Vibrational predissociation spectra and hydrogen-bond topologies of H+(H2O)9–11

Abstract

Vibrational predissociation spectra of protonated water clusters H⁺(H₂O)_n, n = 9–11, are presented. Examination of the spectra in the free-OH stretching region revealed predominance of a single absorption band at ∼3690 cm⁻¹ for three-coordinate H₂O acting as a double-proton-acceptor/single-proton-donor in the n = 11 cluster. In contrast, the intensity of the absorption band of two-coordinate H₂O acting as a single-proton-acceptor/single-proton-donor at ∼3715 cm⁻¹ decreases with cluster size, and that of one-coordinate H₂O acting as a single-proton-acceptor at ∼3740 and ∼3650 cm⁻¹ diminishes nearly entirely at n > 10 in the spectrum. To deduce the information about cluster temperature, we measured the spontaneous dissociation rates of the cluster ions inside an octopole ion trap and fitted the measured rate constants to an empirical Arrhenius equation. Temperatures in the range of 150 K were estimated for all H⁺(H₂O)_9–11, suggesting that the thermal effect may populate the structures other than the true ground state. The results, combined with previously acquired spectra for H⁺(H₂O)_5–8 (J.-C. Jiang, Y.-S. Wang, H.-C. Chang, S. H. Lin, Y. T. Lee, G. Niedner-Schatteburg and H.-C. Chang, J. Am. Chem. Soc., 2000, 122, 1398) and Monte Carlo simulations with the OSS2 model potential (L. Ojamäe, I. Shavitt and S. J. Singer, J. Chem. Phys., 1998, 109, 5547), show a systematic change in hydrogen-bond topology from tree-like, single-ring, multiple-ring to cage-like isomers (and their mixtures) as the cluster size increases from n = 5 to n = 11.