Evidence of zinc superoxide formation in the gas phase: Comparisons in behaviour between ligated Zn(I/II) and Cu(I/II) with regard to the attachment of O-2 or H2O

Singly and doubly charged atomic ions of zinc and copper have been complexed with pyridine and held in an ion trap. Complexes involving Zn(II) and Cu(I) (3d(10)) display a strong tendency to bind with H2O, whilst the Zn(I) (3d(10)4s(1)) complexes exhibit a strong preference for the attachment of O-2. DFT calculations show that this latter result can be interpreted as internal oxidation leading to the formation of superoxide complexes, [Zn(II)O-2(-)](pyridine)(n), in the gas phase. The calculations also show that the oxidation of Zn(I) to form Zn(II)O-2(-) is promoted by a mixing of the occupied 4s and vacant 4p orbitals on the metal cation, and that this process is facilitated by the presence of the pyridine ligands.