Reactions of ozone with olefins and dienes: anharmonic approximation in quantum chemical calculations

The primary step of propene ozonolysis was studied using various quantum chemistry approaches including the M06-2X method, the double hybrid B2PLYP and PBEQIDH methods, and the coupled clusters method at the CCSD level with the aug-cc-pVDZ and aug-cc-pVTZ basis sets. Transition-state geometries, the energies of elementary steps of the reaction, and the activation barriers were determined. The rate constants for the reaction were calculated using the normal vibration frequencies obtained in the harmonic and anharmonic approximations. The rate constants for the ozonolysis of ethylene, fluoroethylene, chloroethylene, butadiene, and isoprene were calculated in the anharmonic approximation. The total rate constant for gas-phase ozonolysis of propene calculated for normal conditions in the harmonic (k(harm)) and anharmonic (k(anh)) approximations for all transition-state configurations taking account of the spatial degeneracy was 2160 and 4714 L (mol s)(-1), respectively. The latter value better agrees with experimental data (5745 L (mol s)(-1)). The rate constants for the ozonolysis of chloroethylene, fluoroethylene, butadiene, and isoprene (characterized by asymmetric arrangement of substituents at the reaction center) calculated with allowance for anharmonicity demonstrate much better agreement with the experiment. The rate constant for ethylene ozonolysis calculated in the anharmonic approximation remains unchanged compared to that obtained in the harmonic approximation, whereas the rate constants for the chloroethylene and fluoroethylene ozonolysis increase two- to fourfold, respectively. Similar results were also obtained for butadiene, isoprene, and propene. For all these systems, taking account of anharmonicity has a stronger impact on the rate constants for the reaction in the case of DeMore mechanism compared to the Criegee mechanism; i.e., the harmonic approximation is insufficient for entropy calculations for molecules with asymmetrically arranged or bulky substituents at the double bond in the reaction center.