Infrared Spectroscopy of the Ag⁺−H₂ Complex: Exploring the Connection Between Vibrational Band-Shifts and Binding Energies

Abstract

Storage of molecular hydrogen in solid media often depends on the interaction between metal cations and H2. Under these circumstances, the strength of the interaction can conveniently be assessed from the frequency of the H2 vibration. Here we investigate the gas-phase Ag+−H2 complex, focusing on the relationship between its binding energy and the H2 subunit’s stretch frequency. The infrared spectrum of Ag+−H2 is recorded in the H−H stretch region by detecting Ag+ photofragments resulting from resonant excitation. Analysis of the partially rotationally resolved spectrum, taking upper state perturbations into account, confirms a T-shaped equilibrium geometry and an intermolecular separation of 2.02 Å. The vibrational band center is shifted by −405.7 ± 1.5 cm−1 relative to the free H2 molecule. A new trend connecting the H−H band shift and H2 binding energy is identified for metal cation−H2 complexes containing filled or half-filled valence d orbitals.