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

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