Room Temperature Gas Phase Infrared Spectra of H-bonded Oligomers of Methanol

Abstract

Formation of various hydrogen bonded oligomers of methanol at room temperature was investigated with the aid of gas phase infrared spectroscopy. In parallel quantum chemical vibrational analyses were performed to identify them and calculate their thermodymical properties. Above 20 torr several weak features appear to the lower wavenumber of the O-H stretch fundamental. Comparing with previous literature reports we ascribe them due to higher (di, tri and tetrameric) oligomeric forms of methanol. Increasing temperature above 298 K in an isochoric condition diminishes the intensity of the observed oligomeric bands, which provides the basis for identification of these bands as originating from H-bonded oligomers. Open and cyclic forms of di, tri and tetrameric forms of methanol were optimized and their anharmonic frequencies were calculated using second order vibrational perturbation theory. Additionally, their binding energies and the enthalpy of formations were also derived from the thermochemical analysis. Experimental enthalpy of formations were evaluated from the variation of integrated band areas with temperature. From the calculated oscillator strengths and the area under each band, the partial pressures of different H-bonded methanol oligomers at room temperature have been obtained for the first time. We obtained in 100 torr total pressure at 298 K the partial pressures of dimer, trimer and tetramer are 0.005, 0.0122 and 0.076 torr, respectively.