Infrared Multiple Photon Dissociation Spectroscopy Confirms Reversible Water Activation in Mn+(H2O)n, n ≤ 8

Controlled activation of water molecules is the key to efficient water splitting. Hydrated singly charged manganese ions Mn+(H2O)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+(H2O)4 is formed directly in the laser vaporization ion source, while its inserted counterpart HMnOH+(H2O)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+(H2O)3 shows that this cluster ion is metastable. During IRMPD, the system rearranges back to the noninserted Mn+(H2O)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.