Theoretical study on reactions of ground state boron atom with ethylene(c 2 H 4 ), allene

Transcription

1 Theoretical study on reactions of ground state boron atom with ethylene(c 2 H 4 ), allene (H 2 CCCH 2 ), and methylacetylen (CH 3 CCH) C. H. Huang 1, C. H. Kao 1, H. L. Sun 1, A. H. H. Chang 1, and R. I. Kaiser 2 (1) Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien, Taiwan, Fax: , air0319@yahoo.com.tw, (2) Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI 96822

2 Abstract The reactions of ground-state boron atom with prototype unsaturated hydrocarbons : ethylene, allene, and methylacetylene, respectively, are investigated theoretically to explore probable routes and dynamics. Both abstraction and insertion channels on C 2 H 4 B and C 3 H 4 B adiabatic doublet ground-state potential energy surfaces for reactions with ethylene, and isomerpair,allene and methylacetylene, respectively, are Characterized by utilizing the unrestricted B3LYP/6-311G(d,p) level of theory and the CCSD(T)/cc-pVTZ calculations. With facilitation of RRKM and variational RRKM rate constants at collision energies of 0-10 kcal/mol, the most probable paths, thus reaction mechanisms, are determined. The corresponding rate equations are then solved such that the evolutions of concentrations of collision complexes, intermediates, and dissociation products versus time are obtained. As a result, the final products and yields are identified-assuming the energy randomization is complete.

3 Strategy Ab initio calculations on doublet C 2 H 4 B and CH 3 CCCHB and H 2 CCCH 2 B ground state surfaces Reaction paths for each collision complex Capturing cross-sections (σ cap '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)/cc-pVTZ energy RRKM and variational RRKM rate constant -- For reaction A * k A, where A*: energized reactant P A P A : transition state P : product A RRKM rate constant: σ W ( E E ) k( E) = h ρ( E) σ where : symmetry factor : number of state of W A : density of state of A* ρ -- For barrierless reactions, ie. simple bond breaking reaction : variational RRKM, the geometry where W is the transition state R = 0 A + A P

5 methods Capturing cross-section σ cap -- For long-range intermolecular potential of a bimolecular reaction, A+B P: V ( R σ cap C ) = 6, where R : distance between centers of mass of two reactants: A-B R R C ( E) = 3π ( ), or σ cap C Langevin model 4E -- now there are 2 or 3 collision complexes: C(1) V1( R1 ) = 6 R C(2) V2( R2 ) =, R 6 R M σ c1 σ c2 M ( C(1) ) 1 ( C(2) ), R : reaction coordinate of forming collision complex C : reaction coordinate of forming collision complex C 2 1 Solve rate equations concentation evolutions product yields

6 B( 2 P) + C 2 H 4 2 collision complexes

7 B( 2 P) + H 2 CCCH 2 3 collision complexes

8 B( 2 P) + CH 3 CCH 2 collision complexes

9 B( 2 P) + C 2 H 4 C1 paths and most probable paths(highlighted)

10 B( 2 P) + C 2 H 4 C2 paths and most probable paths(highlighted)

11 B( 2 P) + C 2 H 4 reaction mechanism ( most probable paths )

12 C1 rate equations based on reaction mechanism: d[c1] = -(k + k + k )[c1] + k [i1] + k [c2] dt d[c2] = -k -3[c2] + k3[c1] dt d[i1] = -(k + k )[i1] + k [c1] + k [i6] dt d[i6] = k [i1] - k [i6] 5-5 dt d[i2] = -(k + k + k )[i2] + k [c1] + k [i4] + k dt d[i4] = -(k + k )[i4] + k [i5] + k [i2] dt d[i5] = -(k + k + k )[i5] + k [i9] + k [i4] dt -19 [i9] d[i9] = -(k k dt d[p4] = k 7[i2] dt d[p9] = k [i5] 20 dt d[p14] = k14[i5] dt -19 )[i9] + k 19 [i2]

13 C1 evolution

14 C1 evolution

15 C2 evolution

16 C2 evolution

17 product yields: B( 2 P) + C 2 H 4 B + (p4) + H 1 : (p9) + H 2.71 : (p14) + H 1.56

18 B( 2 P) + H 2 CCCH 2 i5 paths and most probable paths(highlighted)

19 B( 2 P) + H 2 CCCH 2 i8 paths and most probable paths(highlighted)

20 B( 2 P) + H 2 CCCH 2 C1a paths and most probable paths(highlighted)

21 B( 2 P) + H 2 CCCH 2 reaction mechanism ( most probable paths )

22 i5 evolution

23 i8 evolution

24 C1a evolution

25 product yields: B( 2 P) + H 2 CCCH 2 B + (p13) + H 1 (p24) + H 0.29 :

26 B( 2 P) + CH 3 CCH C1 paths and most probable paths(highlighted)

27 B( 2 P) + CH 3 CCH C2 paths and most probable paths(highlighted)

28 B( 2 P) + CH 3 CCH reaction mechanism ( most probable paths )

29 C1 evolution

30 C2 evolution

31 product yields: B( 2 P) + CH 3 CCH B + (p1) + H

32 summary Barrierless B + C 2 H 4, H 2 CCCH 2,CH 3 CCH 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.