Theoretical calculation of bond dissociation energies and enthalpies of formation for halogenated molecules

The bond dissociation energies and the enthalpies of formation of halogenated molecules were theoretically calculated, and the results were compared with the corresponding experimental values in order to examine the reliability of a large number of levels of theory in thermochemical calculations. Density functional theory using a multitude of exchange and correlation functionals, Moller-Plesset perturbation theory, and QCISD(T) and CCSD(T) methods were employed, with all-electron and effective-core potential basis sets of varying complexity. A small set of 19 molecules was selected, consisting of X-2, HX, and CH3X (X = F, Cl, Br, and I), the mixed-halogen molecules ClF, BrF, BrCl, IF, and ICl, and H-2 and CH4. The calculated bond dissociation energies were corrected for basis set superposition errors and the first-order spin-orbit coupling in the P-2 state of halogen atoms. In addition, the enthalpies of formation of all molecules in the set as well as those of methyl CH3 and halomethyl radicals CH2X were also calculated by using the corresponding atomization reactions, corrected for the spin-orbit coupling in the P-3 State of carbon atom and the 2P State of halogen atoms. Levels of theory employing the B3P86 functional with moderately large basis sets, augmented with diffusion and polarization functions, were found to be sufficiently reliable in the calculation of bond dissociation energies of closed-shell halogenated molecules. In particular, the B3P86/6-311++G(2df,p) level of theory was found to be the most accurate, with an RMS deviation of 6 kJ mol(-1) for 23 bond dissociation energies, with a negligible dependence of the accuracy on the level of theory chosen for the geometry optimization. In addition, the B3P86 functional in combination with small basis sets was found to be superior to B3LYP and MP2 in the calculation of molecular structures. Regarding the calculated enthalpies of formation, G2 theory was the most accurate, with an RMS deviation of 9 kJ mol(-1), followed by several combinations of the B3PW91 and B3LYP functionals with mostly large basis sets. However, the B3P86 functional tends to overbind open shell species, resulting in an underestimation of the enthalpies of formation for polyatomic molecules. Extension of the bond dissociation energy calculations at levels of theory employing the B3P86 functional to a larger set of 60 bends in 41 halogen-containing molecules revealed systematic errors dependent on the molecular size. Therefore, the calculated bond dissociation energies at the B3P86/61311fS G(2df,p) level of theory were empirically improved by increasing the absolute energies of the radicals by the quantity 9 x 10(-5) N, Hartrees (N-e= total number of electrons of the radical), with a subsequent lowering of the RMS deviation in the larger set to 8.0 kJ mol(-l).