Raman and DFT study of polar ν(CN) and non-polar ν(C–H) modes of acetonitrile in aqueous Ag nano-colloids

Raman spectra of acetonitrile (Acn) in different millimolar (mM) concentrations adsorbed on Ag nano-colloids were recorded in the region 2100–3300 cm−1. The ν(CN) and ν(C–H) modes show blue shifts of ∼3 and ∼1 cm−1, respectively, when the concentration of Acn in the mixture is increased from 2 to 8 mM. The blue shift of ν(CN) and ν(C–H) modes is predominantly because of adsorption of Acn molecules on Ag nano-colloids. The wave number shift and variation of intensity of the ν(CN) and ν(C–H) bands have been discussed in terms of the adsorption geometry, which probably changes from flat-on configuration at lower concentration of Acn to an end-on configuration at higher concentration of Acn. The dephasing of ν(CN) oscillator becomes considerably slower at higher concentration of Acn. The adsorption of Acn molecules on the nano-colloids was simulated using the (B3LYP) method and the basis sets used for Acn molecules and Ag atoms were 6-311++G(d,p) and Lanl2dz, respectively.