Hydrogen bonding in methanol—organic solvent and methanol—water mixtures as studied by the vco and voh

Abstract

At low concentrations of methanol in non-polar organic solvents, a splitting of the CO Raman band into monomeric and polymeric components has been observed. From a study of the interactions of CH 3OH monomers with proton-donor and proton-acceptor molecules, it is found that v CO decreases upon H bonding via the lone pairs while it increases upon bonding via the OH proton, and that v co is more sensitive to the latter type of interaction. The frequency shifts of the polymeric component upon dilution are associated with a shortening of methanol chains, the extent of which differs in CCl 4 and CHCl 3, solution. The cooperative properties of H bonding are clearly reflected both in the v oh and v CO band shapes. In methanol—water mixtures a decrease in the CO frequency is observed, accompanied by a gradual removal of the splitting in the I XXX — I ⊥ peak positions. This, together with the increase in the H 2O bending frequency, indicates, that methanol—methanol hydrogen bonds are progressively replaced by intercomponent bonds to water, mainly via the lone pairs of CH 3OH. The excess of lone pairs, introduced by the alcohol component, causes an increase in the overall strength of the H bonds in the mixtures and creates a significant fraction of free lone pairs at the water oxygen atoms.