Ab Initio Modeling of Amide Vibrational Bands in Solution

A combination of molecular dynamics simulations and density functional theory has been used to model the band profiles of the amide I, II, and III vibrational modes of trans-N-methylacetamide in water and acetonitrile. From these calculations, the band shapes can be predicted. Solvent affects the different amide modes differently. The dependence of the solvatochromic shifts in frequency and intensity on the position of the solvent molecules has been studied. These shifts are described by empirical fits of data generated through density functional theory calculations of model systems. Band profiles are simulated on the basis of 8000 structures drawn from molecular dynamics simulations. Frequencies are predicted with an error of less than 13 cm-1. The full width at half-maximum of the predicted bands are also in good agreement with experiment and indicate that hydrogen bonding plays an important role in the broadening of the spectral bands.