Cavity-Enhanced Overtone Spectroscopy of Methanol in Aprotic Solvents: Probing Solute–Solvent Interactions and Self-Associative Behavior

Abstract

Methanol in aprotic solvents can serve as a case study for self-association via hydrogen-bonding, which is an important process in many biological and environmental systems. Incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS), which provides enhanced sensitivity relative to conventional single-pass absorption techniques, has been used to characterize the third "free" O-H stretching overtone of methanol in four aprotic solvents (CCl4, CHCl3, CH2Cl2, and C6H6), including the transition wavenumber, bandwidth, and molar absorptivity. The absorption band characteristics indicate an increasing degree of nonspecific methanol-solvent interaction with increasing solvent dielectric constant, except in the case of benzene, which shows evidence of a specific, H-π interaction. Density functional theory with the polarizable continuum model was used to complement the results by assessing the accuracy of computational methods for calculating anharmonic O-H stretching frequencies. Finally, the self-association of methanol in these solvents at 298 K was also investigated using the concentration dependence of the overtone absorption intensity. The propensity for methanol's self-association in the solvents studied increases in the order: CH2Cl2 ∼ CHCl3 < C6H6 < CCl4.