Barrierless bimolecular reaction: reaction path and branching ratio

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

3
Strategy
Ab initio calculations on triplet C4HN and C4H3N
ground state surface
Reaction paths for each collision complex
Capturing cross-sections (scap'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)/6-311G(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 scap
• -- For long-range intermolecular potential
  of a bimolecular reaction, AB? 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

• 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
  psystems
• ? 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
sc1
C c1
p2
H ( sc1 1 )
c2
p2 H
( sc2 1 )
c3
p2 H
( sc3 1 )
c4
p2 H ( sc4
)

C HCCCN ( sc4 )
c5
p2 H
( sc5 1 )
c6
p2 H
( sc6 1 )
sc2
sc3
sc4
sc5
sc6
product yields p2 HC HCCCN 51
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

???, ???, ???, ???, ???, ???, ???, ???, ???,
???, ???, ???, ???, ???, ???
NSC, NCHC, National Dong Hwa University