1. Formation of interstellar HCCCCC via reaction of ground state carbon atom and diacetylene, HCCCCH B. J. Sun 1, C. Y. Huang 1, H. H. Kuo 1, K. T. Chen 1, H. L. Sun 1, C. H. Huang 1, M. F. Tsai 1, C. H. Kao 1, Y. S. Wang 1, L. G. Gao 1, R. I. Kaiser 2, A. H. H. Chang 1 (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.

2. Why C( 3 P) + HCCCCH ? --carbon chain molecules ubiquitous in interstellar medium --C n H ( n=1-8 ) detected --C( 3 P), everywhere in interstellar clouds --C( 3 P) + C 2 H 2  C 3 H + H, studied intensively --second member of C( 3 P) + HC 2n H  HC 2n+1 H  C 2n+1 H + H What do we know ? -- mechanism: fast, barrierless C addition to πsystems  multiple collision complexes  isomerizations, dissociations

3. 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 ?

4. Strategy Capturing cross-sections (σ cap 's) of forming all collision complexes Ab initio calculations on triplet HC 5 H ground state surface Unimolecular rate constants Solve rate equations Product yields Reaction paths for each collision complex Most probable paths (reaction mechanism)

5. 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

6. Capturing cross-section σ cap -- 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 methods

7. C( 3 P) + HCCCCH  3 collision complexes

8. C 1 paths and the most probable paths C( 3 P) + HCCCCH

9. C 2 paths and the most probable paths

10. C( 3 P) + HCCCCH C 3 paths and the most probable paths

11. reaction mechanism ( most probable paths ) C( 3 P) + HCCCCH

12. C 1 rate equations based on reaction mechanism:

13. C 1 evolution

14. C 2 evolution

15. C 3 evolution

16. product yield ： (C 5 H) + H C + C( 3 P) + HCCCCH

17. summary C( 3 P) + 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 C 5 H 2 isomers identified (32 newly found), HC 5 H crucial 10 C 5 H (2 new). The barrierless and exoergic C( 3 P) + HCCCCH reaction is an efficient route for diacetylene depletion and HCCCCC formation in interstellar medium.

18. 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