Initial hydration processes of magnesium chloride: size-selected anion photoelectron spectroscopy and ab initio calculations

Abstract

To understand the initial hydration processes of MgCl₂, we measured photoelectron spectra of MgCl₂(H₂O)_n^- (n = 0-6) and conducted ab initio calculations on MgCl₂(H₂O)_n^- and their neutral counterparts up to n = 7. A dramatic drop in the vertical detachment energy (VDE) was observed upon addition of the first water molecule to bare MgCl₂^-. This large variation in VDE can be associated with the charge-transfer-to-solvent (CTTS) effect occurring in the MgCl₂(H₂O)_n^- clusters, as hydration induces transfer of the excess electron of MgCl₂^- to the water molecules. Investigation of the separation of Cl^--Mg²⁺ ion pair shows that, in MgCl₂(H₂O)_n^- anions, breaking of the first Mg-Cl bond occurs at n = 4, while breaking of the second Mg-Cl bond takes place at n = 6. For neutral MgCl₂(H₂O)_n clusters, breaking of the first Mg-Cl bond starts at n = 7.