Density functional theory based model calculations for accurate bond dissociation enthalpies. 2. Studies of X-X and X-Y (X, Y=C, N, O, S, halogen) bonds

The bond dissociation enthalpies for a set of 30 compounds containing X-Y (X, Y = C, N, O, S, halogen) bonds an: computed using density functional theory based model approaches with the B3P86 functional. These types of bonds were chosen because of their particular importance in free radical organic and bioorganic chemistry, specifically redox chemistry and atom transfer reactions. A series of test calculations on hydrogen peroxide, propane, and methyl chloride led to the choice of the 6-311G(d,p) basis set for optimum performance in terms of speed and accuracy. Three models are defined and tested. The lowest level model, which is capable of treating systems containing more than 30 non-hydrogen atoms, predicts bond dissociation enthalpies with a mean absolute deviation of 2.38 kcal/mol relative to experiment. For a subset of 21 molecules, the two higher-level models predict results with mean absolute deviations of 1.88 and 2.19 kcal/mol relative to experiment. Test calculations on X-H bond energetics indicate that two separate approaches are required for the accurate treatment of both X-Y and X-H bonds.