Concerted and Stepwise Reaction Mechanisms for the Addition of Ozone to Acetylene: A Computational Study
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- 作者：Wai-To Chan ; Chang'e Weng ; John D. Goddard
- 刊名：Journal of Physical Chemistry A
- 出版年：2007
- 出版时间：June 7, 2007
- 年：2007
- 卷：111
- 期：22
- 页码：4792 - 4803
- 全文大小：134K
- 年卷期：v.111,no.22(June 7, 2007)
- ISSN：1520-5215
文摘
The mechanism of the reaction between acetylene and ozone to form a primary ozonide (POZ) in the gasphase has been studied theoretically. The concerted pathway, HCCH + O3 
POZ, proceeds via a biradicaloidtransition state TS0. The stepwise pathway is a three-step reaction, HCCH + O3 M1 M2 POZ,involving two biradical TSs and two biradical intermediates M1 and M2. The segment of the global potentialenergy surface (PES) for the concerted pathway is characterized as a R-PES, which is obtained from therestricted (R) density functional theory and Hartree-Fock-based methods. The RDFT and RHF solutions ofTS0 and O3 are unstable toward spin-symmetry breaking. The wave function instability for TS0 and O3 resultsin a discontinuity between the R-PES and the region of the global PES encompassing the biradical TSs andthe intermediates of the stepwise pathway, which are characterized with unrestricted (U) methods. The globalPES is characterized separately as an U(R)-PES using a combination of the R and U methods. Several differentvalues of barriers for the concerted pathway and the energy of concert (Ec) can be estimated due to complicationsarising from the discontinuity between the R- and the U(R)-PES and the existence of two different RDFTand UDFT O3 equilibrium geometries. RCCSD(T)//RDFT predicts a barrier of 8.2 kcal/mol. U(R)CCSD(T)/U(R)DFT predicts a barrier of 13.8 kcal/mol for the concerted and 15.3 kcal/mol for the stepwise pathway.Comparison between the R-PES barrier to the concerted pathway and the U(R)-PES barrier to the stepwisepathway suggests the former to be the only significant mechanism. Consideration of the energy differencebetween TS1, the TS for the first step of the stepwise mechanism, and TS0 within the global PES leads toa significantly smaller Ec. Geometry optimization with CASSCF and energy point calculations with MRMP2are employed to characterize TS0 and TS1. MRMP2//CASSCF predicts the energy level of TS1 to be higherthan that of TS0 by 2 kcal/mol. Analysis of experimental and computational data based on the low estimateof Ec shows that the possibility of the stepwise pathway being a secondary channel at elevated temperaturescannot be ruled out.
POZ, proceeds via a biradicaloidtransition state TS0. The stepwise pathway is a three-step reaction, HCCH + O3 M1 M2 POZ,involving two biradical TSs and two biradical intermediates M1 and M2. The segment of the global potentialenergy surface (PES) for the concerted pathway is characterized as a R-PES, which is obtained from therestricted (R) density functional theory and Hartree-Fock-based methods. The RDFT and RHF solutions ofTS0 and O3 are unstable toward spin-symmetry breaking. The wave function instability for TS0 and O3 resultsin a discontinuity between the R-PES and the region of the global PES encompassing the biradical TSs andthe intermediates of the stepwise pathway, which are characterized with unrestricted (U) methods. The globalPES is characterized separately as an U(R)-PES using a combination of the R and U methods. Several differentvalues of barriers for the concerted pathway and the energy of concert (Ec) can be estimated due to complicationsarising from the discontinuity between the R- and the U(R)-PES and the existence of two different RDFTand UDFT O3 equilibrium geometries. RCCSD(T)//RDFT predicts a barrier of 8.2 kcal/mol. U(R)CCSD(T)/U(R)DFT predicts a barrier of 13.8 kcal/mol for the concerted and 15.3 kcal/mol for the stepwise pathway.Comparison between the R-PES barrier to the concerted pathway and the U(R)-PES barrier to the stepwisepathway suggests the former to be the only significant mechanism. Consideration of the energy differencebetween TS1, the TS for the first step of the stepwise mechanism, and TS0 within the global PES leads toa significantly smaller Ec. Geometry optimization with CASSCF and energy point calculations with MRMP2are employed to characterize TS0 and TS1. MRMP2//CASSCF predicts the energy level of TS1 to be higherthan that of TS0 by 2 kcal/mol. Analysis of experimental and computational data based on the low estimateof Ec shows that the possibility of the stepwise pathway being a secondary channel at elevated temperaturescannot be ruled out.