1. Barrierless bimolecular reaction: reaction path and branching ratio

2. Fundamentally challenged :
barrierless reaction with multiple collision complexes:
reaction rate of forming each collision complex?
e.g.
What would we like to achieve ?
with a rigorous theoretical investigation based on first principle,
to provide a prediction for future experimental findings,
to obtain reaction paths, rate constants, branching ratio,
product yields
How ?
What is our edge ?

3. Strategy Ab initio calculations on triplet C4HN and C4H3N
ground state surface
Reaction paths for each
collision complex
Capturing cross-sections (σcap's) of
forming all collision complexes
Unimolecular rate constants
Most probable paths (reaction mechanism)
Solve rate equations
Product yields

4. Theoretical methods
Ab initio electronic structure calculation for reaction paths
B3LYP/6-311G(d,p) optimized geometry, harmonic frequencies
CCSD(T)/6-311+G(3df,2p) 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 : C3H3CN  C + C2H3CN
variational RRKM, the geometry where is the transition state

5. methods Capturing cross-section σcap ------ Langevin model
-- 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 5 or 6 collision complexes:
Solve rate equations  product yields

6. --HCCCN, C2H3CN, prototypes, detected in cold molecular clouds:
Why C(3P) + HCCCN, C2H3CN ?
--HCCCN, C2H3CN, prototypes, detected in cold molecular clouds:
HCCCN (cyanoacetylene), simplest member in cyanopolyynes family
C2H3CN (vinyl cyanide), simplest alkene nitrile
--C(3P), everywhere in interstellar clouds
--potentially important routes to complex carbon-nitrogen bearing species
What do we know ?
-- mechanism: fast, barrierless C addition to πsystems
 multiple collision complexes
 isomerizations, dissociations
-- details not known

7. C(3P) + C2H3CN  5 collision complexes

8. C(3P) + HCCCN  6 collision complexes

9. C(3P) + C2H3CN
C1 paths

10. C(3P) + C2H3CN
C2 paths

11. C(3P) + C2H3CN
C3 paths

12. C(3P) + C2H3CN
C4 paths

13. C(3P) + C2H3CN
C5 paths

14. C(3P) + C2H3CN
C1 most probable paths

15. C(3P) + C2H3CN
C2 most probable paths

16. C(3P) + C2H3CN
C3 most probable paths

17. C(3P) + C2H3CN
C4 most probable paths

18. C(3P) + C2H3CN
C5 most probable paths

19. C(3P) + C2H3CN
reaction mechanism ( most probable paths )

20. C(3P) + C2H3CN
reaction mechanism ( most probable paths )

21. C1 rate equations based on reaction mechanism:

22. C1 evolution

23. C2 evolution

24. C3 evolution

25. C4 evolution

26. C5 evolution

27. C(3P) + C2H3CN
product yields：
C p4 + H
p5 + H

28. C(3P) + HCCCN
C1 paths

29. C(3P) + HCCCN
C2 paths

30. C(3P) + HCCCN
C3 paths

31. C(3P) + HCCCN
C4 paths

32. C(3P) + HCCCN
C5 paths

33. C(3P) + HCCCN
C6 paths

34. C(3P) + HCCCN
most probable paths

35. C(3P) + HCCCN
reaction mechanism ( most probable paths )

36. C(3P) + HCCCN
reaction mechanism ( most probable paths )

37. C(3P) + HCCCN product yields： C + c1 p2 + H ( σc1 × 1 )
C + HCCCN ( σc4 × )
c p2 + H ( σc5 × 1 )
c p2 + H ( σc6 × 1 )
σc2
σc3
σc4
σc5
σc6
σc1 ：
σc2
σc3
σc4
σc5
σc6 ＝
0.16
0.15
0.13
0.23
0.18
product yields：
p2 + H：C + HCCCN ＝ 5：1

38. summary
Barrierless C + HCCCN, C2H3CN reactions have been investigated theoretically by combining ab initio calculation, RRKM and variational RRKM theory, and Langevin model.
Reaction paths, most probable paths (reaction mechanisms), product yields are predicted.

39. acknowledgements 蘇秀芬, 黎慧瑜, 黃建瑜, 孫秉键, 湯明軒, 鄭婉君, 劉雅玲, 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