The reactions of hydroxyl radical with ethene, fluoroethene, and
chloroethene have been studied by quantum
chemical methods. Reactants, prereaction complexes,
transition-state structures, and products were optimized
and vibrational frequencies were calculated at the
UMP2/6-311+G(2d,p) level. Transition-state
structures
are significantly different from the prereaction complexes formed on
the reactant side of the MEP. The
convergence of barrier heights and reaction enthalpies has been
systematically investigated with respect to
the size and quality of basis set and the treatment of correlation
energy. The best agreement with experimental
results is found at the MP2/aug-cc-pVTZ level of theory.
Regioselectivity is discussed in terms of two
properties of the radical and the investigated alkenes. The first
factor is the relative spin density in the
3ππ*
state of the alkene. The second factor is the relative strengths
of the product C−O bond, i.e., relative stability
of the corresponding radical product. In the case of fluoroethene
these two effects oppose each other and
regioselectivity is negligible. In the case of chloroethene spin
density is the dominant factor and the addition
of OH radicals to the unsubstituted carbon atom is
preferred.
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 up to the formation of the primary addition products (primary ozonides). Reactants, transition-state structures, and products were optimized, and harmonic vibrational frequencies were 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 wave functions taken as the zeroth order. The rate constants and Arrhenius kinetic parameters were finally calculated in terms of the conventional transitionstate theory. The favorable 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 the course of the optimizations. For all the reactions thus studied, close agreement is found with the experimental kinetics, which makes the future use of the same approach very promising.
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