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 approximately 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 approximately 3715 cm(-1) decreases with cluster size, and that of one-coordinate H2O acting as a single-proton-acceptor at approximately 3740 and approximately 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. Ojamlie, 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.

First principle study on the structure of H+(H2O)6

“…It is will known that H + (H 2 O) 6 is the smallest protonated water cluster that can carry a fully solvated Zundel type hydronium and very recently Headrick et al have shown conclusive evidences on its existence in Ar-attached protonated water clusters 7 . Previous IR spectra, however, always indicate that bare H + (H 2 O) 6 is a mixture of two topological isomers even temperature as low as 77K. This raises concerns regarding the role of the attached Ar atom on the relative stability of different structural isomers -which will be addressed in this article.…”

“…For H + (H 2 O) 6 , MP2 method with aug-cc-pvDZ (denote as MaDZ) is the highest level ab initio methods with the largest basis set applicable for such system size. In addition, it has been shown that MaDZ allows an accurate characterization of smaller clusters 13,17,18 .…”

“…The structure of protonated water cluster have been the subject of recent experimental studies [1][2][3][4][5][6] . In this work, we have chosen to investigate the structure of H + (H 2 O) 6 by examining the low-lying minima with first principle methods for several reasons.…”

“…Based on the MS-EVB model 10 , Christie and Jordan found that a branched tree structure (402) is more stable 11 . Kuo and Klein 12 identified a single-ring structure (222) as the most stable form of H + (H2O) 6 in the OSS2 model 13 . We should point out that these studies were carried out using different empirical models and zero point energy (ZPE) was not included.…”

“…In this work, we have chosen to investigate the structure of H + (H 2 O) 6 by examining the low-lying minima with first principle methods for several reasons. It is will known that H + (H 2 O) 6 is the smallest protonated water cluster that can carry a fully solvated Zundel type hydronium and very recently Headrick et al have shown conclusive evidences on its existence in Ar-attached protonated water clusters 7 . Previous IR spectra, however, always indicate that bare H + (H 2 O) 6 is a mixture of two topological isomers even temperature as low as 77K.…”