Determination of the intermolecular geometry of the phenol–methanol cluster

Abstract

From the rovibronic spectrum of three isotopomers of the hydrogen bonded binary phenol–methanol cluster the inertial parameters have been determined. The geometry of the hydrogen bond was obtained from a fit of the six intermolecular geometry parameters to the rotational constants of these isotopomers and was compared to the results of ab initio calculations. The hydrogen bond length was found to decrease by 5 pm upon electronic excitation, due to the increased acidity of phenol in the S₁-state. The same fact is responsible for the increased linearity of the hydrogen bond in the S₁-state. The spectra of all three isotopomers are split by the interaction of the internal torsion of the methyl group with the overall rotation. Using the subtorsional splittings and the torsion–rotation parameters, the reduced barrier to internal rotation of the methyl group in the methanol moiety of the cluster could be determined to be 32.2 in the S₀-state and 27.9 in the S₁-state. This reduction can be traced back to the increased acidity of phenol in the S₁-state, which reduces the hyperconjugation between the methanolic oxygen σ-type lone pair and the hydrogen atoms of the methyl group.