Correlation between electronic and molecular structure distortions and vibrational properties. II. Amide I modes of NMA–nD2O complexes

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Abstract
Hydration effects on the molecular structure and amide I mode frequency of a prototype peptide molecule, N-methylacetamide (NMA), when it is solvated by a few water molecules, were investigated by carrying out ab initio calculations for a number of NMA–water complexes. The harmonic frequency shift of the amide I mode in NMA –n D 2 O (n=1–5) complex was found to originate from the combination of the molecular cubic anharmonicity and displacement of the amide I coordinate when the NMA is hydrated. Using a multivariate least-square fitting method, the effective transition charges of six NMA sites were determined. A brief discussion on how this empirical model can be used to quantitatively describe solvatochromic frequency shift of the NMA amide I mode in solution is presented.