Effect of Hydrogen Bonding on the Deformation Frequencies of the Hydroxyl Group in Alcohols

Abstract

The effects of hydrogen bonding on the infrared spectra of aliphatic alcohols have been studied in the range 4000 to 350 cm—1. Twenty-six alcohols were investigated of which 10 were primary, 7 were branched primary, 6 were secondary, and 3 were tertiary. Attention was concentrated on the region of the spectrum between 1500 cm—1 and 350 cm—1 where the deformation vibrations of the OH group occur. Spectra were obtained of the alcohols (a) in dilute solution in nonpolar solvents, (b) in the liquid state, and (c) in the vapor state, the degree of association being followed by observation of the well-known effect of hydrogen bonding in the OH stretching vibration near 3.0μ. Methanol, ethanol, and hexanol-1 were also investigated after deuteration of the OH group. All the alcohols exhibited a broad diffuse association band with a maximum near 650 cm—1. This has been assigned to the out-of-plane deformation vibration of the H atom in the COH group. All the alcohols also exhibited a broad association band which usually had 2 maxima near 1410 cm—1 and 1330 cm—1. This band (which appears to have been missed by previous workers) is assigned to the in-plane deformation vibration of the H atom in the COH group. The corresponding monomeric band varies between 1200 cm—1 and 1330 cm—1 in undeuterated alcohols and between 870 cm—1 and 930 cm—1 in deuterated alcohols. There is in addition a very narrow association band which lies near 1100 cm—1 in primary and secondary alcohols and near 1165 cm—1 in tertiary alcohols. This is assigned to the effect of hydrogen bonding on the skeletal vibrations, which involve stretching of the CO bond. It appears that the effects of hydrogen bonding on the deformation motions of the OH group are quite complex and that steric effects, rotational isomerism, and interaction with C – H deformation frequencies may all be involved to some extent.