Knowledge Bank

Analysis of the rovibrational infrared (IR) absorption spectra of water $(H2O,D2O)$ molecules isolated in solid parahydrogen $(pH2)$ 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 $(oH2)$ 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 $C2v$ structure for the ground state of the $0H2$-water complexes, with the $0H2$ acting as a proton donor to the water oxygen atom. In this picture, the $0H2$-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.