Infrared Multiple Photon Dissociation Spectroscopy Confirms Reversible Water Activation in Mn + (H 2 O) n , n ≤ 8.

Controlled activation of water molecules is the key to efficient water splitting. Hydrated singly charged manganese ions Mn + (H 2 O) n exhibit a size-dependent insertion reaction, which is probed by infrared multiple photon dissociation spectroscopy (IRMPD) and FT-ICR mass spectrometry. The noninserted isomer of Mn + (H 2 O) 4 is formed directly in the laser vaporization ion source, while its inserted counterpart HMnOH + (H 2 O) 3 is selectively prepared by gentle removal of water molecules from larger clusters. The IRMPD spectra in the O-H stretch region of both systems are markedly different, and correlate very well with quantum chemical calculations of the respective species at the CCSD(T)/aug-cc-pVDZ//BHandHLYP/aug-cc-pVDZ level of theory. The calculated potential energy surface for water loss from HMnOH + (H 2 O) 3 shows that this cluster ion is metastable. During IRMPD, the system rearranges back to the noninserted Mn + (H 2 O) 3 structure, indicating that the inserted structure requires stabilization by hydration. The studied system serves as an atomically defined single-atom redox-center for reversible metal insertion into the O-H bond, a key step in metal-centered water activation.