Raman spectroscopic study of nonlinear modulation on Fermi resonance of acetonitrile by hydrogen-bonding network

Abstract Raman spectra of acetonitrile-water mixtures are measured in different acetonitrile concentration fractions. The nitrile stretch ( v 2 ) and the combination ( v 3 + v 4 ) modes are coupled by Fermi resonance. We extract the Fermi resonance parameters and find that the frequency separation shows non-linear dependence as a function of acetonitrile concentration fractions. v 2 ~ v 3 + v 4 Fermi doublets intensity ratio exhibits nonlinearity, which is accelerated with the decreasing of the Fermi coupling strength. The OH stretching region is also investigated in order to reveal the variation of the water structure. Our data show that there are strong relationships between the intensity of ~3225 cm −1 band and the v 2 ~v 3 + v 4 Fermi resonance. Because the nitrile stretch and Fermi resonance are very environmental sensitive, the nonlinear modulation on Fermi resonance and the non-regular intensity ratio are attributed to the change of water hydrogen-bonding network. The results provide useful structural and spectral insights for the design of new experiments and the molecular structure modeling.