Competition between cation-pi interactions and intermolecular hydrogen bonds in alkali metal ion-phenol clusters. II. Phenol trimer

The competition between ion-molecule and molecule-molecule interactions was investigated in M+(phenol)3 cluster ions for M=Li, Na, K, and Cs. Infrared predissociation spectroscopy in the O-H stretch region was used to characterize the structure of the cluster ions. By adjusting the experimental conditions, it was possible to generate species where argon was additionally bound in order to investigate cold cluster ions. From a comparison of the M+(phenol)3 spectra with the M+(phenol)3Ar spectra, it is clear that the relative populations of hydrogen-bonded configurations are significantly higher in the colder (argon-bearing) species. For the cold species, the IR spectra were compared with minimum energy ab initio calculations to elucidate the hydrogen-bonded structures. The experimental spectra are most consistent with a cyclic hydrogen-bonded configuration for Cs+(phenol)3 in which the ion binds to the phenol molecules via cation-pi interactions, and noncyclic configurations for Li+, Na+, and K+.