Structures, vibrational frequencies, thermodynamic properties, and bond dissociation energies of the bromomethanes and bromomethyl radicals: An ab initio study

Reported here is a theoretical study of the entire series of bromomethanes (CH4-nBrn) and bromomethyl radicals (CH3-mBrm) establishing a self-consistent set of structural and thermodynamic information. Ab initio molecular orbital calculations were performed to compute equilibrium geometries for the molecules and radicals initially at the (U)HF/6-31G* and (R)HF/6-31G* levels, respectively, and then refined at the MP2/6-31G* level. Vibrational frequencies were determined for all species at the HF/6-31G* level and comparison with infrared measurements and matrix isolation studies is favorable. Electron correlation contributions were performed by single-point calculations using 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.. The calculations predict the following standard enthalpies of formation in kilocalories per mole (at 298 K and 1 atm): CH2Br2, 1.07 +/- 0.6; CHBr3, 12.16 +/- 0.7; CBr4, 25.23 +/- 0.8; CH2Br(.), 41.63 +/- 0.4; CHBr2., 48.11 +/- 0.6; and CBr3., 55.36 +/- 0.7. These data are then used to tabulate Delta H degrees(f,T), Delta G degrees(f,T), and K-f,K-T for all species over the temperature range 0-1500 K. Comparison is made to existing thermochemical data through calculation of C-H and C-Br bond dissociation energies.