Infrared Spectroscopy of Methanol and Methanol/Water Clusters in Helium Nanodroplets: The OH Stretching Region.

Infrared (IR) spectra of methanol clusters in helium nanodroplets are reported in the OH stretching region for the first time. A simple series of intense bands are seen which almost perfectly match previous gas phase studies of these clusters and which are consistent with cyclic structures for the trimer and larger clusters. This finding differs from an earlier report of (CH3 OH)n clusters in helium nanodroplets, which focused on the CO stretching region and concluded that while the trimer was cyclic, the tetramer and pentamer adopted branched structures based on a cyclic trimer core. We also present preliminary data for small (CH3 OH)n (H2 O) clusters, and in particular, we report the first IR spectra for (CH3 OH)2 (H2 O) and (CH3 OH)3 (H2 O). Supporting ab initio calculations suggest that, like the pure methanol clusters, cyclic structures are adopted by these mixed solvent clusters in helium droplets.