STRUCTURES, HYPERFINE PARAMETERS, AND INVERSION BARRIERS OF CYCLOPROPYL AND OXIRANYL RADICALS

A comparative post-Hartree-Fock study has been performed on cyclopropy l and oxiranyl radicals in order to ascertain the role of the oxygen a tom in modifying the hyperfine structure and height of the barrier gov erning inversion at the radical center. The structural parameters and harmonic force fields obtained for the parent molecules using second-o rder many-body perturbation theory with a large basis set are in good agreement with experiment. The same approach points out significant di stortions upon breaking of a CH bond and a larger pyramidality for the radical center in oxiranyl with respect to cyclopropyl. Also inversio n barriers of both radicals are in remarkable agreement with experimen tal estimates. isotropic hyperfine parameters in good agreement with t hose obtained from electron spin resonance spectra can be computed onl y when using purposely tailored basis sets in the framework of a coupl ed cluster approach and taking into account vibrational averaging effe cts induced by the inversion motion. Interpretation of the results in terms of direct and spin polarization effects points out a number of g eneral trends for germinal and vicinal atoms. Furthermore, it is well evidenced that replacement of a methylenic group by an oxygen atom mod ifies the hyperfine parameters through geometric rather than direct el ectronic effects. (C) 1996 American Institute of Physics.