Ab Initio and Improved Empirical Potentials for the Calculation of the Anharmonic Vibrational States and Intramolecular Mode Coupling of N-Methylacetamide

The second-order Møller−Plesset ab initio electronic structure method is used to compute points for the anharmonic mode-coupled potential energy surface of N-methylacetamide (NMA) in the transct configuration, including all degrees of freedom. The anharmonic vibrational states and the spectroscopy are directly computed from this potential surface using the correlation corrected vibrational self-consistent field (CC-VSCF) method. The results are compared with CC-VSCF calculations using both the standard and improved empirical Amber-like force fields and available low-temperature experimental matrix data. Analysis of our calculated spectroscopic results show that (1) the excellent agreement between the ab initio CC-VSCF calculated frequencies and the experimental data suggest that the computed anharmonic potentials for N-methylacetamide are of a very high quality. (2) For most transitions, the vibrational frequencies obtained from the ab initio CC-VSCF method are superior to those obtained using the empirical CC-VSCF methods, when compared with experimental data. However, the improved empirical force field yields better agreement with the experimental frequencies as compared with a standard AMBER-type force field. (3) The improved empirical force field in particular overestimates anharmonic couplings for the amide II mode, the methyl asymmetric bending modes, the out-of-plane methyl bending modes, and the methyl distortions. (4) Disagreement between the ab initio and empirical anharmonic couplings is greater than the disagreement between the frequencies, and thus the anharmonic part of the empirical potential seems to be less accurate than the harmonic contribution. (5) Both the empirical and ab initio CC-VSCF calculations predict a negligible anharmonic coupling between the amide I and other internal modes. The implication of this is that the intramolecular energy flow between the amide I and the other internal modes may be smaller than anticipated. These results may have important implications for the anharmonic force fields of peptides, for which N-methylacetamide is a model.