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

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.