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

Abstract

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(CH(o)) and C(j,CH)*(o). Using the results obtained for methanol in carbon tetrachloride as a reference, shifts in nu(CH(o)) 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.

New spectral assignment of n‐propanol in the C―H stretching region

“…This may be because of the fact that α‐CH 2 group is directly connected to ―OH group which forms strong O―H…O H‐bond in liquid n‐propanol. As revealed in recent experimental and theoretical studies, the O―H…O H‐bond interaction can cause the blue shift of C―H stretching band in pure alcohols or in alcohol–water mixtures . That is to say, the blue shift is an indirect effect from the interaction of adjacent functional groups with solvent molecules.…”

A Study of the Solvation Structure of l‐Leucine in Alcohol–Water Binary Solvents through Molecular Dynamics Simulations and FTIR and NMR Spectroscopy

“…[33][34][35][36][37][38][39] However,e xperimental results on blue-shifting hydrogen bonds are much scarcer than the theoretical ones. [40][41][42] In particular,b lue-shifting hydrogen bonds have not been observed in the promotion of the secondary structure of biomolecules by TFE and HFIP at the microscopic level, despite the NOE observation of the interactions between the hydrophobic moieties of biomolecules and fluorinated alcohols. [21][22][23][24][25][26] We aimed to clarifyt he solvation structure of the amino acid Leu in aliphatic-alcohol-water andf luorinated-alcohol-water solvents as the first stage of our investigations, because the structures of proteins and peptides are too complex to extract the interactions between their hydrophobic moiety and alcohol molecules by using spectroscopic techniques.…”

Abnormal shifts in Raman spectra of deuterated cytidine and 6-azacytidine

“…In general, blue shifts in Raman spectra of dissolved compounds may occur due to change of the solvent (in our case H 2 O for D 2 O) or due to deuteration of solute molecule itself. Appropriate examples may be found for instance in [4] and [2] where Raman blue shifts are connected with transformation of intermolecular or intramolecular H-bonds into D-bonds, respectively. To discriminate effect of deuterated solvent and deuteration effect of the nucleoside molecule itself we have made recrystallization of 6-azaC and cytidine from their H 2 O and D 2 O solutions.…”