Non-covalent interactions between metal cations and molecular hydrogen: spectroscopic studies of M+–H2 complexes

Abstract

We describe recent infrared spectroscopic investigations of charged complexes consisting of a hydrogen or deuterium molecule attached to an atomic metal cation, including complexes containing alkali metals (Li+–H2, Li+−D2, Na+–H2, Na+–D2), alkaline earth metals (Mg+–H2, Mg+–D2), main group metals and metalloids (B+–H2, B+–D2, Al+–H2) and transition metals (Cr+–D2, Mn+–H2, Zn+–D2 and Ag+–H2). These species have been probed spectroscopically through their infrared absorptions in the H–H or D–D stretch regions (3700–4160 cm−1 and 2700–3000 cm−1 ranges, respectively) by monitoring metal cation photofragments as the infrared wavelength is scanned. The spectra, which typically display resolved rotational structure, provide fundamental information on the M++H2 interaction, and can be used to develop and test intermolecular potential energy surfaces. The extended data-set for M+–H2 and M+–D2 complexes allows systematic explorations of the way in which the size and electronic structure of the metal cation influences the properties of the intermolecular M+ H2 bond. Aside from their fundamental interest, spectroscopic and theoretical investigations of M+–H2 complexes provide insights into the intermolecular forces underlying hydrogen storage in porous materials containing metal cations, including zeolites and metal organic frameworks.