| Abstract
| - Bromo- and iodomethanes and the corresponding halogenated methyl radicals have been investigated by abinitio methods. Geometries and vibrational frequencies were derived with quadratic configuration interactionmethods at the QCISD/6-311G(d,p) level of theory, and energies via QCISD(T)/6-311+G(3df,2p). Coreelectrons were represented with relativistic effective potentials. Anharmonicity of the out-of-plane bendingmodes in the methyl radicals was taken into account by numerical integration of the Schrödinger equationwith potentials derived from relaxed scans of these modes. The results are in good accord with experimentaldata where available. Thermochemistry derived via isodesmic reactions referenced to CH3, CH4, andmonohalomethanes yields excellent accord with new experiments on dihalomethanes and provides recommendations for the more poorly characterized tri- and tetrahalomethanes and halomethyl radicals. For themethanes CH2Br2, CHBr3, CBr4, CH2I2, CHI3, CI4, CH2BrI, CHBr2I, and CHBrI2 we compute ΔfH°298 valuesof 4.3, 51.6, 110.6, 108.1, 208.5, 321.3, 56.8, 104.8, and 157.1 kJ mol-1, respectively. For the methyl radicalsCH2Br, CHBr2, CBr3, CH2I, CHI2, CI3, CHBrI, CBr2I, and CBrI2 we compute ΔfH°298 values of 166.6, 191.7,224.0, 217.2, 290.4, 369.1, 241.6, 320.8, and 272.3 kJ mol-1, respectively. Recommended confidence limitsare ±3 kJ mol-1 per Br or I atom. Trends in these values and the corresponding C−H bond strengths arediscussed and compared with prior experiments, empirical estimation schemes, and ab initio calculations.
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