Density functional theory based model calculations for accurate bond dissociation enthalpies. 3. A single approach for X-H, X-X, and X-Y (X, Y=C, N, O, S, halogen) bonds

Molecule and radical enthalpies were computed using five model chemistries, which are differentiated by the method used for calculating geometries and scaled frequencies. For all the models, electronic energies were calculated using density functional theory (DFT) at the B3P86/6-311G(2d,2p) level of theory, which was selected following tests involving six hybrid functionals and three basis sets. The models were assessed for their ability to accurately predict the bond dissociation enthalpies (BDEs) of 34 X-H bonds and 28 X-X and X-Y bonds, where X, Y = C, N, O, S, and halogen. The mean absolute error (MAE) of the BDEs relative to experiment predicted using each of the five models is: AM1 = 2.1, PM3 = 1.7, HF/3-21 G(d) = 1.6, B3P86/3-21G(d) = 1.4, and B3P86/6-31G(d) = 1.5 kcal/mol. The B3PS6/6-311G(2d,2p)//B3P86/3-21G(d) and B3P86/6-311G(2d,2p)//B3P86/6-31G(d) models perform as well as G3(MP2) (MAE = 1.5) for the bonds in the test set and with a substantially lower computational cost. The models also perform well for Si-H bonds and for Si-X (X = C, N, 0) bonds in radicals but not for Si-X bonds in closed-shell molecules. Comparisons are also made to a reparametrized version of B3LYP, which is also shown to perform well for most bonds in the test set. The models are shown to be applicable to the study of olefin line growth on silicon surfaces, an area of research in which we are currently involved. The basis set dependence of the X-H BDEs is examined. The shortcomings of the present models are discussed, with particular emphasis on the failure of various DFT methods to adequately describe molecules with extensive delocalization.