Molecular dynamics simulation study of N-methylacetamide in water. I. Amide I mode frequency fluctuation

Abstract

Carrying out molecular dynamics simulations of an N-methylacetamide (NMA) in H2O and D2O solutions, we investigated the amide I mode frequency fluctuation and dynamics. The ensemble averaged amide I mode frequency shift was found to be −78 cm−1 in comparison to that of the gas-phase NMA molecule, which is in excellent agreement with the experimental value of −81 cm−1. Similar to the solvation correlation function of a polar solute in liquid water, the correlation function of the fluctuating amide I mode frequency exhibits a bimodal decaying pattern and both the hindered translational and the librational motions of the water molecules directly hydrogen-bonded to the NMA are found to play critical roles in the pure dephasing of the amide I mode. The pure dephasing constant is estimated to be 11 cm−1. The vibrational broadening mechanism is mainly determined by the motional narrowing process. The vibrational Stokes shift of the amide I mode was estimated to be as small as 1.2 cm−1. The amide I IR absorption spectrum thus calculated without any adjustable parameters except for the lifetime of the first excited state has a full width at half maximum of 26.9 cm−1 and is found to be in good agreement with the experiment.