w h o se v i b ra t i on a ltemperat ures ha ve been determined t o be 700-1500K .109 With the exception of SiC 2, S iC 3, a n d S i C 4, a l lmolecules in CSE have been observed in interstellarclouds. No ion ha s been identified so far in a ny C SE .

However, th is simplified cat egoriza tion of a car bon-and oxygen-dominated chemistry in S- and C-stars,respectively, is incomplete. Herpin and Cernicharop rese n ted com p ellin g e vid en ce th a t tw o oxyg en -bearing species, wa ter (H 2O) and the hydroxyl radical

(OH), a re a b u n d an t in t h e in n erm o st re g ion of t h ecircumstellar envelope of the carbon-rich, preplan-e tary n e b u la CRL 681.110 A s th e in n e r re g io n s arevery close to the photosphere of the centra l sta r, t heubiquitous carbon monoxide molecule (CO) can bephotodissociated to carbon and oxygen atoms. 110,111

T h e lat te r we re su g g e ste d to re act with m o le cu larhydrogen (H 2) to form H 2O an d OH rad icals . L ik e -wise, the methanol molecule (CH 3OH) as detected int h e C S E of I R C +10216 could be the product of thereaction of oxygen atoms with methane.

Finally, the dust formation needs to be addressedb rief ly . D u st f or m s i n h ig h m a s s los in g AG Bs t a r s .112-114 Typically, the temperature and densities

at the outer edge of the st a rs photosphere reach upto 4500 K a n d 6 1015 at om s cm -3. S i n ce t h e s t a rcan be approximated as a point source, the numberdensity drops roughly w ith t he inverse squa re of theradius from the center. I f a region above the photo-sphere is cooled t o a bout 1700-2000 K, gas-phasemolecules are allowed to seed to dust grains. In thein n er re g ion s of t h e C S E, th e d u st cools t o 1000-1200 K. Ca rbon-rich gra ins a re of part icular impor-ta nce, an d tw o mechanisms ha ve been postula ted onh ow t h e y ca n b e f or m ed . Th ey i n vol ve e it h e r asuccessive build-up of neat carbon clusters via se-quential addition steps or are speculated to compriseth e a cety lene molecule to form benzenesthe very firstar omat ic moleculesvia multiple rea ctions (sectionsI I .A an d I I .D ).115

6. Planetary Nebulae

P laneta ry nebulae (P Ne) ar e the descenda nts fromAG B stars . 116,117 This phase is char a cterized by a veryefficient mass loss of the dying star. The reshapingof the circumstellar envelope which is initially formedby ma teria l expelled during the AGB phase over 106

ye ars is d ictate d b y h ig h - ve lo ci ty win d s fro m th ece n tral s tar . A n in cre asin g p h o to n f lu x in th e UVran g e acco m p an ie s th is p ro ce ss as th e re m ain in ghydr ogen shell burns up by nuclear fusion. While the

ejecta move away from the central star, the enhancedphoton emission will gradually photodissociate and-ionize the circumstellar shell which at the sa me timeis growing a s more ma teria l is swept up.118,119 Thesefast stellar winds and photon field sweep the circum-stellar envelope into the ring-shaped, planetary-likestructure w e see in planeta ry nebulae. At some sta geof their evolution, dust erosion might play an impor-tant role. Despite their unique name, these objects

a r e n ot cor r el a t e d t o p la n e t s . Wh en t h e s t el la rhydr ogen envelope is burn ed up completely, t he corewil l cool , d e cre ase s in lu m in osity , an d b ecom e s aw h i t e d w a r f . D u r i n g t h i s p ha s e , t h e c i r cu m s t el la rm at te r e xp an d s m ore , f in al ly r e plen ish in g in to th ein te rclou d re g ion s. D u e to th e low d e n sity of th elat t e r (


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or radical BC which leads to the generic products ABan d C o r AB plus C (Figure 2). On the basis of the

energetics, tw o ca ses ca n be distinguished. If t he freeenergy of th e products is lower t ha n t he free energyof t h e se par at e d re acta n ts , th e r e action is e xoe rg ic(AB + C); if the situa tion is reversed, th e reaction isendoergic (AB + C). B e sid e s th e e n e rg e tics , twolim itin g case s o f th e re actio n d yn am ics h ave to b ea d d r es s ed . I f A a n d B C r e a c t v i a a t r a n s i t i on s t a t e[ABC] to th e products, t his rea ction is ca lled direct.On th e o th e r h an d , th e re actio n can g o th ro u g h ani n t er m ed i a t e [AB C ] *; t h e se d y n a m i cs a r e ca l l edindirect . The int ermediate can be formed eitherb arrie rle ss o r via an e n tran ce b arr ier (TS in ). Com-pared t o the sepa ra ted rea cta nts, [AB C]* is lower inenergy. D ue to energy a nd momentum conserva tion,

t h e f r ee r ea c t ion en er g y m u st ch a n n el i nt o t h einternal (rotation, vibration, electronic) degrees offreedom. Hence, this intermediate is internally ex-ci t ed a n d ca n u n d er g o v a r i ou s r ea c t i on s . F i r st , aunimolecular decay might form AB + C or AB + C .These fra gmenta tions may proceed via a n exit t ra n-sition st a te (TS exit ) or without exit barrier. Second, ifthe num ber density of th e gas-pha se molecules andhence the collision freq uency is la rge, th e interm edi-ate can collide with another bath molecule M. Thisthree-body collision diverts the excess energy of theenergized intermediate int o the t ra nslat iona l/interna ldegrees of freedom of the third-body collider forminga stable ABC species. This process is of particularimportance in dense planetary atmospheres or in thesolid sta te. In solids, the interna lly excited interme-diate can couple with the phonons of the solid target.L a s t l y , [AB C ]* c a n r e a ct b a ck t o t h e r e a c t a n t s ormight be stabilized via photon emission; the latterprocess converts (parts of) the internal energy intoelectromagnetic radiation. Note that [ABC]* mightisom e riz e p rior to f rag m e n tat ion or s ta b i liz at ion .Multiple electr onic surfa ces a nd/or int ersyst em cross-in g m ay com p licat e th e sce n ario. Th is s im plif iedtre at m e n t of a ch em ical re action h e lp s to e xtra ctgeneralized concepts on th e a str ochemical processingof distinct interstellar environments.

1. Solid State

Although th e dust component embodies only 1%ofthe interstellar ma tt er, these predominant ly silicate-a nd car bona ceous-based gr a in nuclei play a key rolei n t h e f or m a t i on of n ew m ol ecu le s. D e ep i n t h einterior of dense clouds, grain particles effectivelyshield newly synt hesized molecules in the ga s pha sefrom th e d e stru ctive exte rn al UV ra d iat ion f ie ld .Most importa nt , th ese submicrometer-sized part icles

p rese n t valu ab le n u rse ries to syn th e siz e n e w m ol-e cu le s. I n d e n se clo u d s, th e se g rain s h ave typ icaltemperatures of 10 K.139,140 Once molecules, r a dicals,o r ato m s fro m th e g as p h ase co l l id e with th e so l idp a r t i cl e, t h e y a r e a c cr et e d on t h e g r a i n s u r fa c e,r es u lt i n g i n a n i cy m a n t l e u p t o 0 .1 m thick. Atultra low temperat ures, a ll species except H, H 2, a n dHe hold sticking coefficients of unity. This means thate ach col lis ion of a g as-p h ase sp ecies with a coldsurfa ce leads t o an a bsorption and hence thickeningof the ice layer. Here, solid mixtures containing H 2O(100), CO (7-27), CH 3OH (


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low -tempera tur e icy gra ins, but a lso on car bonaceousan d si l icate g rain p art icle s.142-153 Re call th at re ac-tion s b etw e en tw o at om s or ra d icals d o n ot in volveany entra nce barrier. Gas -grain models suggest thatt h e r ecom b in a t i on of t w o h y d r og en a t o m s i s t h ecent ra l source of molecula r hy drogen,154-157 which hasbeen detected via pure rotat iona l lines158,159 in emis-sion a round T Ta uri st a rs, 160 in th e Orion b a r ,161 ins u pe r n ov a r e m n a n t s ,162 t o w a r d s t a r -f or m i n g r e -

gions,163

ar ou n d circu m ste llar accretion d isks ofyo u n g stars ,164 an d to war d you n g st e llar ob je cts . 165

Vibrationally excited hydrogen has been identifiedfu rth e r in e m ission to war d th e d i ffu sive clou d Ophiuchi.166 The surface recombination of two ad-sorbed hydrogen atoms via the Langmuir -Hinshel-wood mechanism might account for the molecularhydrogen abun da nces a s found in dense clouds. Sincet h e H -H b o n d i s 4 . 5 e V s t r o n g , t h e H 2 moleculewh ich is formed on the gra in surfa ce is vibrat iona llyexcited a nd hence ca n be desorbed from the gra in intot h e g a s p h a s e .167,168 A lte rn ative ly, an Ele y -Rid e almechanism , w hich involves t he collision betw een onegas-phase hydrogen atom and another on a surface 169

or a negative ion route via H- 170,171

present alterna-tive routes t o molecular hydr ogen. The rea der shouldk ee p in m i nd t h a t t h e g r a i n a ct s a s a ca t a l y s t forthe recombinat ion of tw o hydrogen at oms; in t he gasp ha s e t h i s r e a c t i on i s n o t f ea s i b le , a s t h e n ew l yformed hydrogen molecule is highly rovibra tionallyexcited and cannot get rid of its internal energy viaa th ree-body react ion due to the low -density medium(Table 2). Therefore, any internally excited H 2 mol-ecule which is formed in t he ga s pha se will dissociat eb ack to th e re actan ts . Ho we ve r, in te ractio n o f th einternally excited hydrogen molecule on the grainwith th e p h on on s can tra n sfer a s ign if ican t am ou n tof interna l energy to the solid mat ter, thus st a bilizing

the new ly formed H 2 species on t he gra in or releasingit in to th e g as p h ase .

Th e m e ch an ism to form m o le cu lar h yd rog en issu g g e ste d to b e clo se ly re late d to th e syn th e sis o fsimple hydrides on icy gra ins. Since at omic hydr ogeni s e v en m ob il e a t t e m pe ra t u r es a s l ow a s 10 K ,h y d r og en a t om s ca n r ea c t w i t h a c cr e t ed h ea v i era t o m s s u c h a s C , N , a n d O v i a m u l t i p l e r e a c t i o ns eq u en ces . R ea c t i on s 1 a n d 2 p r es en t a t y p ica lexample to form the water molecule on interstellarg rain s

These processes can expla in t he forma tion of CH 4,NH 3, and H 2O t ogether with their deuterat ed counter-p a r t s .172-174 I t sh o u ld b e stre sse d th at cu rre n tly n oscientific proof exists wh ich support s tha t speciesheavier tha n deuterium ar e mobile on grain surfa cesat 10 K . He n ce , re actio n n e two rk s 175-177 involvingsu rface reaction s o f th e rm aliz ed C , N , a n d O at om sas well as polyat omic ra dicals such as H CO, CH, C H 2,C H 3, OH, N H, an d N H 2 with species except H and Datoms must be regarded as purely speculative. 178

T h e ad d it io n o f h yd ro g e n ato m s to u n satu rate dm ol ecu le s r ep r es en t s a t h i r d i m por t a n t cl a s s of

therma l rea ctions involving mobile hydrogen a toms.Alth ou g h th e in it ial ad d it ion ste p to close d-sh ellspecies such a s C 2H 2, C 2H 4, CO, a n d H CN in volvesentra nce bar riers of up to 20 kJ mol-1, th e lat te r canbe overcome due to the unique ability of hydrogenat oms t o tunnel through barr iers.179 These successivehydrogenation processes can lead ult imat ely to fullysat u rat e d m o le cu les on g rain su rfaces an d cou ldsynt hesize, for exa mple, CH 3OH an d CH 3NH 2 via H 2-

C O a n d C H 2NH, respectively. The suggested forma-tion route to methanol via hydrogenation of carbonmonoxide and forma ldehyde is outlined in reactions3-6179

I t sh o u ld b e em p h asiz ed th a t o n ly H a n d D at om scan tunnel through entrance barriers. Reactions of,for example, CH 3 a n d N H 2 ra dica ls adding t o doublea nd tr iple bonds involve significa nt ent ra nce bar riersof up to 30 kJ mol-1. E ven if current m odels shouldu n d e r e s t i m a t e t h e m o b i l i t y o f t h e s e r a d i c a l s , t h el a t t e r ca n n o t ov er com e t h e e nt r a n c e b a r r i er v iatunneling. Therefore, rea ctions of therma lized CH 3a n d N H 2 species wit h closed-shell molecules a reclearly irrelevant at tempera tures as low a s 10 K a ndcan be disregarded.

Ho we ve r, in te rste l lar sp ace is in te rsp e rse d withultraviolet photons (e13.59 eV) a nd energetic par-ticles from T-Ta uri w inds, st ella r jets, carbon st a rs,

and galactic cosmic ray particles. Therefore, pristineice ma ntles a re processed chemica lly by the cosmicray -in d u ce d in te rn al u l traviolet ra d iat ion p rese n teven in the deep interior of dense clouds ( ) 103

photons cm -2 s -1) an d , in p art icu lar , th ro u g h p ar-ticles of t he ga lactic cosmic ra diat ion field. This ca nlead to the formation of new molecules in the solidsta te via n on e qu il ibriu m (n on th e rm al) ch em istrye ve n at te m p e ratu re s as lo w as 10 K . T h e p art iclecomponent of the cosmic ray ra diat ion field consistsof 97-98%protons (p, H +) a n d 2-3%helium nuclei(R-particles, He2+) in the low-energy range of 1-10MeV (1 MeV ) 106 eV) w ith ) 10 part icles cm -2

s -1 a nd higher energies up to the P eV limit (1 PeV )

1015

eV) ( ) 10-12

particles cm-2

s-1

). The chem ica levolution of interstellar ices by bombardment s w ithultraviolet (UV) photons180-184 an d cosm ic ray (CR)particles is well established. 185-200 A photon can bea bsorbed by a single molecule in th e ice; this processis followed predominantly by a selective bond ruptureof a s in gle b on d . I f a h yd rog e n at om is re lease d inthis photodissociat ion, it can hold kinetic energies upto a few electronvolts. The radical formed is inter-n a l l y e x ci t ed a n d m i gh t r ea c t w i t h a n ei gh b or i ngmolecule. Since the interna l energy of the ra dical canb e cou p le d in to th e re action coord in ate , e n tran cebar riers could be passed. For exa mple, vibra tionallyexcited CH 3 a n d N H 2 r a d i ca l s cou ld n ow a d d t o

O(3P j) + H (2S 1/2) fOH(X

2

) (1)

OH(X2) + H (2S 1/2) fH 2O(X1A1) (2)

CO(X1+

) + H (2S 1/2) fHCO(X2A) (3)

HCO(X2A) + H (2S 1/2) fH 2CO(X1A1) (4)

H 2CO(X1A1) + H (

2S 1/2) fH 2COH(X2A) (5)

H 2COH(X2A)+ H (2S 1/2) fC H 3OH(X

1A) (6)



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double and t riple bonds; this is in st rong contr a st t oth e ir th e rm al re actio n s in wh ich th e e n tran ce b ar-riers cannot be overcome.

C om p a r ed t o U V p hot on s w h i ch a r e a b s or b edwit hin a bout 0.05 m of the ice, CR particles such asa 10 MeV H + penetrate the grain easily and depositu p t o 1 M eV in sid e th e icy m a n tle . S in ce ch e m icalbonds are up to 10 eV strong, this energy exceedst h e s t a b i li t y of t h e m ol ecu le . F o r i n st a n ce , a C R

p article can in te ra ct w ith a m e th an o l m olecule an dknocks off an H atom which carries kinetic energyup t o 10 keV. The C H 2OH or CH 3O counterfra gmentremains excited interna lly. This H a tom intera cts insu b se qu en t col l is ion s with th e sol id ta rg e t a n d re -l ea s es i t s ex ces s en er g y t o t h e t a r g et a t o ms i nsuccessive collisions via elast ic and inela stic intera c-tions to generate further high-energy carbon, hydro-g e n , an d o xyg e n ato m s. He re , th e e last ic p ro ce sstran sfe rs e n e rg y to th e n u cle i o f th e targ e t ato m signiting pr i m a r y knock-on p a r t i cl es (P K O s ; f ir s tgeneration of knock-on particles) if this amount isl a r g e r t h a n t h e b i n d i n g e n e r g y o f t h e a t o m . M e V

R-part icles, for exa mple, generate ca rbon P KOs w ithk in etic e n erg ies u p to 10 k eV. Th e se k n ock-onp article s can tran sfe r th e ir e n e rg y in co n se cu tiveencounters to the ta rget a toms resulting in a collisioncascade of seconda ry, t ertia ry, a nd higher knock-onato m s. M o d e rate d to ab o u t 1-10 eVsthe so-calledchemical energy ra ngesthese atoms are not in ther-m al e qu il ib riu m with th e 10 K targ e t (h e n ce , n o n -equilibrium or supra therma l part icles) and can reactfinally w ith t he ta rget molecules via elementa ry st epsof bond insertion, addition to double or triple bonds,or hy drogen a bstra ction. The power of supra therma lre action s to fo rm n e w m olecules at te m p era tu re sev en a s l ow a s 10 K i s b a s ed on t h ei r a b il it y t o

overcome reaction barriers in the entrance channeleasily, since suprathermal species can impart theirexcess kinetic energy int o the tra nsition sta te of ther ea c t i on . E v en r ea c t i on s en d oe r gi c a t 10 K a r ef ea s i b le a n d e xt e n d t h e s y n t h et i c p ow e r of t h i sreaction class beyond thermal processes on interstel-lar g rain s. T h e se u n iqu e asp e cts o f su p rath e rm alre action s r e sult in re action rat e con sta n ts k ordersof magnitude larger tha n their therma l counterpart s.D e tai le d calcu lat io n s o n th e re actio n s o f 1 e V su -p r a t h er m a l h y d r og en a t om s w i t h H 2O a n d C H 4 t oform H 2 a n d O H a s w e ll a s C H 3, r espectively, depictk(1 eV H, H 2O) ) 1.7 10-11 cm 3 s -1 a nd k(1 eV H,

C H 4) ) 5.0

10

-11

cm

3

s

-1

v e r s u s t h e r m a l r a t econstants of k(293 K, H + H 2O) ) 9.6 10-27 cm 3

s -1 a n d k(293 K, H + C H 4) ) 2.5 10-19 cm 3 s -1, ad i f fe re n ce o f u p to 16 o rd e rs o f m ag n itu d e . 201 I r -rad iat in g , fo r e xam p le , so l id CH 4, C 2H 2, a n d C 2H 4samples with MeV protons or helium nuclei leads toa broad product spectrum of synthesized species suchas a tomic a nd molecular hy drogen, H a nd H 2,202 C H n(n ) 1-4), C 2H n (n ) 1-6), C 3H n (n ) 4-8),203 largeralk an e s a n d a lk en e s with u p t o 18 car b on a to m s, aswell a s polycyclic a romat ic hydroca rbons (P AHs) upto coronene.204 Once molecules are formed on inter-ste l lar g rain s, g rain h e atin g can re d istr ib u te th e semolecules into the gas phase.205-209

L a s t l y , b a s i c d i f fe r en ce s b et w e en p h ot o n a n dchar ged par ticle processing of interst ellar ices shouldbe addressed. Compared to a photon, a CR-tr iggeredbond rupture does not follow optical selection rules.Furthermore, a CR particle generates suprathermalca r b on a n d ox y ge n p a r t i cl es f r om , f or i n st a n ce,metha nol via a n elast ic intera ction potentia l. Thesesp ecies can n ot b e form e d via p h oto lyse s. He n ce ,p hot o n a n d C R p r oce ss i ng of i ce s a r e e xp ect e d

to synthesize different molecules. A comparison ofMeV particle processing with photolyzed CH 4 ices at10 K o f fe rs fu rth e r in sig h ts in th e cru cial ro le o fs upr a t h er m a l a t o ms s uch a s ca r b on i n i ces . U Virradia tion experiments indicat e tha t 38%of the C H 4w a s converted t o methy l ra dicals (0.01%), C 2 speciesethylene (2.6%) a nd etha ne (8%), the C3 speciespropane, and higher hydrocarbons holding structuralu n its R -C H 3, R -C H 2-R, R 2CdC H 2, R 2CdC R H , a swell as HCCR; R indicates an organic group. Mostimporta ntly, neither a cetylene, C 2H 2, nor vinyl radi-cals, C 2H 3, could be sa mpled. S ince, however, C 2H 2m olecules we re fo rm ed in M e V irrad iat ion via aninitial insertion of a suprathermal carbon atom into

a carbon -hydrogen bond of a methane molecule, wem u s t con cl ud e t h a t U V e xp os u r e o f C H 4 cannotg en e rate su p rat h e rm al C at om s. He re , UV p h oto n si n t er a c t i n s i n g le q u a n t u m p r oces s es w i t h a C H 4molecule an d a re una ble to ignite collision cascadesto form suprathermal carbon atoms. In strong coin-cid en ce , G e rak in es an d co-wo rk er p ostu late d th eforma tion of ethylene a nd etha ne via rea ctions of UVphoton-induced CH 2 a n d C H 3 radicals. Since CH 2 a sw e ll a s C H 3 a re not mobile at 10 K, these processesmust be restricted to neighboring radicals in the UV-processed CH 4 ta rg e t . Alth ou g h calcu lat ion s sh owthat the CR particle flux in dense molecular cloudsi s 2 o r d e r s o f m a g n i t u d e l o w e r t h a n t h e i n t e r n a l

ultraviolet flux, each particle can generate about 100su pra th e rm al sp ecies in a 0 .1 m th ick icy laye r.Hence, the flux advantage of the UV field is clearlye lim in at e d b y th e a b i li ty of o n e CR to in d u ce u p t o100 energetic particles.

2. Gas Phase

The chemistry in t he ga seous int erstellar m ediumd e p e n d s stro n g ly o n th e e f fe ctive p h ysical p aram -eters. Since the kinetic energy of interstellar speciesis confined from ty pica lly 0.8 (diffuse clouds) to 0.08kJ mol-1 (dense clouds) on average, gas-phase reac-tions under thermodynamical equilibrium conditionsin in te rstel lar clou d s sh ou ld b e e xoe rg ic or on ly

slightly endoergic, exhibit l itt le or n o entra nce bar-rie rs , an d in vo lve e xit b arrie rs wh ich are lo we r ine ne r gy t h a n t h e s e pa r a t e d r e a c t a n t s . F u r t h e r , d u eto the low density of the interstellar cloud media , onlytwo-body collisions (binary reactions) are relevant.A three-body reaction occurs only once in a few 10 9

year s a nd is negligible considering ty pical lifetimesof d en s e cl ou d s of 105-106 ye ars. Circu m ste llare nv el op es of ca r b on s t a r s a n d p la n e t a r y n eb u la edepict a more diverse chemistry. Here, the temper-a tures can r ise to a few t housand Kelvin close to thephotosphere of the central star. Therefore, reactionswh ich are e i th e r e n d o e rg ic o r in vo lve an e n tran ceb arrie r b ecom e in cre asin g ly sig n if ican t . Th e e n -



12/50

hanced number density of up to 1015 at om s cm-3 int h e ou t er e dg e of t h e ca r b on s t a r s p hot os ph er edictates further that three-body encounters are im-p o rtan t .

The classical Arrhenius equation postulates thatt h e r a t e c on s t a n t k of a chemical reaction dependson th e t e m pe ratu re T a ccording to eq 7

Here, A denotes the preexponential factor, R t h e g a sco n stan t , an d Ea ct the classical activation energy. I fthis relation holds even down to typical interstellart em per a t u r es a s l ow a s 10 K a n d t h e a ct i va t i onenergy is positive, ra te consta nts of gas-phase r eac-t i on s w o ul d b e v a n i sh i n gl y s m a l l i n i n t er s t el la rclou d s an d n o ch e m ical p rocessin g sh ou ld occu r.However, the large number of molecules detected sofar su g g e sts th at th is co n sid e ratio n is n o t e n tire lyaccu rate .

2.1. Ion-Molecule Reactions. In the early 1970s,a s ign if ican t s te p forwa rd w as m ad e t o accou n t forthe formation of simple interstellar molecules and

io n s. I t was p o stu late d th at b im o le cu lar , e xo e rg icion -molecule rea ctions ha ve no entra nce ba rrier a ndproceed wit hin orbit ing limit s. This hy poth esis fulfillsa l l b a s i c r e q u ir em en t s f or a r e a ct i on t o occu r i ninterstellar clouds. Early chemical equilibrium mod-els of interstellar clouds incorporated this assumptionan d focuse d on ion -m olecule re action s, rad ia t ivea ssociat ions, a nd dissociat ive recombinat ion betw eencations a nd electr ons of the cosmic ra diat ion field toad va n ce in te rstel lar ch em istry.210-230 These modelsp rop ose d a cosm ic ra y (CR) d riven ion iz at ion ofm o le cu lar h yd ro g e n fo l lo we d b y an H 3+ ion-domi-na ted chemistry to form sma ll molecules via protontra nsfer to a n a cceptor molecule X231-234

Since the proton a ffinity of H 2 is low, H 3+ donatesH + wit h a la rge variety of a strochemica lly importa ntmolecules such as CO, H 2O, NH 3, a n d C H 4 a s w e l la s nit rogen and oxygen at oms. These models expla insurprisingly well the formation of small moleculessu ch a s wa te r , am m o n ia, a n d m e th an e , in p art icu lar

in diffuse clouds with their high number densities ofion s. At th is s ta g e, n e u tral -n e u tral re a ction s h a vebeen considered to be unimporta nt beca use they w erethought t o involve significa nt ent ra nce bar riers. Notet h a t t h e k ey m ol ecu le i n i on -molecule reactionschemessH 3+swa s detected only very recently in theinterstellar medium.235-239

However, the following years revealed that not allion -molecule rea ctions a re bar rierless. For inst a nce,the reaction of c-C 3H 3+ with atomic nitrogen, whichw a s t h ou gh t t o b e t h e k ey r ou t e t o i nt e rs t el la rnitrogen chemistry, is strictly forbidden in the coldin te rstel lar m e d iu m .240 As complex hyd rogen-defi-cient molecules were detected in dense clouds and

in th e o u tf low of carb on st ar s , i t b e cam e clear th a tion -molecule rea ctions a lone ca nnot reproduce theobserved gas-phase abundances of complex, carbon-b ear in g m olecules. Th e form a tion of in te rstel larisomers, in pa rt icula r t hose of C 3H , C 3H 2, a n d H C 3N,p rese n ted a se ve re p rob lem .,241-243 I on -moleculerea ction schemes incorporat ing a dissocia tive recom-binat ion of HC 3NH + wit h a n electron followed by a nisomerization of the neutral HC 3NH molecule prior

to d issociat io n p red icte d th a t HC CCN , H N CCC, an dHCCNC should be present in relative amounts of 240:1:8 to war d th e d e n se clou d TM C-1.244 However,actual observations depicted a ratio of 1000:1:8salowe r p rod u ction of th e th e rm od yn am ical ly m oststa ble cya noacetylene isomer. Therefore, importa ntproduction routes t o cyan oacetylenespossibly viahitherto neglected r eactions of tw o neutra l speciessa re obviously missing.

2.2. Neutral-Neutral Reactions. The inclusionof alternative, bimolecular exoergic neutral-n e u t r a lreactions into chemical models of, for instance, thedark cloud TMC-1 occurred only gradually, 245 pre-dominant ly because labora tory dat a w ere absent a nd

e nt r a n ce b a r r i er s w e r e a s s u m ed t o h i nd er e ve nexoergic reactions. Kinetic experiments verified thatt h i s s ce na r i o h ol d s p a r t i cu l a r l y f or r e a ct i on s ofg rou n d -sta te n i trog e n (N (4S 3/2))246-252 an d oxyg en(O(3P j)) a t o m s w i t h cl os ed -s h el l h y dr oca r b onmolecules.253-267 L ik e wise , re actio n s o f CH 3, C H 2,NH 2, a n d N H in th e ir e lectron ic g rou n d sta te s w ithhydrocarbons were found to ha ve significa nt ent ra nceb arrie rs as h ig h at 35 k J m ol-1.268,269 Note, however,that oxygen and nitrogen reactions with hydrocarbonrad icals270-272 a n d r e a ct i on s of t w o h y d r oca r b onradicals were found to be barrierless. 273-275

As dense clouds and evolved carbons stars contain

a grea t va riety of highly unsa tura ted carbon-bearingmolecules and n itriles, pa rticular a tt ention ha s beendevoted on how th ese complex species can be formedin the interstellar medium via neutra l-neutral reac-tions. Very often, kinetic models of distinct int erstel-l a r e nv ir on m en t s a r e d es ig n ed a n d t h e ou t pu t ofthese reaction networks is then compared with actualastronomical observations. However, each networkr el ies h ea v i ly on l a b or a t o r y d a t a . Th es e a r e (i )temperature-dependent rate constants over a broadtempera tur e ra nge from 10 K in dense clouds t o hightempera tur es in circumst ella r envelopes, (ii) rea ctionproducts, (iii) branching ratios, and (iv) intermediatesinvolved. Kinetic mea surements a t r oom temperat ure

an d u ltralo w te m p era tu re s (se ction I I I ) im p rove dt h e u n d er s t a n d i n g o n t h e r ol e o f n e ut r a l -n e u t r a lre action s in e xtra te rre strial e n viron m e n ts. Th e sekinetic studies demonstra ted explicitly t ha t r eactionso f g ro u n d - state carb o n ato m s, cyan o rad icals , an de th in yl rad icals w ith u n sat u rat e d h yd rocar b on s a reindeed ra pid and ba rrierless and proceed wit h a lmostunit collision efficiency within gas kinetics limits.Alth ough th ese kinetic experiments provided unpre-cedented informat ion on temperat ure-dependent ra tecon s t a n t s , t h e ir cr u ci a l d r a w b a ck i s t h e l a ck in ginformation on intermediates involved, reaction prod-ucts, a nd bra nching ra tios. Therefore, informa tion onthe chemical reaction dynamics under single-collision

k(T) ) A e-(Eact /RT) (7)

H 2 + C R fH 2+

+ e- (8)

H 2+

+ H 2 fH 3+

+ H (9)

H 3+

+ X fH 2 + H X+. (10)



13/50

conditions as provided in crossed molecular beamexperiments are crucial to provide these information(section I V). These experiments give a n intima teinsight into the underlying reaction mechanisms atth e molecular levelsa crucial piece of information tountangle the chemical processing of distinct inter-s t el la r e nv ir on m en t s . H e r e, t h e n eu t r a l -n e u t r a lreactions of C(3P j), C 2(X1g+), C 3(X1g+) the isoelec-tronic radicals C N(X2+) an d C 2H (X2+), and phenyl

rad icals C 6H 5(X2

A) with u n sa tu ra te d h yd ro carb o n san d hydr ogen sulfide ar e of part icular importa nce inastrochemistry.

II. Key Reactants in the Interstellar Medium

A. Atomic Carbon, C(3Pj), Dicarbon, C2(X1g+),and Tricarbon, C3(X1g+)

Atomic ca rbon is the fourth most a bunda nt elementin u n ive rse an d u b iqu ito u s in th e in te rste l lar m e -dium. The 609 m 3P 1 - 3P 0 tran si t io n p re se n ts ava luable probe to detect C(3P j) in significant amountsin circumstellar envelopes of evolved carbon stars

I RC +10216 a nd R Orionis.276-279

A furt her identifica -t i on w a s m a d e t ow a r d t h e p r ot op la n e t a r y n eb u la eCRL 618 an d CR L 2688,280 th e diffuse cloud Oph,281

an d th e d e n se clo u d OM C- 1. 282 The dicar bon a ndtricar bon molecules C 2(X1g+) a nd C 3(X1g+) h a v ebeen detected in diffuse clouds Oph and Pe r,283-285

in translucent clouds such as HD147889,286,287 a n din th e circu m ste l lar sh e ll o f carb o n s stars su ch asI RC +10216.288-292 Additionally, tricarbon w a s foundto be abundant toward the hot core source SgrB2. 293

N o t e t h a t C 2 a n d C 3 molecules ha ve been a ssignedin comets as well.294-298

I n ve stig at in g th e d yn am ics o f th e se re actio n s isstrongly expected to provide a unique knowledge on

the formation of hydrogen-deficient carbon moleculesin the interstellar medium such as hydrogen-termi-n at e d carb on clu sters C nH, cummulenes C nH 2, a n dhydrogen-deficient, sulfur-carrying molecules (Table1).299-311 These investigations are of further impor-tance to resolve synthetic routes to interstellar cyclicmolecules C 3H a n d C 3H 2 to ge th e r with th e ir l in eari s o m e r s C C C H a n d C C C H 2. L in e ar an d cycl ic C 3Hrad icals are u b iqu ito u s in th e in te rste l lar m e d iu ma n d h ol d h i gh f r a ct i on a l a b u n d a n ce s, u p t o 1 0-9

relative to atomic hydrogen. l-C 3H was d e te cte d in1985 by Tha ddeus et a l. via microwa ve spectr oscopytoward the dark Taurus molecular cloud 1 (TMC-1)an d th e carb o n star I RC +10216.312 Two years later,

Yam a m o to et a l . id en ti f ie d ro tat ion al tra n si t ion s o fthe cyclic isomer in TMC-1 prior to la bora tory syn-th esis via ra diofrequency discha rge of H e/CO/C 2H 2mixtures.313 The aromatic c-C 3H 2 molecule is verysta ble towa rd decomposition; h ence, it is observa blenot only toward shielded regions as dense clouds sucha s TMC-1 [c-C 3H /c-C 3H 2 ) 0.1; c-C 3H 2/l-C 3H 2 )5-13], 314-319 but a lso in diffuse clouds, 320 aro u n d th ecircumst ellar envelope of IR C +10216, an d in t he hotmolecular cores SgrB2 [c-C 3H 2/l-C 3H 2 ) 150:1].321,322

The cumm ulene isomer H 2CCC is less abundant t hanth e cycl ic s tru ctu re, an d isom e r rat ios of c-C 3H 2/l-C 3H 2 ) 100:1 have been determined in TMC-1. 323

These hy drogen-deficient ra dical s pecies a re t hought

to be key int ermedia tes in t he synt hesis of polycyclicar omatic hydroca rbons (PAHs) and their derivatives,carbonaceous grain particles in circumstellar enve-lopes of car bon a nd planeta ry n ebula e, and possiblyfullerenes.324-326 N ote t h at so far n o fu llere n e m ol-e cu le s h ave b e e n d e te cte d in th e g as p h ase o f th einterstellar medium unam biguously. However, on th ebasis of the 3H e/4He isotopic composition of endohe-dra l helium tra pped inside a sa mple of C 60 obtained

from a meteorite, the authors provided compellinge vid en ce o f a n e xtra te rre strial e vid en ce of C 60.327

Nevertheless, th e synt hetic routes to form fullerenesin the interstellar medium remain elusive.

B. Cyano Radicals, CN(X2+)

Ever since the very first detection of cyanoacetylene(HCCCN) in interstellar environments, the synthesisof highly unsaturated cyanopolyynes up to HC 10C Nhas been a challenge for astronomers and chemists.Since early models of interstellar cloud chemistryemploying ion -molecule rea ctions cann ot reproducethe abundances of these long-chain molecules, neu-

t r a l-n e u tral re action s o f cyan o ra d icals w ith u n sa t-u rat e d h yd rocarb on s w e re su gg e ste d a s a p ote n tialalternative. These elementary reactions might verifynot only the synthetic routes to polycyanoacetylenes,but a lso ast ronomically observed nitr iles vinylcyanide(C 2H 3CN) and cyanomethyla cetylene (CH 3CCC N ) inthe circumstellar envelope surrounding the carbons ta r I RC +10216, h o t m olecular core s, a n d d arkmolecular clouds such as TMC-1. Therefore, detailedlab o rato ry in ve stig at io n s u n rave lin g th e ch e m icalreaction dynam ics a re crucial t o test t his hypothesis.Th e cyan o rad ical is u b iqu ito u s in th e I S M a n d h a sbeen observed in the dense clouds TMC-1328,329 a n dOMC-1330 an d ar ou n d circum ste l lar e n velop es of

I RC + 10216.331,332

N o t e t h a t t h e s e r e a c t i o n s h a v estro n g t ie s to o u r so lar syste m as cyan o ace tyle n eto g eth e r with u n sat u rat e d h yd ro car b on s ace tylen e ,ethylene, and methylacetylene have been observedin Sa turn s moon Tita n. Underst a nding the forma tionof nitriles has further importance to astrobiologicalproblems since nitriles are suggested to representp re cu rso rs to am in o acid s wh ich can b e fo rm e d inm u lt i pl e r e a c t ion p a t h w a y s v ia h y d r ol y si s a n daminolyses.333-337

C. Ethinyl Radicals, C2H(X2+)

T h e e th in yl rad ical is iso e le ctro n ic to th e cyan o

ra dica l. After its first det ection338

it became clear thatC 2H is u b iqu ito u s in th e in te rste l lar m e d iu m an dmight be the key species involved in the formationof the polyynes C H 3CtC -CtC -H a n d H (CtC )n a sdetected in cold molecular clouds TMC-1 and OMC-1339 as we ll as in circu m ste l lar e n ve lo p e s aro u n dcarbon-rich asymptotic giant branch stars such as ther e d g i a n t s t a r I R C +10216.340 F u rth e r, d iacetyle n e(HCCCCH) has been observed in the atmosphere ofSa tur ns moon Tita n 342 an d in S atu rn .341 Whereas t hema in source of the interstellar a nd planeta ry ethinylrad icals h ave b e e n u n am b ig u o u sly id e n ti f ie d as aphotolyses of a cetylene (C 2H 2)342 a n d t o a m in ora mount of dia cety lene, the mecha nisms to synt hesize



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the observed polyacetylenes is far less clear. Neutral-neutra l reactions of ethinyl ra dicals w ith unsa tura tedh yd rocar b on s h ave b ee n p ostu late d as a p ossiblesource, and experiments will be discussed in sectionI V.

D. Phenyl Radicals, C6H5(X2A)

Ele m en ta ry re action s o f p h en yl ra d icals are su g -g es t ed t o pl a y a m a jor r ol e i n t h e f or m a t i on ofpolycyclic ar omat ic hydrocar bons (P AHs),343-360 theircations,361-385 hydrogenated 386 an d d e h yd rog en at e dPA Hs,387-391 an d p ossibly P AHs with al ip h atic s id echains 392-395 in circumst ellar envelopes of car bon-richstars and planetary nebulae. The very first postula-tion of their interst ellar relevance as t he missing linkbetween sma ll ca rbon clusters C 2-C 5 and carbon-richgrain material fueled enormous scientific research.Curr ently, P AH-like species a re presumed t o tie upabout 18%of t he cosmic carbon.396,397 These speciesa r e f ur t h e r t h ou g h t t o b e t h e ca r r i er of d if fu s e,in te rstel lar ab so rption b an d s (D I B s) coverin g th evisible spectr um from 440 nm t o the near -infra red398

an d ma y contribute t o the infrared emission feat uresin t he spectrum of comet P /Ha lley.399 PAH-like spe-cies also might be the emitt er of unidentified infra redba nds (U IRs ) observed a t 3030, 2915, 2832, 1612,1300, 1150, and 885 cm -1.400 These bands dominateth e sp e ctra o f a g re at varie ty o f o b je cts su ch as TTauri Stars,401 HII regions,402 pla netary nebulae,403-405

circumstellar envelopes of carbon stars, 406-411 youngstellar objects, 412,413 and even in the diffuse interstel-lar medium.414 The survival of ar omat ic units in theh arsh co n d it io n s o f th e in te rste l lar rad iat io n f ie ldsuggests that distinct PAH-like structures could besta ble towa rd ionizing radia tion and possibly towa rdphotodissociation.415-417 These large molecules may

su rvive th e h arsh p h oto n f lu x b e cau se t h e y h a ve asufficiently high density of states to undergo internalconversion; these rovibrationally excited species areconsidered for t hese UI Rs a s discussed above. H ow-ever, th is sta bility depends st rongly on t he molecula rstru ctu re o f th e in d ivid ual aro m at ic sp ecie s a n d inp art icu lar on t h e ch arg e sta te .418-429 P AH-like mol-ecules a re furt her considered a s precursors leadingultimately to carbon-rich interstellar grain materialwit h va rious degrees of hydrogenat ion,430-438 possiblyfullerenes and their ions,439-444 an d p e rh ap s in te r-stellar diamonds as detected toward carbon-rich pre-p lan e ta ry n e bu lae .445-447 N o te th at al th o u g h n o in -dividual P AH molecules ha ve been identified in th e

in te rstel lar m e d iu m so far , ar om a tic m o ie ties h avebeen detected in meteorites. 448-450

However, despite the crucial importance of PAH(like) molecules in var ious extra terrestria l environ-ments, experiment a lly an d theoretically w ell-definedm e ch an ism s to syn t h e siz e th e se sp e cies in t h e g a sphase have not been elucidated. All chemical reactionnetworks modeling PAH formation suggest a step-wise extension of monocyclic rings via benzene (C 6H 6),phenyl (C 6H 5), a nd/or cyclopenta dienyl r a dica ls (C5H 5)to polycyclic systems.451-459 However, even t he mostfundamenta l dat a on th e forma tion of these very firstcyclic building blockssp os s ib ly v ia s m a l l C 2-C 4ra dicals such a s propargyl,460-465 allyl,466 or C 4H 3 a nd

C 4H 5 intermediates 467-476 as investigated in crossedmolecular beam experimentssand their subsequentreactions are lacking. Alternative formation routesmight involve meta l-cata lyzed reactions in circums-tellar envelopes477 as supported by laboratory experi-ments478 an d ma ybe ion-molecule reactions in denseclouds.479 Therefore, not only kinet ic stud ies of phenylra dical reactions,480-484 but in particula r a systema ticinvestigat ion of reaction pa ths lea ding to these sma ll

precursor molecules and the elementary reactions ofcyclic building blocks such as phenyl on the mostfu n d am e n tal , m icroscop ic leve l a re of p ara m ou n tim p o rtan ce . B e fo re p lu n g in g in to th e re actio n d y-n am ics, th e k in e tics o f t h e se im p orta n t n e utra l-neutral reactions are discussed briefly.

III. Kinetics

A. Room-Temperature Kinetic Studies

K in etic s t u d ies on th e re activity of g rou n d -sta tecar b on a to m s C(3P j) with u n sa tu ra te d h yd ro carb o n sre volu tion iz ed th e con ve n tion al th in k in g of ast ro -

chemists.

485-492

Hu sain e t a l . p ro vid e d fo r th e ve ryfirst time sound experimental data on the potentialim porta n ce o f n e u tra l-n e u tral re actio n s in th e in -terstellar medium and determined absolute reactionrat e con sta n ts a t room te m p era tu re (T ) 300 K).493

T h e sch e m atic se tu p is sh o wn in F ig u re 4. I n th e

early measurements, two lithium fluoride (LiF) opti-

cal windows (a) sea led both ends of a squa re sha pedP yrex cell (b) thr ough wh ich a pulsed photolysis ofprecursor to atomic carbon was initially effected (> 120 nm). Subsequently, a coa xial cylindrica l lam pand reactor assembly was employed with photolysisth ro u g h h ig h - p u ri ty qu artz ( > 160 nm). The gasmixture, which contained the carbon atom precursor(carbon suboxide; C 3O2), helium buffer (He), a nd t hereacta nt ga s, wa s introduced through port 1 into thereactor. The a ctual experiments employed t he prin-ciple of time-resolved a tomic a bsorpt ion spectr oscopyw i t h m on i t or i n g o f a t om i c c a r b on C (3P j) i n t hevacu u m u ltra violet at 166 n m fol lowin g a p u lse dirrad iat ion of car b on su b oxid e in th e p resen ce o f

Figure 4. Experiment a l s et up a nd s implif ied rea ct ordesign employed in kinetic investigations of reactions ofground-s t a t e ca rbon a t oms w it h va rious uns a t ura t ed hy-drocarbons at 300 K.



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excess helium buffer gas. Vacuum ultraviolet photonswere generat ed from a microwa ve-pow ered discha rgein a microwa ve cavity (c) a nd m onitored in a bsorptionvia a photomultiplier tube (PMT); port 2 served asth e g a s in let in to th e m icro wave cavi ty .485

The experiments indicated that all reactions pro-ceeded w ith second-order kinetics, bar rierless, an d

very rapidly (k ) 2.0 10-10 to 1.5 10-9 cm 3 s -1;T ab le 3) . Clary e t a l . p re se n te d a syste m atic d ataan a lysis com b in in g t h e ab so lu te ra te con sta n ts withclassical cap tu re th e ory d om in ate d b y an isotro picdispersion term.494 In t he case of a lkenes and a lkynes,this a pproach lea ds to an expression in w hich th e rat e

constant k scales w ith one-third power of t he polar -izability of the colliding molecule a nd h ence w ith N1/3

(N ) number of ca rbon at oms in the alkene or a lkyne;Figure 5). Note tha t C (3P j) showed no reaction towa rdwate r an d m e th an e , su g g e stin g th at p o ssib ly o n lyu n satu ra te d m o lecules re acted rap id ly w ith carb onato m s.488-494 Th is h yp oth e sis wa s su pp orte d veryrecently. Kinetic studies on the collisional removalof g rou n d -sta te carb on at om s with form a ld eh yd e ,ace tald e h yd e , an d ace to n e at ro o m te m p e ratu re i l-lu strate d th at th e se re actio n s are in d e e d ve ry fast(k ) 3.8-6.6 10-10 cm 3 s -1) and proceed withoutb arrie rs .495

These kinetic da ta demonstr a ted explicitly th a t a tl ea s t a t e le va t e d t e m pe ra t u r es p r ev a i li n g i n h otmolecular cores, circumstellar envelopes of car bons t a r s , a n d p l a n e t a r y n e b u l a e , r e a c t i o n s o f a t o m i ccarbon with unsaturated molecules can occur. Theab se n ce o f a n y e n tran ce b arrie r u n d erl in es fu rth e rthe potential role of these neutra l-neutral reactionsin dense clouds w here temperat ures of 10 K prevail.Ho we ver, t h e ir actu a l e f fe ct on th e in te rste l lar h y-droca rbon chemistry a t low tempera tur es depends onthe a bsence of even minor entra nce bar riers an d theunambiguous identification of the reaction products.Therefore, further investigations are clearly neces-s a r y a n d s h ou ld a c q u ir e d a t a on l ow -t e m per a t u r e

rat e con sta n ts an d th e p rim ary re action p ro du cts ofa s t r o ch em i ca l l y i m por t a n t n eu t r a l -n eu t r a l r e a c-tions.

V e ry re ce n tly, B e rg e at e t a l . e xte n d e d Hu sain sinvestigations of reactions involving atomic carbonto detect the atomic hydrogen reaction product viaresonance fluorescence.496,497 Figure 6 depicts a sche-m a t i c v ie w of t h e e xp er i m en t a l s et u p . Th e l ow -p ressu re fast-f low re actor con sisted of a h ollows t a i n l es s s t e el b lock i n w h i ch a Tef lon t u b e i sinserted. P orts for th e at omic resonan ce fluorescencedetection of carbon and hydrogen atoms were locatedd own st re am . Ato m ic car b on , C(3P j) , was g e n e rate din situ passing tetrabromomethane (CBr 4) in helium

Figure 5. N1/3 dependence (N ) number of ca rbon a t oms ) of ra t e const a nt s of a t omic ca rbon rea ct ions w it h a lkenes a nda lkynes a t 300 K.

Table 3. Rate Constants of Reactions of CarbonAtoms with Unsaturated Hydrocarbons at 300 K

r ea ct a nt r a te const a nt , 10-10 cm 3 s-1

alkeneset hylene 2.0 ( 0.1pr opylene 4.0 ( 0.2

1-but ene 4.3 ( 0.11-pent en e 6.5 ( 0.31-hexene 6.9 ( 0.41-hept en e 8.0 ( 0.41-oct ene 8.5 ( 0.51-nonen e 8.9 ( 0.51-decene 9.6 ( 0.51-undecene 9.8 ( 0.51-dodecene 10.4 ( 0.61-t r idecen e 11.2 ( 0.61-t et r a decene 11.7 ( 0.61-pent a decene 12.0 ( 0.6

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