Formation of interstellar HCCCCC via reaction of ground state carbon atom and diacetylene, HCCCCH

B. J. Sun1, C. Y. Huang1, H. H. Kuo1, K. T. Chen1, H. L. Sun1,C. H. Huang1, M. F. Tsai1, C. H. Kao1, Y. S. Wang1, L. G. Gao1, R. I. Kaiser2, A. H. H. Chang1

(1)Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan

(2)Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI 96822, U. S. A.

• Why C(3P) + HCCCCH ? --carbon chain molecules ubiquitous in interstellar medium

--CnH ( n=1-8 ) detected

--C(3P), everywhere in interstellar clouds

--C(3P) + C2H2 C3H + H, studied intensively

--second member of C(3P) + HC2nH HC2n+1H C2n+1H + H

• What do we know ? -- mechanism: fast, barrierless C addition to πsystems

multiple collision complexes

isomerizations, dissociations

• What would we like to achieve ? with a rigorous theoretical investigation based on first principle,

to obtain reaction paths, rate constants, reaction mechanism,

intermediate lifetimes, product yield

• Difficulty : barrierless reaction with multiple collision complexes:

reaction rate of forming each collision complex?

e.g.

• How ?

Strategy

Capturing cross-sections (σcap's) of forming all collision complexes

Ab initio calculations on triplet HC5H ground state surface

Unimolecular rate constants

Solve rate equations

Product yields

Reaction paths for each collision complex

Most probable paths (reaction mechanism)

Theoretical methods

• Ab initio electronic structure calculation for reaction paths B3LYP/6-311G(d,p) optimized geometry, harmonic frequencies CCSD(T)/cc-pVTZ energy

• RRKM and variational RRKM rate constant -- For reaction , where A*: energized reactant : transition state P : product RRKM rate constant: where : symmetry factor : number of state of : density of state of A*

-- For barrierless reactions, ie. simple bond breaking reaction : variational RRKM, the geometry where is the transition state

PAA k *

A

)(

)()(

E

EEW

hEk

W

A

0

R

W

A

AP

A

P

A

• Capturing cross-section σcap

-- For long-range intermolecular potential of a bimolecular reaction, A+B P :

, where R : distance between centers of mass of two reactants: A － B R

------ Langevin model

-- now there are 3 collision complexes:

• Solve rate equations concentation evolutions product yields

31

31

or , )4

(3)( CE

CE capcap

methods

6)(R

CRV

31

3

31

2

31

1

3363

33

2262

22

1161

11

)3(

)2(

)1(

complex collision forming of coordinate reaction: , )3(

)(

complex collision forming of coordinate reaction: , )2(

)(

complex collision forming of coordinate reaction: , )1(

)(

C

C

C

CRR

CRV

CRR

CRV

CRR

CRV

c

c

c

C(3P) + HCCCCH 3 collision complexes

C1 paths and the most probable paths

C(3P) + HCCCCH

C(3P) + HCCCCH

C2 paths and the most probable paths

C(3P) + HCCCCH

C3 paths and the most probable paths

reaction mechanism ( most probable paths )

C(3P) + HCCCCH

C1 rate equations based on reaction mechanism:

]4[]1[

]3[]11[]3[

]11)[(]4[]11[

]4)[(]11[]1[]4[

]2[]3[]1[]2[

]1)[(]2[]4[]1[

10

615

151414

10114141

262

1221

ikdt

pd

ikikdt

id

ikkikdt

id

ikkkikckdt

id

ckikckdt

cd

ckkckikdt

cd

C1 evolution

C2 evolution

C3 evolution

product yield ：

(C5H) + HC +

C(3P) + HCCCCH

summary

• C(3P) + HCCCCH reactions have been investigated theoretically by combining ab initio calculation, RRKM and variational RRKM theory, and Langevin model.

• Reaction paths, rate constants, most probable paths (reaction mechanisms), intermediate lifetimes, and product yield are predicted.

• 38 C5H2 isomers identified (32 newly found), HC5H crucial

10 C5H (2 new).

• The barrierless and exoergic C(3P) + HCCCCH reaction is an efficient route for diacetylene depletion and HCCCCC formation in interstellar medium.

acknowledgements

黃建瑜 , 孫秉键 , 孔憲和 , 陳寬澤 , 高志豪 , 黃瓊惠 , 蔡閔豐 , 王奕翔 , 高立均 , 孫慧倫

NSC, NCHC, National Dong Hwa University