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 the ory 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 re actions. A series of test calculations on hydrogen peroxide, propane, and m ethyl chloride led to the choice of the 6-311G(d,p) basis set for optimum p erformance in terms of speed and accuracy. Three models are defined and tes ted. The lowest level model, which is capable of treating systems containin g more than 30 non-hydrogen atoms, predicts bond dissociation enthalpies wi th 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 m ean absolute deviations of 1.88 and 2.19 kcal/mol relative to experiment. T est calculations on X-H bond energetics indicate that two separate approach es are required for the accurate treatment of both X-Y and X-H bonds.