Electrostatic DFT Map for the Complete Vibrational Amide Band of NMA

Abstract

An anharmonic vibrational Hamiltonian for the amide I, II, III, and A modes of N-methyl acetamide (NMA), recast in terms of the 19 components of an external electric field and its first and second derivative tensors (electrostatic DFT map), is calculated at the DFT(BPW91/6-31G(d,p)) level. Strong correlations are found between NMA geometry and the amide frequency fluctuations calculated using this Hamiltonian together with the fluctuating solvent electric field obtained from the MD simulations in TIP3 water. The amide I and A frequencies are strongly positively correlated with the C=O and N-H bond lengths. The C=O and C-N amide bond lengths are negatively correlated, suggesting the solvent-induced fluctuations of the contribution of zwitterionic resonance form. Sampling the global electric field in the entire region of the transition charge densities (TCDs) is required for accurate infrared line shape simulations. Collective electrostatic solvent coordinates which represent the fluctuations of the 10 lowest amide fundamental and overtone states are reported. Normal-mode analysis of an NMA-3H(2)O cluster shows that the 660 cm(-1) to 1100 cm(-1) oscillation found in the frequency autocorrelation functions of the amide modes may be ascribed to the two bending vibrations of intermolecular hydrogen bonds with the amide oxygen of NMA.