Fermi-Resonance Raman Spectra of Acetonitrile in Water and Water-Acetonitrile Mixture Solutions Based on the Reference Interaction Site Model Self-Consistent Field Method Coupled with Constrained Spatial Electron Density Distribution and Vibrational Quasi-Degenerate Perturbation Theory.

We present an approach for calculating three-mode-coupling Fermi-resonance Raman spectra of acetonitrile molecules in solution by combining the Reference Interaction Site Model Self-Consistent Field method coupled with the constrained Spatial Electron Density Distribution (RISM-SCF-cSED) and second-order vibrational quasi-degenerate perturbation theory (VQDPT2). By describing solvents using integral equation theory and modeling anharmonic vibrations with VQDPT2, this method enables efficient computation of three-mode-coupling Fermi resonance Raman spectra of acetonitrile in solution at a modest computational cost. We demonstrate its application to acetonitrile in water and to acetonitrile-water mixture solutions, showing that the predicted peak positions and solvent-induced shifts in the Raman spectra agree well with experiment. Analysis of radial distribution functions indicates that the packing of water molecules around the cyano group of acetonitrile influences the nonlinear, high-frequency shift of its cyano stretching vibration in solution.