Experimental and theoretical studies on the electronic properties and dissociation reactions of gas-phase metal oxide nitrate complexes

Abstract

This dissertation discusses dissociation studies performed on various gas-phase transition and main group metal nitrate anion complexes using a tandem mass spectrometer. The dissociation of these complexes, via consecutive loss of NO2•, results in the formation of multiple metal-oxygen bonds. The dissertation focuses primarily on the metal oxide products in an attempt to gain a better understanding of the nature of metal-oxygen bonds, with an emphasis on the assignment of oxidation states for the atoms in these species. The dissociation behavior for each system was found to be characteristic to the metal in that system. Specifically, the elimination of NO2• results in abstraction of O•− by the metal and formation of a metal-oxygen bond. The formation of this bond can result in oxidation of the metal, reduction of the metal, or show an absence of redox activity for the metal. The main group metals, such as aluminum, gallium, and the pseudo-main group metal zinc, have valences lower in energy than that of the O•− ligand and do not undergo oxidation or reduction upon metal-oxygen bond formation. The oxygen ligand retains the radical and is the reactive site in those systems, illustrated by the observed high degree of dissociation. The late transition metal, copper(II), has a low-lying 3d vacancy that allows metal reduction upon metal-oxygen bond formation. The copper(I) system, with its full 3d valence shows dissociation behavior identical to that of the main group metals. The nickel(II) cation has a nearly degenerate valence to that of the O•− ligand, resulting in dissociation similar to that of the main group metals. The dissociation patterns are similar because the O•− ligands are the primary reactive sites in the nickel system. Metal reduction can occur upon elimination of atomic and molecular oxygen from some complexes. This metal reduction occurs at relatively low energy due to the low-lying 3d vacancies for nickel. There are two instances of electron transfer that result in partial metal and partial ligand reduction, due to the near degeneracy of the valences for the nickel cation and O•− ligand. The unusual, half-integer values for the oxidation state assignments in these two cases introduce a larger unanswered question of, “What do the oxidation states tell us about these types of metal oxide systems?” Early-to-mid transition metals, such as chromium through cobalt, undergo metal oxidation upon metal-oxygen bond formation. These metals have high energy valence occupancies relative to the valence of the O•− ligand. An electron transfers from the metal 3d subshell to the 2p subshell of the oxygen ligand, reducing it to an O2− ligand. Metal reduction can occur for these systems upon elimination of atomic and molecular oxygen, but it occurs at high energy due to the high energy 3d subshell.