Using the C–O stretch to unravel the nature of hydrogen bonding in low-temperature solid methanol–water condensates

Abstract

Transmission infrared spectroscopy has been used in a systematic laboratory study to investigate hydrogen bonding in binary mixtures of CH₃OH and H₂O, vapour deposited at 30 K, as a function of CH₃OH/H₂O mixing ratio, R. Strong intermolecular interactions are evident between CH₃OH and H₂O with infrared band profiles of the binary ices differing from that of the pure components and changing significantly with R. Consistent evidence from the O–H and C–H band profiles and detailed analysis of the C–O stretch band reveal two different hydrogen bonding structural regimes below and above R = 0.6–0.7. The vapour deposited solid mixtures were found to exhibit behaviour similar to that of liquids with evidence of inhomogeneity and higher coordination number of hydrogen bonds that are concentration dependent. The C–O stretch band is found to consist of three components around 1039 cm⁻¹ (‘blue’), 1027 cm⁻¹ (‘middle’) and 1011 cm⁻¹ (‘red’). The ‘blue’ and ‘middle’ components corresponding to environments with CH₃OH dominating as a proton donor (PD) and proton acceptor (PA) respectively reveal preferential bonding of CH₃OH as a PA and H₂O as a PD in the mixtures. The ‘red’ component is only present in the presence of H₂O and has been assigned to the involvement of both lone pairs of electrons on the oxygen atom of CH₃OH as a PA to two PD H₂O atoms. Cooperative effects are evident with concurrent blue-shifts in the C–H stretching modes of CH₃OH below R = 0.6 indicating CH₃ group participation in hydrogen bonding.