Near-Infrared Spectroscopy and Anharmonic Theory of Protonated Water Clusters: Higher Elevations in the Hydrogen Bonding Landscape.

Near-infrared spectroscopy measurements are presented for protonated water clusters, H+(H2O) n, in the size range of n = 1-8. Clusters are produced in a pulsed-discharge supersonic expansion, mass selected, and studied with infrared laser photodissociation spectroscopy in the regions of 3600-4550 and 4850-7350 cm-1. Although there is some variation with cluster size, the main features of these spectra are a broad absorption near 5300 cm-1, a sharp doublet near 7200 cm-1, as well as a structured absorption near 4100 cm-1 for n ≥ 2. The vibrational patterns measured for the hydronium, Zundel, and Eigen ions are compared to those predicted by different forms of anharmonic theory. Second-order vibrational perturbation theory (VPT2) and a local mode treatment of the OH stretches both capture key aspects of the spectra but suffer understandable deficiencies in the quantitative description of band positions and intensities.