ROVIBRATIONAL DYNAMICS OF ORTHOHYDROGEN-WATER COMPLEXES IN SOLID PARAHYDROGEN
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Date
2004
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Ohio State University
Abstract
Analysis of the rovibrational infrared (IR) absorption spectra of water $(H_{2}O, D_{2}O)$ molecules isolated in solid parahydrogen $(pH_{2})$ reveals their existence as very slightly hindered rotors, with rotational constants reduced by only 2-5% from their gas phase values. Clustering of residual orthohydrogen $(oH_{2})$ molecules with water monomers results in the appearance of several new IR absorption features. For Type B water monomer bands (e.g. $\nu_{1}, \nu_{2}$) most of the new features appear near the vibrational origin, and were originally interpreted as indicating the presence of ``non-rotating'' water $molecules.^{a}$ However, for Type A bands (e.g. $\nu_{3}, \nu_{2} + \nu_{3}, \nu_{1} + \nu_{3}$) very little IR activity is observed near the vibrational origin, refuting this appealingly simple explanation. Here we propose a new interpretation which assumes a semi-rigid $C_{2v}$ structure for the ground state of the $0H_{2}$-water complexes, with the $0H_{2}$ acting as a proton donor to the water oxygen atom. In this picture, the $0H_{2}$-water complex spectra can be understood as parallel and perpendicular bands of an asymmetric top near the prolate symmetric top $limit.^{b}$ Thus, the features bunching near the Type B vibrational band origins arise from the $\Delta K = 0$ selection rule for parallel bands, while the more widely separated features in the Type A bands arise from the $\Delta K = \pm 1$ selection rule for perpendicular bands.
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$^{a}$ M.E. Fajardo, 57th International Symposium on Molecular Spectroscopy, paper MF01 (2002). $^{b}$ M. E. Fajardo, S. Tam, and M.E. DeRose, J. Mol. Struct. accepted (2004).
Author Institution: AFRL/MNME, Energetic Materials Branch, Ordnance Division, U.S. Air Force Research Lab
Author Institution: AFRL/MNME, Energetic Materials Branch, Ordnance Division, U.S. Air Force Research Lab