Ab initio study of [Mg,nH2O]− reactive decay products: structure and stability of magnesium oxide and magnesium hydroxidewater cluster anions [MgO,(n − 1)H2O]−, [HMgOH,(n − 1)H2O]− and [Mg(OH)2,(n − 2)H2O]−

Abstract

Geometrical as well as electronic structures of hydrated magnesium oxide and hydroxide anions with the stoichiometry [MgO,(n − 1)H₂O]⁻, [HMgOH,(n − 1)H₂O]⁻ and [Mg(OH)₂,(n − 2)H₂O]⁻ are investigated by ab initio calculations at the MP2 level of theory. Different structural archetypes emerge and the excess electron localization modes within them are elucidated. The oxidation state of the central Mg atom is found to correlate with the coordination number, which exhibits a strong influence on the localization mode of the excess electron. Only in [HMgOH,(n − 1)H₂O]⁻, n = 1–2, and [Mg(OH)₂,(n − 2)H₂O]⁻ , n = 2–3, a valence bound state of the excess electron is favored, whereas for all larger cluster sizes the excess electron resides within the hydration shell and is localized by dangling O–H bonds. These act as molecular tweezers which trap the hydrated electron. The investigated magnesium oxide and hydroxide water cluster anions are discussed as potential products of a reactive decay of anionic magnesium water clusters [Mg,nH₂O]⁻, elucidating the chemical reactivity of these species. The minimum energy pathway of the O–H insertion by Mg in [Mg,nH₂O]⁻ is investigated in detail.