Contents lists available at ScienceDirect Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec Room Temperature Gas Phase Infrared Spectra of H-bonded Oligomers of Methanol Bedabyas Behera*, Shubhadip Chakraborty** Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, 560012, India ARTICLE INFO Keywords: FTIR Hydrogen bonding Gas phase Methanol Van’t Hoff equation Partial pressure Enthalpy 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. 1. Introduction In the last few decades a large number of IR spectral studies have been devoted to various hydrogen bonded oligomers of methanol [1–11]. Methanol is an ideal molecule to form intermolecular H- bonding in the exclusively single hydrogen-donor configuration, unlike water. The H-bonded oligomers of methanol are important for in- vestigation since supercritical methanol (at T c = 513 K, P c = 8.1 MPa) can be used as reaction medium for the decomposition of hazardous organic compounds and recovery of useful chemicals from industrial wastes in a green environment friendly way [12]. The O-H stretching mode has been proven to be a sensitive indicator of H-bond formation in methanol oligomers. Errera et al. [13] observed a band at 3350 cm -1 whose intensity decreased with increasing tem- perature and dilution in a nonpolar solvent. They interpreted this phenomenon due to the aggregation of methanol. Inskeep et al. [14] performed IR spectroscopy of methanol vapor and for the first time reported the presence of dimer and tetramer but with little or no trimer. They assigned three bands at 3682, 3590 and 3380 cm -1 to the monomer, dimer and cyclic tetramer O-H stretch. Lee et al. [15] recorded high resolution IR spectrum of methanol vapor and assigned the 3681.5 cm -1 band to the Q-branch center of methanol monomer but did not assign any bands for the higher oligomers. Hussein et al. [16] recorded IR spectra of methanol at 10 to 110 torr. At 10 torr, exclusively methanol monomer is present with some P Q R features of the O-H stretching band appearing. With increasing pressure, an un- resolved shoulder at 3600 cm -1 appears and its intensity starts growing. They assigned this band to the methanol dimer. At a very high pressure a broad band appeared at 3410 cm -1 whose intensity in- creased rapidly with pressure. They tentatively assigned it to higher oligomeric forms of methanol. Barnes et al. [17] recorded the IR spectrum of methanol vapor using a 1 meter path-length cell with an accuracy of ± 5 cm -1 . A band observed at 3690 cm -1 was assigned to the methanol monomer although the presence of rotational structure made the assignment near the OH stretching region difficult. Two more bands observed at 3895 and 3485 cm -1 were combination bands of O- H stretching with O-H torsion in methanol. They also assigned a band at 3590 cm -1 to methanol dimer but did not see any other higher oli- gomer. Dixon et al. [18] recorded the gas phase FTIR spectrum of methanol vapor between 4000-2500 cm -1 and performed ab initio https://doi.org/10.1016/j.vibspec.2019.102981 Received 22 July 2019; Received in revised form 11 September 2019; Accepted 6 October 2019 ⁎ Corresponding author. ⁎⁎ Corresponding author at: Institut de Physique de Rennes, UMR CNRS 6251, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France. E-mail addresses: bedabyas88@gmail.com (B. Behera), shubhachemu@gmail.com (S. Chakraborty). Vibrational Spectroscopy 106 (2020) 102981 Available online 09 November 2019 0924-2031/ © 2019 Elsevier B.V. All rights reserved. T