Determination of the Hydrate Structure of an Isolated Alcoholic OH in Hydrophobic Medium by Infrared and Near-Infrared Spectroscopy

Abstract

This paper reports the structure of the hydrate complex of an isolated alcoholic OH, produced in a small amount in hydrophobic solution in heptane. The structure was determined from the changes, caused by hydration, in the infrared and near-infrared spectra of 2-nonanol in the solution. The changes were exhibited in the "difference" spectrum, in which the spectrum of the solution before hydration was subtracted from that after hydration. The difference spectrum showed a "plus" or "minus" peak at the frequency of the stretching band of a free OH, depending on whether the concentration was below or above about 2%(v/v), respectively. The plus peak appears because the OH stretching band of the isolated OH that acts as an acceptor does not change in frequency but significantly increases in intensity, in agreement with theoretical calculations. In contrast, the stretching band of an isolated OH that acts as a donor shifts downward. This shift decreases the intensity at the stretching frequency of a free OH, giving rise to a minus peak at the frequency in the difference spectrum. It was concluded that an isolated OH is hydrated in the manner as HO...HOH and OH...OH(2) at a concentration below and above about 2%, respectively, in the hydrophobic solution of 2-nonanol.