Characterization of Infrared and Near-Infrared Absorptions of Free Alcoholic OH Groups in Hydrocarbon

Abstract

We have demonstrated that the near-infrared and infrared absorptions in the 8000–3200 cm−1 region of an OH group of 2-nonanol, 1-nonanol, etc., in n-heptane are excellently separated by subtraction without any serious interference down to very low concentrations at which OH groups are completely free. The separated sharp absorptions are assigned to the fundamental, combination, and overtone bands that are concerned with the OH stretching of free OH. Two components of a sharp overtone band around 7100 cm−1, which are observed for primary and secondary alcohols, are assigned to coexisting internal rotational isomers of an OH group around the O–C bond. The frequencies of the OH stretching fundamental and overtone bands that are assigned to internal rotational positions are consistent for all the investigated alcohols, including methanol and tertiary butanol. Comparison of the separated spectrum of 2-nonanol in n-heptane with that in 1-chlorooctane or in carbon tetrachloride makes it clear that hydrocarbon is an inert solvent that does not disturb the intrinsic nature of an alcohol OH group. There actually exists a constant anharmonicity shift of 169–175 cm−1 between the double frequency (2 v(OH)o) of the observed fundamental and the observed overtone frequency ([2 v(OH)]o) for free OH of various alcohols in n-heptane.