Abstract
The performance of a large variety of contemporary density functional theory (DFT), double‐hybrid DFT, and high‐level Gaussian‐n (G__n__) procedures has been evaluated for the calculation of bond dissociation energies (BDEs) and radical stabilization energies (RSEs) associated with NX bonds (X = H, Cl). The chosen set of 62 NX systems (31 NH and 31 NCl) 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 G__n__ schemes, the modified G4 procedures (G4‐5H and G4(MP2)‐6X) perform somewhat better than the corresponding standard G4 procedures for the NX BDEs of these systems. For the NH 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 0.9–1.4 kJ mol^–1^. However, for the NCl RSEs, G4 is the best performer, with an MAD of 1.7 kJ mol^–1^. The BDEs of both NH and NCl 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 NH and NCl 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 H~2~NH or H~2~NCl bond. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011