Zero-Point-Energy Driven Isotopic Exchange of the [H 3 O] - anion Probed by Mid-Infrared Action Spectroscopy.

We present the first observation of vibrational transitions in the [H 3 O] - anion, an intermediate in the anion-molecule reaction of water, H 2 O, and hydride, H - , using a laser-induced isotopic H/D exchange reaction action spectroscopy scheme applied to anions. The observed bands are assigned as the fundamental and first overtone of the H 2 O-H - vibrational stretching mode, based on anharmonic calculations within the vibrational perturbation theory and vibrational configuration interaction. Although the D 2 O·D - species has the lowest energy, our experiments confirm the D 2 O·H - isotope to be a sink of the H/D exchange reaction. Ab initio calculations corroborate that the formation of D 2 O·H - is favored, as the zero-point-energy difference is larger between D 2 and H 2 than between D 2 O·H - and D 2 O·D - .