Improper Hydrogen-Bonding CH•Y Interactions in Binary Methanol Systems As Studied by FTIR and Raman Spectroscopy

Fourier Transform infrared spectroscopy and Raman spectroscopy have been used to investigate hydrogen bonding of methanol in different solvents with an aim to explore potential experimental evidence for improper hydrogen bonding involving the methyl group of methanol as suggested by various computational studies. Pure methanol and solutions of methanol in water, acetonitrile, carbon tetrachloride, deuterium oxide, and deuterated acetonitrile have been studied over a range of concentrations. Wavenumber shifts of the CH stretching vibrations were examined to determine if the CH from methanol participates in hydrogen bonding. New concepts of the vibrational wavenumber and integrated intensity at infinite dilution are proposed and given the respective symbols (nu) over tilde (o)(CH) and C(j,CH)*(o). Using the results obtained for methanol in carbon tetrachloride as a,CH reference, shifts in (nu) over tilde (o)(CH) of methanol to higher wavenumbers (blue shifts) were observed in each of the other solvents studied, with the shifts being greatest for the methanol-water interactions. The shifts in vibrational wavenumber suggest possible improper hydrogen bonding, although at this stage a definitive conclusion is not possible. The C(j,CH)*(o) results show that there is no distinguishable change in the methanol CH stretch integrated intensity in carbon tetrachloride and acetonitrile, while there is a significant decrease in the methanol CH stretch integrated intensity in the water solutions.