Application of the Local Vibrational Mode Framework for the Spectroscopic Estimation of Free Hydroxyl Group Fractions in Pure Liquid Methanol-d3.

The temperature dependence of the nondonor hydroxyl group fractions of hydrogen-bonding compounds is of paramount importance to the thermodynamics community for the development of advanced equations of state that can accurately account for association behavior. We adapt the vibrational theory of local modes for the investigation of the vibrational properties of methanol clusters in an explicit solvent environment. The calculated intensities are then utilized alongwith near-infrared direct absorption spectra of partially deuterated methanol counterparts for the extraction of free hydroxyl group fractions in a temperature range of 10-50 °C. The estimated free OH fraction values are validated through the application of two approaches for the processing of the measured spectroscopic data, presenting large deviations from previous works and aligning with experimental studies of dilute methanol/inert solvent solutions. The results can be extended to CH3OH and are consistent with the notion of methanol forming a network of hydrogen bonds in the liquid phase with very low amounts of free OH groups. This work establishes a proof-of-concept for the synergy of the vibrational local mode framework with simple liquid-phase spectroscopic measurements, paving the way for the quantification of hydrogen-bonding equilibria in a wide variety of associating liquids.