N-H and N-Cl homolytic bond dissociation energies and radical stabilization energies: An assessment of theoretical procedures through comparison with benchmark-quality W2w data

The performance of a large variety of contemporary density functional theory (DFT), double-hybrid DFT, and high-level Gaussian-n (Gn) procedures has been evaluated for the calculation of bond dissociation energies (BDEs) and radical stabilization energies (RSEs) associated with N?X bonds (X = H, Cl). The chosen set of 62 N?X systems (31 N?H and 31 N?Cl) span a wide range of biologically relevant species. As reference values, we used benchmark-quality W2w data that we recently obtained as part of a systematic thermochemical study of substituent effects in these systems. Of the Gn schemes, the modified G4 procedures (G4-5H and G4(MP2)-6X) perform somewhat better than the corresponding standard G4 procedures for the N?X BDEs of these systems. For the N?H RSEs, G3X, G3X(MP2), G3X(MP2)-RAD, G4-5H, and G4(MP2)-6X emerge as excellent performers, with mean absolute deviations (MADs) from the benchmark W2w values of 0.91.4 kJ mol1. However, for the N?Cl RSEs, G4 is the best performer, with an MAD of 1.7 kJ mol1. The BDEs of both N?H and N?Cl bonds represent a challenge for DFT procedures. In particular, only a handful of functionals (namely, B3P86, M05-2X, M06-2X, and ROB2-PLYP) perform well, with MADs = 4.5 kJ mol-1 for both bond types. Nearly all of the considered DFT procedures perform significantly better for the computation of RSEs, due to a significantly larger degree of error cancelation compared with the BDEs. For the RSEs, BH&HLYP, M05-2X, M06, M06-2X, BMK, PBE0, B2-PLYP, B2GP-PLYP, B2T-PLYP, and ROB2-PLYP are the best performers, with MADs = 4.2 kJ mol-1. Reliable values of N?H and N?Cl BDEs may be obtained by using the RSEs calculated by these functionals in conjunction with a thermochemical cycle involving an experimental (or high-level theoretical) BDE for the H2N?H or H2N?Cl bond. (c) 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011