Evaluation of N-H bond dissociation energies in some amides using ab initio and density functional methods

The performance of ab initio and DFT methods in the evaluation of N-H bond dissociation energies of formamide, N-methyl formamide, urea, and acetamide molecules has been analyzed. Both restricted and unrestricted HF and NIP2 fail to provide reliable results because of unreliable spin localization of the radical. The composite methods G3, G2MP2, and CBS-Q, however, provide fairly accurate bond dissociation energies, the results for urea and acetamide agree with the experimental values. The ROB3LYP/6-311 + + G(d,p) method is observed to be the alternative to composite method keeping in view the relatively lower computational effort and a small loss in accuracy. The N-H bond dissociation energy of all the amides is higher than N-H bond dissociation energy for NH3 molecules at all the levels of theory. (c) 2005 Elsevier B.V. All rights reserved.