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We have detected the methanol dimer, trimer, and tetramer at equilibrium conditions at room temperature in the gas phase using direct absorption Fourier transform infrared spectroscopy. The infrared intensity of the OH-stretching transitions are enhanced upon hydrogen bonding and are increasingly red-shifted with increasing cluster size, facilitating identification and quantification of the various clusters. We calculate the intensities of the bound OH-stretches, OH b , for all clusters with a range of reduced dimensional vibrational models with different levels of electronic structure theory. Partial pressures of the clusters are obtained by scaling the measured integrated absorbance of the OH b -stretching bands by the calculated intensities of the associated vibrational transitions. We estimate the methanol dimer equilibrium constant, K D , to be 0.033, at 298.15 K, which is comparable to that of the water dimer. For the methanol trimer and tetramer, we estimate equilibrium constants for aggregation of monomers of K T ∼ 0.04 and K Q ∼ 0.6, respectively.

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 reduced-dimensional vibrational model, which includes the two high-frequency 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.

We have detected the tert -butyl hydroperoxide dimer, ( t -BuOOH) 2 , in the gas phase at room temperature using conventional FTIR techniques. The dimer is identified by an asymmetric absorbance band assigned to the fundamental hydrogen-bound OH b -stretch. The weighted band maximum of the dimer OH b -stretch is located at ∼3452 cm -1 , red-shifted by ∼145 cm -1 from the monomer OH-stretching band. The gas-phase dimer assignment is supported by Ar matrix isolation FTIR experiments at 12 K and experiments with a partially deuterated sample. Computationally, we find the lowest energy structure of ( t -BuOOH) 2 to be a doubly hydrogen bound six-membered ring with non-optimal hydrogen bond angles. We estimate the gas-phase constant of dimer formation, K , to be 0.4 (standard pressure of 1 bar) using the experimental integrated absorbance and a theoretically determined oscillator strength of the OH b -stretching band.