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 + O
3 ![](/images/entities/rarr.gif)
POZ, proceeds via a biradicaloidtransition state TS0. The stepwise pathway is a three-step reaction, HCCH + O
3 ![](/images/entities/rarr.gif)
M1
![](/images/entities/rarr.gif)
M2
![](/images/entities/rarr.gif)
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 O
3 are unstable toward spin-symmetry breaking. The wave function instability for TS0 and O
3 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 O
3 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 estimate
of Ec shows that the possibility
of the stepwise pathway being a secondary channel at elevated temperaturescannot be ruled out.