Dimetal hydride generated from dimetalated glycine: Formation mechanism and electronic structure

Collision-induced dissociation (CID) of [MaMb(Gly-H)]+ precursors (where Ma = Ag, Cu; Mb = Ag, Cu) results in formation of novel dimetal hydrides, [MaHMb]+ and elimination of MaH in the gas phase. Deuterium labeling studies reveal that the hydride only originates from the CH2 group. Energy-resolved CID of [Ag2(Gly-H)]+ shows that the formation of [Ag2H]+ and the elimination of AgH are two primary dissociation channels, and the latter is always more prevalent than the former. H2 loss, only a minor channel, was also observed. This was confirmed by the correlation of the relative abundance of dimetal hydride with the difference of silver ion affinity between the neutral (Gly-2H) and AgH. Dimetal hydrides are of particular interest due to the three-center two-electron (3c-2e) bond. DFT calculations predict that [Cu2H]+ and [CuAgH]+ have bent structures, while [Ag2H]+ has linear structures. However, bending deformation energies are only 2.6 kcal/mol for [Cu2H]+. Population analysis and atom in molecule (AIM) calculations show that in [Cu2H]+ the metal-metal bond is present, while for [Ag2H]+ there is no metal-metal bond. Energies from high-level theory calculations provide direct evidence to explain why the dimetal radical cation cannot be observed in the CID of the dimetal hydride.