Probing the dynamics of N-methylacetamide in methanol via ab initio molecular dynamics.

Two-dimensional infrared (2D IR) spectroscopy of amide 1 vibrational bands provides a valuable probe of proteins as well as molecules such as N-methylacetamide (NMA), which present peptide-like H-bonding possibilities to a solvent. To assist in rationalizing a large body of experimental 2D IR data on NMA in both aqueous and non-aqueous solvents, we have performed an ab initio molecular dynamics simulation of NMA in methanol. Our study enables us to compare and contrast our findings with earlier calculations on NMA in water, and thereby explore the effect of solvent on the structural and dynamical properties of the NMA solute. We explicitly focus on the dynamics associated with the amide mode I (mainly C[double bond, length as m-dash]O stretch) and amide mode A (mainly N-H stretch) and its interaction with solvent through hydrogen bonding. Our results show that NMA exhibits faster hydrogen bond dynamics in methanol than in water, with an accompanying blue shift of the vibrational frequencies with respect to water. The observed faster diffusion of NMA in methanol signifies a weakening of hydrogen bonding between solute and solvent compared with water.