Co-existence of hydrated electron and metal di-cation in [Mg(H2O)n]+Dedicated to Professor F. Dörr on the occasion of his 80th birthday.

DFT/BLYP based investigations of hydrated magnesium mono-cations [Mg(H2O)n]+, 1 ≤ n ≤ 19, reveal an electronic structure of [Mg(H2O)n]+ that evolves from a valence state in Mg+(H2O)n, n ≤ 5, via a contact ion pair state Mg2+(H2O)n−, 6 ≤ n < 17, to a solvent separated ion pair state Mg2+(H2O)ne⊖ for n ≥ 17. In [Mg(H2O)n]+, n ≥ 9, O–H bonds of adjacent H2O ligands form “molecular tweezers” HO–H⋯e⊖⋯H–OH, which induce a localization of the odd electron within the hydration sphere. The shape of the singly occupied molecular orbitals (SOMOs) and their position within the clusters indicate the co-existence of a hydrated electron and of a magnesium di-cation in [Mg(H2O)n]+, n ≥ 8. The now calculated energetics of previously observed competing decay channels — reactive hydroxide formation versus H2O monomer evaporation — predict the evaporation process to be favored for [Mg(H2O)n]+, 6 ≥ n ≥ 17, in very nice agreement with experiment. The switchings in the decay propensities are analysed on the basis of the computed electronic and geometrical structures of product and reactant clusters.