engleski

CASSCF/CASPT2 study of the mechanism and kinetics of the gas-phase ozone additions to ethene, fluoroethene and chloroethene

Ab initio multiconfigurational CASSCF and CASPT2 methods were employed in studying the reaction mechanisms and kinetics of the gas-phase ozone additions to ethene, fluoroethene and chloroethene. Reactants, transition state structures and products were optimized and harmonic vibrational frequencies calculated at the CASSCF/cc-pVTZ level. For kinetic calculations the electron energies of all the stationary points were further refined by utilizing the CASPT2 method with the optimized CASSCF/cc-pVTZ wavefunctions taken as the zeroth-order. The rate constants and Arrhenius kinetic parameters were finally calculated in terms of the conventional transition state theory. The favourable conformations of the ozone approach to the two asymmetrically substituted haloalkenes are at first governed by the electrostatic repulsion in the transition state structures and later by the gradually predominating anomeric effect. The bond formation in the primary haloozonides was analyzed by monitoring the changes in the occupation numbers of the active orbitals in course of the optimizations. For all the reactions thus studied a close agreement is found with the experimental kinetics, which makes a future use of the same approach very promising.

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