AB-INITIO STUDIES OF HALOGENATED METHYL AND METHYLENE RADICALS - MOLECULAR-STRUCTURE, VIBRATIONAL FREQUENCIES, AND ENTHALPIES OF FORMATION

Fluorine-and chlorine-substituted methyl and methylene radicals have b een studied by ab initio quantum chemical methods in order to determin e the molecular structure and vibrational frequencies as well as the e nthalpies of formation. The equilibrium geometries of radicals have be en optimized at various levels, and the vibrational frequencies have b een calculated at the optimized geometries. The calculated results, pa rticularly at the MP2/6-311G(d) level, are in good agreement with the experimental data reported previously, suggesting that the present lev els of theory can yield reasonably accurate estimates for the molecula r properties of halogenated radicals when the experimental data are la cking. The standard enthalpies of formation at 298 K, Delta H-f degree s(298 K), of these radicals have been obtained, utilizing the electron ic energies calculated by the G2 or the CBS method. Enthalpies of form ation for methyl radicals calculated with the original G2 method are c lose to the experimental values although the claimed accuracy of 2 kca l/mol is not quite achieved, but the errors in enthalpies of formation from other methods are ra?her high. For methylene radicals whose enth alpies of formation show large discrepancies among the values reported , some assessments have been made based on the calculated enthalpies o f formation.