Identification of Intermediates in the Photolysis of Mercaptans in Dilute Glass Matrices. I1

ALLEN JOHN ELLIOT A N D FRANK CUTHBERT ADAM Department of Chernist~y, University of Calgary, Calgary, Alberm T2N IN4

Received June 5, 1973

Dilute solutions of aliphatic thiols have been prepared and photolyzed at 253.7 nm in 3-methylpentane glasses at 77°K. The major products formed initially are thiyl radicals RS. and hydrogen atoms. Both of these are hot and yield solvent radicals by abstraction reactions, and relatively small amounts of RS,., RS., and possibly R,R2C=S. Thermal annealing of the matrices is accompanied by loss of hydrocarbon radicals through cage reactions and concommitant increase in the RS. concentration. An intermediate radical of the type R~R,CSH may also be produced in the annealing process. The photometric and spin resonance characteristics of RS., RS2. and R ~ ~ H - S H are discussed and related to other systems in which these intermediates are observed. RS. is characterized in the amorphous phase by an absorption at 405 nm and a broad asymmetrical e.p.r. resonance near g 2 2.0. RS2. has a non-axial s-tensor with principal values of about 2.06,2.02, and 2.00and has no detectableabsorptions between 350and 1200 nm.

The photolytic behavior of thcse metastable radical species is also investigated.

Des solutions de thiols aliphatiques ont 6te preparees et irradiies B 253.7 nm dans des matrices de methyl-3 pentane a 77 OK. Les principaux produits formes au debut de la photolyse sont des radicaux thiyle RS. et des atomes d'hydrogene. Ces deux produits sont chauds et conduisent des radicaux du solvant par des reactions d'abstraction. On detecte egalement de faibles quantites de RS,., RS. et peut &tre de R,R2C=S. Le rechauffement des matrices s'accompagne d e la disparition des radicaux hydro- carbure par des reactions de cage et de I'accroissement simultane de la concentration en RS.. Un radical intermediaire du type R , R ~ C S H peut aussi Etre forme au cours du rechauffement. Les caracttristiques photometriques et de rksonnance de spin de RS., RS2. et RICH-SH sont discutees et comparees a celles des autres systemes dans lesquels ces intermediaires sont observis. RS. est caracterise dans la phase amorphe par une absorption a 405 nm et une large bande asymetrique en r.p.e. proche de g - 2.0. RS,. prisente un tenseur-s non axial dont les principales composantes sont environ 2.06, 2.02 et 2.00. Ce radical ne prtsente pas d'absorptions dktectables entre 350 et 1200 nm.

La conduite photolytique de ces espices radicalaires metastables a aussi t te etudike. [Traduit par le journal]

Can. J. Chem., 52, 102(1974)

Introduction The role of sulfur containing substances in the -

decomposition pathways of biological materials subjected to high energy radiation has been the topic of many investigations since the observa- tion was first made by Gordy et al. (1) that, regardless of what the initial process of dis- ruption might be, the damage site eventually ended up on sulfur atoms contained in such amino acid constituents as cysteine or cystine present in the parent organic material. This transfer was inferred by the appearance of terminal radical species characterized by a large variation in the principal g-tensor values, not obtained in biological systems which do not con- tain sulfur atoms. In fact, irradiation of single crystals of cystine.HC1 at room temperature gives oriented radicals with non-axial g-tensor elements (2.052, 2.029, and 2.003) similar (2) to those found in larger biological molecules. These

radicals appear to be much the same as thos~ obtained by 253.7 nm photolysis (3) and b: y-radiolysis (4) of cysteine.HCl.H,O O~L-Penicill amine.HC1 (5) at room temperature. Radical with analogous g-tensor elements are also ob tained from the y-irradiation of pure polycrystal line alkyl disulfides (6) and thiols (7 ) , and b photolysis of neat mercaptans (8) at 77 OK. I: most cases the observed resonance has bee. attributed to the thiyl radical RS. as wa originally suggested by Kurita and Gordy (2: and that the stability of this species at roor temperature was due to its lack of mobility an extreme unreactivity in the solid state. 0 melting or dissolving the crystallites, the thi: radicals combine to yield disulfide, which, in th case of thiols, is one of the major products (9 In several of the above single crystal studies th initial irradiation was also carried out at lowc temperatures, generally 77 OK. It was shown th:

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ELLIOT AND ADAM: PHOT(

while the room temperature radical was indeed common to both disulfides and thiols, it was never the initial product formed, but results from a sequence of solid state reactions which appear to be peculiar to the individual sub- stances and depend upon the particular mode of radiation damage or upon the precise experimen- tal conditions. By way of example, y-irradiated single crystals of cysteine.HCI.H,O at 77 OK produces a radical which also has a strongly an- isotropic, but axial, g-tensor withg -- 2.29 and g - 1.99, and a roughly isotropic proton hyperfine structure characteristic of electron coupling to two non-equivalent hydrogen nuclei (10) (AH - 14 G, 38 G). No such resonance is observed in the cystine system even though the final radical at high temperature appears to be much the same for both compounds. This low temperature axially-symmetric radical clearly has the large g-shifts expected for the RS. radical and the hyperfine splitting is what might be expected from the P-coupling of the analogous hydrocar- bon fragment RcH,-CH,. Neglecting the small anisotropy, and the B, term, in the familiar

, expression (1 1)

I one obtains a value for B2 = 33-40 G which is 1 comparable to the value of 45-50 G obtained for hydrocarbon radicals (12). The chief dif- ference between the low and the high tempera- ture form of R S is that the latter has been assumed to have "relaxed" due to the thermal treatment of the crystal, and to have taken on a more stable non-linear configuration in the fields of the crystalline environment (5). The

1 marked difference in proton hyperfine structure is however a little difficult to rationalize using the same argument. Yet another assignment of e.p.r. signals to thiyl radicals arises in the aqueous phase oxidation of simple thiols where a resonance is obtained with averaged g-values near 2.01 1 and proton hyperfine structure appro-

, priate to the RS. radical (13, 14). The magnitude I of the coupling (AH - 9.5 G) seems low, while the rotationally averaged g-value of 2.0 1 corre- sponds to neither of the assignments given above for thiyl in the solid state determinations.

1 Radiolysis of thiols in the basic p H range yields ,nothing at all which may be attributed to RS. I radicals (15). Regardless of whether the irradia- 'tion or photolysis of thiols is carried out in the

3LYSIS OF MERCAPTANS 103

solid (9), vapor (16), or liquid phase (using either yrotic (17) or aprotic (18) media) the major product appears t o be the disulfide mole- cule RSSR, which suggests that somewhere along the line, the thiyl radical has indeed been produced in the system, and that its eventual fate is combination with its own kind.

The non-axial g-tensor elements characteristic of the solid state high temperature form of RS. may also be obtained when dilute glassy solu- tions of thiols in inert matrices are photolyzed at low temperatures. For instance, 0.1 M ethanethiol in 3-methylpentane at 7 7 ° K has been photolyzed at 253.7 nm to give a radical which is characteristic of the solvent and a second species characterized by g-tensor ele- ments of 2.063, 2.027, and 1.999, and a poorly resolved hyperfine doublet of about 9 G (19). Although one might have expected the "field- free" thiyl radical in this particular matrix, there is obviously a much closer resemblance to the "thermalized" configuration of the single crystal studies. Since the thiol molecules are presumably physically isolated in the glassy solid solution, being present at - 1.2 molz , then the major products of photolysis (19) (H,, thioethers, and disulfides) have been explained by the following simple scheme :

hv [I] EtSH + EtS + H*

[2al H* + MH -> M. + Hz

Pal H. + EtSH -> EtS. + H2

[3bI H. + H. + Hz

On slight warming of the matrix, cage reactions begin to occur

[4] M. + EtS. -+ EtSM

Warming to the melting point of the matrix allows diffusion reactions to begin so that

[51 2 EtS -> EtSSEt

In these equations, H* is taken to be a "hot" hydrogen atom and MH is, in the reference cited, 3-methylpentane. The primary assump- tion made is that while initially hot hydrogen atoms are produced, some of these may be thermalized by third body collisions and diffuse out of the cage in which the photolytic absorp- tion took place.

The use of dilute rigid glasses has one distinct

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104 CAN. J . CHEM. VOL. 5 2 , 1974

advantage over the studies employing single Cysteine.HC1.H20 was recrystallized several times crystals, pOlycrystalline samples, or thin films from air-free water. The X-band e.p.r. homodyne spec-

trometer has been described elsewhere (21). The cavity of thiols (20) in that large concentrations of was cooled using a conventional nitrogen gas flow system thiyl radicals may be built up without, at the with a liquid nitrogen heat exchanger directly below the same time, having to deal with the possible cavity. Special care was required in this system in order biomolecular reactions between the reaction to catch some of the earlier processes occurring in the intermediates and the parent thiol molecules. ~f photolyzed glasses. Only Pure liquid nitrogen (containing

no oxygen) was used and every advantage was taken of such processes are of little importance, then in the altitude of Calgary (ambient atmospheric pressure dilute glassy solutions the scheme outlined above -660Torr) to obtain these initial spectra. The entire should be reasonably independent of thiol con- range of temperatures available for 3MP is less than centration with results similar to those obtained in 15 'C, lying between the temperature of boiling nitrogen the solid state. In order to test the relative irnpor- a n ~ ~ ~ ~ ~ ~ ~ : t ~ ~ ~ ~ b " , ~ ~ ~ ~ i ~ ~ ~ ~ , " ~ ~ tezcarried out tance of such reactions and to further determine ~~~k~~~ D"2400 spectrop~otometer, the physical properties of the thiyl radical, a Glasses could be photolyzed, melted, and brought t o variety of alkyl thiols at different concentrations room temperature so that g.1.c. analyses could be carried

have been photolyzed in various hydrocarbon out on a Varian series 1800 chromatograph. Liquid phase analyses were effected on a 3% Squalane or SE-30 silicon glasses. "lid state may be induced rubber column. The vapors were also investigated using

following ~ h o t o l ~ s i s by careful thermal an- a Poropak Q column. Isothermal analysis at 90 "C was nealing of the glasses and may be followed by used jn each case. Hydrogen analyses were carried ou t both e,p.r. and optical spectroscopic techniques. using a palladium thimble technique described elsewhere

warming the glasses to the melting point results (22). We will not deal extensively with the results of these analyses at the present time except to note that the main

in solutions which have been subjected to prod- products of photolysis of RSH are H,, RSSR, and R S M uct analysis at room temperature.

Experimental The 3-methylpentane (3MP) was Aldrich Chemicals

"puriss" grade and was dried over lithium aluminum hydride. Before use it was run through an activated silica gel column and then distilled in small amounts (-50 ml). Ethanethiol (EtSH) and isopropylmercaptan (i-PrSH) were obtained from the Aldrich Chemical Company, tert-butyl mercaptan (t-BUSH) from Eastman Organic Chemicals, and n-butyl mercaptan (n-BUSH) was ob- tained from the J. D. Baker Chemical Company. The thiols were fractionally distilled before use. H,S gas was used as supplied from the Matheson Company. Thiol solutions were prepared in either of two ways. In one, the solutions were mixed in volumetric flasks and sub- sequently transferred to Suprasil e.s.r. tubes (3 mm id.) , or tubular optical cells (16 mm i.d.) on the vacuum sys- tem. The samples were then degassed by repeated freeze- pump-thaw cycling and then sealed off from the line under high vacuum. All glassware was heated to annealing temperature before use. Other samples were prepared using only high vacuum techniques in which the sulfhy- dry1 or 3MP vapors were metered through calibrated bulbs and frozen in the sample cell, sealed off, and mixed after the components had been returned to room tem- perature. The samples were then refrozen ready for use. Both techniques produced the same spectral effects, s o that what follows cannot be related to the presence of oxygen or water in the thiol-hydrocarbon solutions.

Photolyses were carried out a t 253.7 nm in a clear quartz Dewar filled with fresh pure liquid nitrogen, with the cell situated 3 cm away from a Hanovia 87A45 low pressure resonance mercury lamp enclosed in a housing with a I + in. aperture. A GatesP-109 high pressure 100 W mercury lamp or a 500 W tungsten projection lamp, each with appropriate Corning filters, were used for photo- bleaching experiments.

where M represents incorporation of the solvent. Chro- matographic peaks corresponding to the last named change when different solvents o r different thiols a re used.- he ratio of peak areas, RSSR to RSM, increase a s the concentration of the thiol in the glass increases. There are a t least three RSM peaks corresponding to different isomers. Subsidiary peaks are also observed which corre- spond to fragmentation of the solvent molecule (lower alkanes and alkenes) and of the solute molecules. T h e latter is indicated by the presence of H2S peaks detected using a Varian H 3 electron capture detector. In any case, the amount of these subsidiary products represents less than 10% of the total hydrogen yield.

Results We have repeated the experiments of Wheaton

and Ormerod (4) on both single crystals and polycrystalline cysteine.HC1 samples, to further determine the characteristics of the axial form of RS.. We find in agreement with these authors that the integrated e.p.r. signal obtained in- creases as the crystals are warmed from 77 O K

to about 180-200°K and then decreases at higher temperatures. Not noted by Wheaton and Ormerod was the fact that while the crystals af 77 and 300 OK are pale yellow, the color intensi- fies quite markedly at intermediate temperatures near 180 OK. The results are in agreement witk Henriksen (22) who found that while y-irradi. ated polycrystalline cysteine.HC1 crystals wen yellow at low temperatures, those irradiated a. room temperature were colorless. Sample: irradiated a t low temperatures and warmed alsc

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1 ELLIOT AND ADAM: PHOTOLYSIS OF MERCAPTANS 105 a 2 0597 i show the characteristic fading exhibited by single b C 2 2 0203 0005

: crystals. While the e.p.r. spectrum of single ' crystals shows the axial g-tensor symmetry and : hyperfine structure already referred to, the

polycrystalline samples exhibited e.p.r, spectra i similar to those obtained by Henriksen; the low

field absorption near g = 2.2 obtained in single crystals is broadened beyond detection and an

FIG. 1. Radical A ; obtained from a 3 h 253.7 nm unsymmetrical resonance devoid of resolved photolysis of neat ethanethiol at 77

hyperfine structure remains near g = 2.0. From these results one can conclude that the axially

I symmetric RS. is probably colored and should 8 2.0596

, be difficult to detect by e.p.r. in a homogeneous b 2.0023

I rigid solution except that perhaps a broad signal I with no distinct hyperfine structure may be ob- 1 tained near g = 2.00. The resonance will occur

with considerably reduced sensitivity since using 50 OC

differential recording techniques, one normally "sees" only a small fraction of the radicals

b

actually present because of the extreme aniso- tropic broadening.

We have also repeated the work reported FIG. 2. A 4 h 253.7 n m photolysis of 0.1 Methane-

1 earlier from this laboratory (19) paying much thiol, 3-methylpentane glass a t 77 OK.

; greater attention to the temperature control and , details of the thermal annealing processes. In trum B is similar to that obtained from

1 view of the color changes mentioned above, we y-radiolysis of 3MP (19) except that there is have also obtained low temperature absorption additional detail in the photolytically produced spectra to follow these processes optically as spectrum which has been attributed t o a small closely as is possible using different sample amount of ethyl radicals (23). This additional sizes. featuring does not depend on the particular

Generally speaking the 77 OK photolysis of substrate, and photolysis of a 0.02 M H,S in dilute glassy solutions containing alkyl thiols 3MP or any of the alkyl thiols glasses gives gives results wh~ch are similar to those obtained essentially the same spectrum if the temperature

I by y-irradiation of the crystalline cysteine sys- is kept low enough. I tem insofar as the most intense yellow coloration As the matrix is warmed, loss of the hyperfine

and broad e.p.r. absorption near g = 2.00 detail occurs first and the total intensity of B i occur only after thermal annealing, being either steadily decreases; the glasses become more I absent or substantially reduced in freshly intensely yellow, and the e.p.r. signal becomes I photolyzed samples. As before (19) a resonance asymmetric relative to the baseline near g = 2.0. ) A is obtained, characterized by an anisotropic Other intermediate hyperfine patterns may also I S-tensor with principal values near 2.06, 2.02, grow in, as for example in 0.01 EtSH in 3MP, , and 2.00. The spectrum is similar to that ob- where the odd line spectrum shown in Fig. 3 is

tained by photolysis of the pure thiol at 77 OK. obtained at the expense of B. These intermediate For reference, the spectrum obtained for neat species are characteristic of the solute and will ethanethiol (EtSH) appears in Fig. 1 ; it is similar be designated as type C absorptions. As the to the spectrum obtained elsewhere (8) and the matrix is further warmed, eventually all proton 9 G hyperfine splitting seen is characteristic of hyperfine structure is lost and there remains only a normal alkyl substituent ; iso- and tertiary- the unchanged absorption due to A, along with a alkyl thiols do not exhibit any such hyperfine broad asymmetrical signal near g = 2.00, which structure. Superimposed on A initially is a six- will be called a type D resonance. A typical line proton hyperfine spectrum B characteristic terminal spectrum appears in Fig. 4. The signals I of the solvent and giving a total spectrum such of both A and D vanish as the melting point of as in Fig. 2 which is that obtained for 0.1 M the glass is approached. None of the above EtSH in 3MP. The six line solvent radical spec- transformations is thermally reversible.

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106 CAN. J . CHEM. VOL. 5 2 , 1974

2.4

1.6

QD. 1.4..

FIG. 3. Sample of Fig. 2 warmed slightly. Solvent radical replaced by species C.

FIG. 4. Sample of Fig. 3 warmed to -85 "K.

Following the initial photolysis, glasses of increasing concentration show greater optical absorption at 400 nm, and greater intensity in the low-field A absorption relative to B. Regard- less of the color initially present, all glasses become more intensely yellow upon warming as can be seen from Fig. 5 which can be con- sidered as representative. Both color and e.p.r. spectrum appear to disappear simultaneously as the matrix is warmed to the melting point.

Glasses containing EtSH were also photolyzed at 77 OK and then warmed until the glass had become an intense yellow. The sample was then returned to 77 OK and photolyzed again. Wave- lengths greater than 300 nm produce no change but photolysis at 253.7 nm for an additional period results in a considerable reduction in intensity of the 400 nm peak, loss of the five line spectrum and a substantial increase in the A resonance at g = 2.06. The optical results in Fig. 5 and the e.p.r. spectrum in Fig. 6 are typical results. The lines which appear in Fig. 6 resemble the ethyl spectrum (24) rather more than the 3MP radicals seen in Fig. 2. The latter spectrum would have been expected if resump- tion of the primary photolysis were the only process occurring.

Photolysis of thiols in ethylene glycol - water matrices (80-20) produces no visible coloration.

0 3!0 400 510 6.0 700 800 goo 1000 1100 1200

nm.

FIG. 5. Optical spectra of 0.01 M ethanethiol, 3-methylpentane glass photolyzed a t 253.7 nm, for I h at 77 "K (I). O n warming (2) is obtained. This is recooled to 77 "K and photolyzed for an additional half hour to obtain (3).

FIG. 6. Electron spin resonance spectrum corre- sponding to (3) of Fig. 5.

Discussion The Thiyl Species RS. (Radical C)

The results cited for EtSH raise a number of perplexing questions. Regardless of whether one chooses the yellow coloration or the low field resonance A to be indicative of the presence of EtS., because of the delayed appearance of the former and constant intensity of the latter, neither is consistent with the simple reaction scheme cited in the Introduction which suggests that the maximum E t S concentration should be present immediately after photolysis. The thiyl concentration should then decrease as the matrix is warmed due to cage reactions with the solvent radicals as recorded by the changes in e.p.r.

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ELLIOT AND ADAM: PHOTOLYSIS O F MERCAPTANS 107

spectrum. The difficulty can be resolved if it may be assumed that the thiyl radicals formed in the photolytic act are "hot", and can either react or rearrange in such a way as to disguise their presence. Since the product analysis re- quires that it is the S-H, rather than the C-S bond which breaks, then the energy available to the fragments RS and H is about 1.1 eV, which is the difference in energy between the incident proton (-4.9 eV), and the S-H bond energy (25) (-3.8 eV). Ordinarily the hydrogen atom would carry off over 95% of this energy, because of the fragment mass ratios, and would appear t o be "hot", while the thiyl part will remain kinetically "cool". If the excess energy were more equally distributed the following reaction might be expected to occur and should be placed in the scheme presented in the Intro- duction

hv [I] EtSH -> EtS* + H*

[2c1 EtS* + MH -> EtSH + M.

If this process were efficient enough (as appears to be required for certain rate studies carried out at higher temperatures (26), the presence of the thiyl radical would be completely masked but would appear again by relaxation of the matrix through the cage reaction

[4bI M + EtSH -> MH + EtS,

As noted above, for this t o occur EtS must carry away an unusually large share of the photon energy. That this should indeed be the case can be justified by the following argument based on the magnitude of the g l l tensor element for the RS. radical.

The g-shifts so characteristic of these sulfur containing radicals are given in a molecular fixed frame by the general expression (27)

I Agij = -2 z z 5 k <OILIIn)<nILjlO) PL

k 11 En - Eo I

wherein the sums are over the atomic centers k and energy states n, with pr, being the spin

I density, and 5 , the spin-orbital parameter for the atom k. Since 6 , = -28 cnl-I while 5 , =

I 382 cm-', only that spin density on sulfur 1 ;oms will be important. In the one electron

orbital approximation for which this equation , is valid, radicals with z-axis symmetry (e.g.

CH,S. or t-BUS., with C, symmetry) have isoenergetic p, and p, atomic orbitals containing

three electrons. Accordingly two degenerate configurations will result and the resonance denominator E,, - Eo tends to zero for these two configurations. Since the numerator will not vanish, Ag,, = Agll will become infinite. One can assume that the Jahn-Teller effect (28) or other non-axial field effects will lift this degen- eracy with the result that Agll will remain finite, but presumably large. On the other hand, Ag,, = Agyy = Ag, will be determined by con- figurational mixing of states of sufficiently high energy, so that g, will be relatively unchanged from the free electron value at 2.0023. The ob- served shift AgZZ = Agl, is 0.21 for the low tem- perature cysteine radical. Using the appropriate parameters with p, -- 1, one calculates an energy difference of E,, - Eo -- 3000 cm- ' which means that R S must have at least one low-lying excited electronic slate in the range 0 to I. 1 eV and may well be "hot" in the sense that it can carry off excess electronic energy in this meta- stable state. Naturally enough the occurrence of hot thiyl will decrease the energy available to the H atom, making it less reactive and so increasing the probability that it will be able to diffuse out of the cage in which it was formed initially. The increasing yellow coloration of successively more concentrated thiol glasses reflects the increased probability that a diffusing hydrogen atom will encounter a thiol molecule before becoming completely thermalized. Accordingly we assign the yellow coloration of the glasses to signify the presence of the RS. radical in agree- ment with the observations made on the single crystal experiments, and ascribe the tardy appearance of this absorption in thermal an- nealing experiments t o be a result of hot reac- tions involving the R S radical, resulting in the reformation of the thiol nlolecule. The low field resonance A must therefore arise from a dif- ferent sulfur radical since its concentration does not increase and fall with that of RS., as would be expected of a "thermalized" isomer.

The thiyl radical may be characterized by an axial g-tensor with g ~ 2 . 2 , g -2.0 a n d hyper- fine coupling following the P-coupling relation- ship a, - 36 cos2 8. Distinctive hyperfine struc- ture is not observed in randomly oriented RS. and only a broad resonance may be detected at g = 2.0 when the radical is present in high concen- tration. Optically, RS. has a forbidden n, + i ~ ,

transition near 3000 cm- l , and appears yellow due t o a transition near 400 nm. T h e latter

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108 CAN. J . CHEM. VOL. 52 , 1974

absorption does not contribute to the g-tensor elements and is accordingly assigned to be a n, -t np transition involving the 3s electrons in sulfur.

The Hydrocarbon Radicals B The spectrum B is similar to that obtained

from photolysis of HI in 3MP containing ethylene (23) and is considered to be the 3MP~adical (AH = 24 G) in the presence of some ethyl radical (AH = 27 G). In the photol- ysis of the thiols, ethane and ethylene are among the most plentiful of the subsidiary by-products according to g.1.c. analysis and arise in a large part from fragmentation of the solvent. Al- though more mobi!e (23) in glassy 3MP matrices, ethyl will behave in much the same manner as the more abundant 3 M P and react to produce RS. and the other observed inter- mediates such as C.

The Radical Species C (R'CHSH) The five line spectrum which is obtained for

EtSH in Fig. 3 appears to be characteristic of this particular solute. Glasses of the other thiols do not produce the same five-line spectrum but give changing proton hyperfine spectra which suggest formation of analogous intermediate species at the expense of the solvent radical spectrum, which all exhibit initially, and growth of yellow RS. color, which is also common to all of the systems. Since the temperature at which these processes occur is that at which cage reac- tions are observed, we presume that the reaction which produces thiyl also results in the formation of other radicals. Thus

~ 4 ~ 1 M. + EtSH -t EtS. + MH

Reference to Fig. 3 shows that each hyperfineline is sharper on the high field side, and is slightly asymmetrical relative to the baseline. This general shape is indicative of a small near-axial g-tensor anisotropy which is slightly greater than the line width but smaller than the hyperfine splitting, sug- gesting a small but finite spin density on the sulfur atom; so our choice for absorption B is the radical C H , ~ H S H . This is in agreement with other work (29) which finds the four protons of the

a-isomer to be coupled more or less equally while those of p-isomer are not. The abstraction which has occurred to produce Fig. 3 is not the low energy process (30) (S-H abstraction, AE,,, -4 kcal mol-') but rather a higher energy, cage-controlled reaction (aC-H abstraction E,,, 5 8 kcal mol-') which has occurred. The C signal so produced then disappears by thermally induced irreversible isomerization to thiyl

or perhaps by "repair" reaction due to thermal relaxation of the cage

Resonance C is therefore ascribed to radicals of the type R'CHSH.

The Radical Species A (RS,.) So far no mention has been made of the struc-

ture appropriate to radical A, except that it is unlikely to be the RS. radical. In dilute thiol glasses it is formed only during the initial photolysis and in spite of the fact that it is para- magnetic, it does not undergo any subsequent cage reactions with other radicals. This suggests that radical A is relatively isolated in the matrix and is formed as a result of attack by diffusing hydrogen atoms which are present in the glass only during the photolysis period. The concen- tration dependence of A suggests its precursor competes satisfactorily against RSH for the available hydrogen atoms which in turn sug- gests that it-has even a lower energy of activation for reaction than does the thiol molecule itself. Bearing in mind that solid disulfides, as well as thiols, yield this very stable free radical, one must consider the possibility that the radical A contains two skeletal sulfur atoms rather than just one. We believe the most likely structure for the A type radical is RS,. and that it results from reactions involving dimers of the parent mer- captan (which will explain its concentration dependence) or by cleavage of the C-S bond in the case of disulfides. In the present situation, RS,. could result by means of two separate pathways involving a thiol-thiyl complex.

H + (RSH)z + [RS. ... HSR] + Hz

[RS. ... HSR] -t RSZ. + H R

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E L L I O T A N D ADAM: P H O T O 1LYSIS O F MERCAPTANS 109

Alternatively hv

(RSH), + RSSR + Hz

and subsequently

H + RSSR + [RS ... HSR] -> RS2. + HR

The effect of the last reaction has been observed in the aqueous phase (31). While association of thiols in non-polar media was once thought to be unlikely (32), recent studies (33) show the asso- ciation constant may be as high as K,, = 0.01 for EtSH in hydrocarbon solutions at room temperature. This constant is presumably larger at lower temperatures.

The general ingredients for the intermediate, a thiyl radical in close proximity to an un- damaged thiol molecule, is a general feature of all solid state investigations involving the photolysis and radiolysis of pure thiols. In addi- tion the radiolysis of disulfide systems is known to produce disulfide negative ions (34) and the above intermediate is simply the proton adduct of such a negative ion. In most single crystal studies there is a plentiful source of protons. For instance most cystine studies are performed using the double hydrochloride salt. These fac- tors may account in part for the ubiquitous appearance of this radical in many of the sys- tems subjected to photolytic or ionizing radia- tion such as those studied by Gordy et al. (1).

The observed properties of the radical A are consistent with the structure RS,. since the molecule is almost surely nonlinear with the spin density most likely distributed over both sul- fur atoms in a n* molecular orbital. This in itself would reduce the proton hyperfine cou- pling by a half and g-shift by at least this amount if any stability at all is gained by x-bond forma- tion. With the introduction of the S-S bond, other low energy excitations (xss* -+ o,,*) not found in RS. become possible and these will contribute to the non-axial character of the 9-tensor. The g-value anisotropy should be relatively independent of the particular alkyl group, as is observed, and the stability of RS,. can be accredited to the relatively low potential for the formation of bonds at this position. For instance (35) the HSS-H bond energy is 72 kcal mol-' compared to HS-H energy of about 90 kcal mol-', making H-atom abstrac- tion by RS,. extremely unlikely. This radical also appears to be the stable entity in amine solutions of elemental sulfur (36).

Photochemical Belzavior of RS., RS,., and R'CHSH

As noted in the Results section, when a photolyzed glass containing EtSH was warmed to produce RS. and RCHSH, cooled, and re- photolyzed at 253.7 nm both of these species disappeared while the spectrum of RS,. grew rapidly. On this basis one can presume that C H , ~ H S H and CH,CH,S. are photochemically active in the 250 nm region while RS,. is not.

The following reactions appear feasible at 77 OK:

hv cH3CHSH + CH3CH2S.**

and h v

CH3CHzS. -> CH3CH2S**

CH3CH2S.** -> CH3CH2 + .S.

followed by

If sulfur atoms are indeed produced, there is enough energy that either 'D or ground state atoms can be formed. The fate of 'D atoms in hydrocarbon solvents is immediate insertion (37) or thermalization to the lower states. The latter are known to be unreactive toward hydro- carbon matrices (37). The solvent mercaptan MSH has not been detected among the prod- ucts, which suggests that photolysis of EtS. produces only atoms, presumably from a lower energy excited quartet state of the radical. Under the conditions of the second photolysis, S(,P) would most probably react with the excess RS- present in the matrix and this accounts for the increase in the RS,. signal.

During the primary photolysis, since there are relatively few RS. radicals free in the matrix, but a high concentration of solvent radicals, the most likely product would be MS. with M being the 3MP radical. These molecules would be spectroscopically indistinguishable from RS. and subject to further photolysis with the eventual formation of (S),. Since this radical has a prin- ciple g-value (38) of 2.042, an absorption maxi- mum should be observed in the low temperature glasses (Figs. 2-4) or in frozen neat thiols (Fig. 1). Its absence shows that secondary photolysis of RS. is unimportant during the primary photol- ysis and supports the idea that "hot" thiyl radicals are indeed formed and that most of

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110 C A N . J . CHEM.. VOL. 5 2 , 1974

these react b y an immediate abstraction f r o m the solvent .

The authors would like to express their gratitude to the Department of National Defence whose financial help initiated this project and to the National Research Council of Canada for continuing s ~ ~ p p o r t . Thanks are also due to Mr. Glen Smith, Mr. L. Peters, Mr. R. Wickson, and Mrs. R. Sniatycka whose technical help was much appreciated during certain phases of the work.

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