Spectroscopic study of the benchmark Mn+-H2 complex

We have recorded the rotationally resolved infrared spectrum of the weakly bound Mn--H2 complex in the H-H stretch region (4022-4078 cm-1) by monitoring Mn- photodissociation products. The band center of Mn+-H2, the H-H stretch transition, is shifted by -111.8 cm-1 from the transition of the free H2 molecule. The spectroscopic data suggest that the Mn--H2 complex consists of a slightly perturbed H2 molecule attached to the Mn+ ion in a T-shaped configuration with a vibrationally averaged intermolecular separation of 2.73 ̊. Together with the measured Mn +⋯H2 binding energy of 7.9 kJ/mol (Weis, P.; et al. J. Phys. Chem. A 1997, 101, 2809.), the spectroscopic parameters establish Mn+⋯2 as the most thoroughly characterized transition-metal cation-dihydrogen complex and a benchmark for calibrating quantum chemical calculations on noncovalent systems involving open d-shell configurations. Such systems are of possible importance for hydrogen storage applications.