Effect of Temperature on the OH-Stretching Bands of the Methanol Dimer

Abstract

We present a conceptually simple model for understanding the significant spectral changes that occur with the temperature in the infrared spectra of hydrogen-bound complexes. We have measured room-temperature spectra of the methanol dimer and two deuterated isotopologues in the OH(D)-stretching region. We correctly predict spectral changes observed in the gas phase for the bound OH stretch in the methanol dimer from jet-cooled to room temperature and corroborate this with experimental and theoretical results for deuterated isotopologues. The origin of the observed spectral features is explained based on a reduceddimensional vibrational model, which includes the two highfrequency OH stretches, the two methyl torsions, and the six intermolecular low-frequency vibrations. Key to the success of the model is a new coordinate definition to describe the intrinsic large-amplitude curvilinear motion of low-frequency vibrations. Despite the deceivingly simple appearance of the room temperature bound OH-stretching fundamental band, it consists of ∼10 7 vibrational transitions.

Room Temperature Gas Phase Equilibrium Constants of the Methanol Dimer, Trimer, and Tetramer

“…The peak location of the cold clusters are supported by other studies including cavity ringdown and predissociative size selected molecular beam depletion experiments. ,,, These three maxima all appear at the onset of the fitted band profiles in our room temperature spectra, which we assign as the MeOH tetramer, trimer, and dimer. In general, it has been observed that cold OH b -stretching absorption bands are positioned at the low energy onset of the thermal band counterparts in bimolecular hydrogen bound complexes. ,,, For (MeOH) 2 , spectra at both cold and warm conditions are available with limited interfering absorbance from other clusters, ,, and ab initio simulated band profiles at various temperatures have recently been presented . In Figure , we show a comparison between our fitted (MeOH) 2 band profile and an experimental and simulated band profile at room temperature from ref .…”

“…The change in absorbance between the high pressure spectrum and the low pressure reference spectrum becomes apparent when magnified by a factor of 10, and clearly reveals a wide cluster absorption band with three maxima located at 3390, 3526, and 3598 cm –1 , respectively. The spectrum resembles previous spectra recorded at room temperature and consists of absorbance contributions from OH b -stretching transitions in the MeOH dimer, trimer and tetramer. , We deconvolute the band by fitting three band profiles, one for each order of cluster, with each cluster band profile consisting of the sum of two Voight profiles slightly shifted from each other to allow for asymmetric band-shapes …”

“…The experimental setup and approach has been described previously ,,, and is summarized. Spectroscopic grade methanol, MeOH ≥ 99.9%, was purchased from Sigma-Aldrich.…”

“…To do this, we use a hybrid theoretical/experimental spectroscopic approach, , which we have used previously to determine small partial pressures of molecular complexes in gas mixtures. ,,,,, The partial pressure of a cluster can be determined from the ratio of a measured integrated absorbance and the corresponding calculated oscillator strength of the band: p c l u s t e r = 2.6935 × 10 − 9 Torr m cm normalK T f c a l c · l · prefix∫ A ( ν̃ ) .25em normald ν̃ where p cluster is the cluster partial pressure, f calc is the calculated oscillator strength, l the path length of the cell, and A (ν̃) the wavenumber dependent base-10 absorbance of the band. We focus on the absorption band associated with the bound OH b -stretch, thus taking advantage of the OH-stretching redshift and intensity enhancement upon hydrogen bonding. , We approximate the oscillator strength of the band as the oscillator strength of the ground-state OH-stretching transition . The clusters are expected to be progressively more red-shifted with the size of the cluster due to the increasing cooperative effects in the cyclic hydrogen bonding networks. − This aids the separation and identification of the different clusters.…”

“…The OH b -stretching band from each of the three clusters is fitted with the sum of two normalized Voight profiles. The two Voight profiles are shifted slightly from each other simply to allow for some asymmetry in the absorbance band profiles, which has been observed for the MeOH dimer at room temperature. , In total, 21 parameters (Gaussian and Lorentzian widths, peak positions, and pairwise relative amplitudes) are optimized for the 3 pairs of Voight profiles by fitting the sum of the profiles to the recorded spectra. The parameters are fitted to the spectra recorded at all different pressures in parallel by letting the absolute amplitudes relax for each pressure with a set of fixed profile parameters.…”