Hydrogen bonding dynamics and two-dimensional vibrational spectroscopy: N-methylacetamide in liquid methanol

The distribution of the fluctuating amide I mode frequency of N- methylacetamide (NMA)-methanol solution at room temperature was recently found to be non-Gaussian and can be decomposed into two Gaussians that are associated with two different solvation structures. In the present paper we discuss the temperature dependences of hydrogen bonding dynamics and their influences on one- and two-dimensional vibrational spectra by carrying out molecular dynamics simulations of the same solution at five temperatures from 230 to 310 K. The radial distributions reveal the detailed solvent configuration around the NMA. The two-species model where conformational transitions between the two distinct solvation structures are treated as a reversible reaction and the Onsager regression hypothesis was used to estimate the hydrogen bond association and dissociation rates and to study thermodynamic properties. By using the time evolution of the amide I mode frequency fluctuation, both IR absorption and two-dimensional IR photon echo spectra were numerically simulated and compared with approximate two-species model results. An approximation method that can be used to extract kinetic information of the hydrogen bonding dynamics from the spectra is briefly discussed.