Unconventional O-H···C Hydrogen Bonding and Effects of Conformational Changes on Infrared Spectroscopy of o-Cresol in Solutions.

Abstract

The unconventional O-H···C intramolecular hydrogen bonding and the effect of conformational changes on IR spectra of o-cresol in aqueous solutions were investigated by using molecular dynamics (MD) simulations, density functional theory (DFT), and experiments. A facial rotational isomerization between global minimum with trans conformation and the cis isomer is predicted to take place in gas phase with a low barrier of about 3.7 kcal/mol through a vertical-like transition state. Upon solvation in aqueous solution, the contents of energetically high-lying vertical and cis conformations of neutral o-cresol are increased to 19% and 57%, respectively, in comparison with those (vertical, 0%; cis, 27%) in vacuum. The IR spectra of aqueous solution are closely related to the relative population of the different conformations, especially for the cis conformation with hydroxyl group facing to alkyl group. The appearance of cis conformations and unconventional O-H···C intramolecular hydrogen bond (HB) caused the low-frequency shift in OH stretching vibration of the IR spectra, which was also correlated with cation-like charge distribution and the decrease in s-component of oxygen hybridization orbital. However, the intermolecular HB between the o-cresol and surrounding water (o-cresol) molecules gave rise to more evident shifts in IR spectra than that caused by the intramolecular HB contributions in cis isomer. Further identification of intramolecular HB effect is performed through the comparison of spectrum changes that occurs on passing from aqueous solution to less interacting (carbon tetrachloride and cyclohexane) solvents. The absence of the intermolecular HB interaction between o-cresol and carbon tetrachloride (and cyclohexane) solvents leads to the weaker intensity and narrower width of OH stretching vibration region (around 3407 cm-1) in the IR spectra than that in aqueous solution.