Reaction mechanisms for size-dependent H loss in Mg+(H2O)n: solvation controlled electron transfer

Abstract

Mg⁺(H₂O)_n clusters ions are known to undergo a hydrogen loss reaction to produce (MgOH)⁺(H₂O)_n−1, when the cluster size n falls between 5 and 15. It indicates that the microscopic solvation environment has a strong effect on the reaction mechanisms, which have now been elucidated. Above n = 6, Mg⁺(H₂O)_n exists in the form of an ion pair, Mg²⁺(H₂O)_l⋯e⁻(H₂O)_n−l, and H⁺ is produced by the autoionization of H₂O, enhanced by the hydrolysis of Mg²⁺. Production of the H atom is due to a reductive half reaction H⁺ + e → H, with the electron being the solvated electron in the ion pair. The reaction barrier is reduced when the electron is in the first or second solvation shell, promoting the H loss reaction. As the number of H₂O molecules increases, the electron moves beyond the third solvation shell and the reaction barrier increases significantly, which is responsible for the subsequent switch-off of H loss. It has been one of the important goals in cluster sciences to understand the microscopic correlation between solvation and chemical reactivity by in-depth study on clusters, which has now been achieved in the case of Mg⁺(H₂O)_n, linking electron solvation with reaction paths for electron transfer.