Studies of alkaline earth and transition metal M++ gas phase ion chemistry

Abstract

A breakthrough into the hitherto inaccessible alkaline earth and transition metal M⁺⁺ gas phase ion chemistry is reported. Ions M⁺⁺(L)_n, where M⁺⁺(Mg⁺⁺, Ca⁺⁺, Sr⁺⁺, Ba⁺⁺, Mn⁺⁺, Fe⁺⁺, Co⁺⁺, Ni⁺⁺, and Zn⁺⁺) and L=H₂O could be produced. The hydrate equilibria M⁺⁺(H₂O)_n−1+H₂O=M⁺⁺(H₂O)_n (n−1, n), were determined for Mg⁺⁺, Ca⁺⁺, Sr⁺⁺, Mn⁺⁺, and Co⁺⁺. These lead to successive ion–H₂O binding energies for high n, i.e., n=8–13 which are in the 15 kcal/mol range. The above hydrates and many other ion–ligand complexes could be produced by transferring the ions from liquid solution into the gas phase by means of electrospray. The ions were detected with a triple quadrupole mass spectrometer. The much stronger inner shell ion–ligand interactions can be studied by collision‐induced dissociation in the triple quadrupole. Single ligand loss gives way to charge reduction at low n. Thus the M⁺⁺(H₂O)_n give MOH⁺(H₂O)_k+H₃O⁺ at a low n. The n for which reduction occurs decreases as the second ionization energy of M decreases. Ligands such as DMSO and DMF lead to charge reduction at a lower n than that observed for H₂O. For these ligands, the charge reduction occurs via simple charge transfer, i.e., Cu⁺⁺(DMSO)₃=Cu⁺(DMSO)₂+DMSO⁺.