Theoretical calculation of heats of formation, bond dissociation energies, and gas-phase acidities of fluoromethanes, chloromethanes, and eight other monoderivatives of methane

The heats of formation (HOFs), bond dissociation energies (BDEs) and gas-phase acidities (GPAs) of small methane halogenated (F, Cl) and monoderivatives of methane molecules were theoretically calculated, and the results were compared with the available experimental values in order to examine the accuracy of the six high levels of ab initio calculations CBS-Q, CBS-QB3, G2, G3, G3B3 and G4 in thermochemical studies. The HOFs were derived from ab initio calculations and thermodynamic cycles, using experimentally measured reactions and formation enthalpies. The HOFs of all the radicals of CH₃, CH₂X and X (X=F, Cl, C, O, N) were calculated by atomization reactions. The bond dissociation energies of homolytic cleavage of C-H and C-X (X=F, Cl, C, O, N) bond at 298.15K were calculated. The gas-phase acidities were measured for the H-contained molecules, and the values were combined with the previously reported gas-phase deprotonation enthalpies. Regarding the calculated HOFs, BDEs and GPAs of the set of molecules, G4 theory was found to be the most accurate, with an RMS deviation of 1.62, 5.39, 3.61kJmol^-1 respectively.