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# 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

journal contribution

posted on 2003-11-20, 00:00 authored by Erin R. Johnson, Owen J. Clarkin, Gino A. DiLabioMolecule 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/3-21G(d) =
1.6, B3P86/3-21G(d) = 1.4, and B3P86/6-31G(d) = 1.5 kcal/mol. The B3P86/6-311G(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, O) 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.