An ultraviolet photoelectron spectroscopic study of the hydrogen bonded dimers of methanol, ethanol, and dimethylamine in the vapor phase

Abstract

Hydrogen bonded dimers of a few molecules have been studied in the free state by vibration spectroscopy and cognate techniques, particularly in relation to their structure and thermodynamic properties ( 1, 2). Alcohol dimers cannot be examined in the vapor phase as readily as the dimers of carboxylic acids because of the small fraction of the associated species and low association enthalpies. Vital information related to the electronic structures of homodimeric hydrogen bonded species by means of ultraviolet photoelectron spectroscopy (UVPES) is therefore essentially limited to carboxylic acids (3, 4). In order to obtain higher concentrations of alcohol dimers for UVPES investigations, it becomes necessary to have molecular beams generated through an appropriate jet (5, 6). In the present study, we have investigated the electron states of the dimers of methanol and ethanol by employing an indigenously built molecular-beam photoelectron spectrometer (7). More importantly, we have studied, for the first time, the vapor phase UVPE spectrum of the hydrogen bonded dimer of dimethylamine with a N-H ... N bond and made assignments based on ab initio molecular orbital (MO) calculations. Unlike the alcohol dimers there is little or no information in the literature on amine dimers in the free state (1).