CASSCF/CASPT2 Study of Mechanism and Kinetics of the Gas-phase

Ozone Additions to Ethene, Fluoroethene and Chloroethene

Ivan Ljubic, Aleksandar Sabljic

Rudjer Boskovic Institute, Zagreb, Croatia

Studies into reactivity of ozone towards ethene and its monohalogenated derivatives play an important role in evaluating the persistance of these widespread class of pollutants in the troposphere. Multiconfigurational CASSCF and multireference CASPT2 quantum-chemical methods were used in studying mechanism and kinetics of the gas-phase ozone additions to ethene, fluoroethene and chloroethene. Geometrical structures and harmonic vibrational wavenumbers of the reactants, transition states and products were calculated at the CASSCF/cc-pVTZ level of theory. All the electron energies were further refined at the CASPT2/cc-pVTZ level with the optimized CASSCF wave functions taken as the zeroth order. The rate constants and Arrhenius kinetic parametres were then calculated in terms of the conventional transition state theory (C-TST). The overall reliability of the CASPT2 approach was tested by performing a numerical calculation of the equlibrium geometry, harmonic vibrational wavenumbers and force field of the ground state ozone. Use of the Dunning's correlation consistent basis sets enabled a two-point extrapolation of the examined properties to the limit of infinite basis. The mechanism of ozonolyses includes formation of the primary addition products, their decomposition and possible re-arrangement to the secondary addition products in condensed phase. Also studied was the unimolecular decomposition of (halo)carbonyl oxides, ozonolytic fragments relevant to the processes in the atmosphere.