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.