Thermodynamic Properties of Bromomethanes and Bromomethyl Radicals: An ab Initio Study

Thermochemical data on volatile organic compounds containing bromine are needed tor atmospheric modeling, in view of their ozone depletion potential upon photodissociation and the release of atomic bromine. Yet even for the bromomethane series, with the exception of CH3Br, thermodynamic properties are not well established. Similarly, structural and thermochemical information on brominated methyl radicals is incomplete or not available. In this paper we have adopted a computational approach to obtain this needed information. Equilibrium geometries for the molecules CH4_nBrn (n = 0-4) and radicals CH3_mBrm (m = 0-3) were optimized at both HF/6-31G* and MP2/6-31G* levels of theory. Moments of inertia, harmonic vibrational frequencies, and thermodynamic functions were determined at the HF/6-31G* level. Electron correlation contributions were performed by single-point calculations at both second- and fourth-order Moller-Plesset perturbation theory for derived MP2/6-31G*geometries. Enthalpies of formation were obtained from a consideration of applicable isodesmic reactions using the derived MP4/6- 31G**//MP2/6-31G* total energies in conjunction with experimentally established enthalpies of formation for CH3Br, CH4, and CH3. These data were then used in the determination of AH°f,T, xG°f,T, and Kf, T for all species over the temperature range 0 to 1500K. A comparison was made to the existing standard enthalpies of formation at 298 K, both experimentally measured and theoretically estimated, for CH2Br2, CHBr3, CBr4, CH2Br., CHBr2 . , and CBr3..