Intermolecular interactions involving noble-gas hydrides: Where the blue shift of vibrational frequency is a normal effect

The intermolecular interactions of noble-gas hydride molecules (HNgY) are overviewed with emphasis on the experimentally observed species. These interactions include molecular complexes, interaction with surrounding matrix (matrix-site effect), and librational motion in a solid matrix. The developed models qualitatively explain all experimental IR spectral features of the matrix-isolated HNgY molecules. A number of complexes between noble-gas hydrides and other molecules have been experimentally and computationally studied. These complexes often involve hydrogen bonds and they show unusual spectral effects, such as large blue shifts of the H–Ng stretching frequencies. The blue shift seems to be the normal effect for the complexes of HNgY molecules. Remarkably, the complexes without hydrogen bonding also show substantial vibrational blue shifts. The H–Ng stretching blue shift presumably originates from the enhanced (HNg)+Y– charge separation upon complex formation. It is possible that the solvation and intermolecular interaction of the HNgY molecules can stabilize them in condense phases at normal (not cryogenic) conditions. Preparation of helium and neon compounds can be also discussed in terms of complexation (solvation) induced stabilization. In particular, the complexation-induced stabilization of hypothetical HHeF is computationally studied.