Anharmonicity and Hydrogen Bonding. The in-plane OH Bending and its Combination with the OH Stretching Vibration

The infrared spectra of isopropanol, tert-butanol, and their deuterated homologues were measured between 1600 and 1160 cm −1 in a mixture of CCl 3 F and methylcyclohexane-d 14 at temperatures ranging from 20 to −182 °C. The assignments of the in-plane OH deformation (δ OH ) bands of alcohols are confirmed by these low temperature measurements. An explanation is proposed for the great apparent differences between the δ OH frequencies of primary and secondary alcohols on the one hand, and tertiary alcohols on the other. The effect of temperature on the δ OH frequency of hydrogen bonded alcohols is found to be slight. Hydrogen bonding increases significantly the anharmonic coupling constant between ν OH and δ OH It remains, however, much smaller than the anharmonicity constant of ν OH itself.