Electronic spectroscopy and dissociation dynamics of gas-phase transition metal containing cations and dications

Abstract

Studies of gas-phase ionic clusters have become an integral component in understanding microsolvation and catalysis by transition metal cations. Further interest in this field is due to the possibility of bridging the gap between the condensed and gas phases by developing our understanding of clusters and the possibility that small clusters can have unique chemical and catalytic properties. Most gas phase studies have focused on singly charged ions. Electrospray allows for the production of multiply charged ions solvated by a few solvent molecules. Understanding smaller reactive species such as metal centered clusters with well-defined, gas phase conditions also allows for detailed comparison between theory and experiments. In these studies the main focus is to understand bond activation by transition metal cations and solvation of transition metal dications. The gas phase ions of interest are studied using an electrospray-ionization or laser-ablation dual time-of-flight mass spectrometer and are characterized using photofragment spectroscopy in the visible and ultraviolet regions of the spectrum. Photofragment spectroscopy is a powerful method that can be used in gas phase studies to gather a wealth of information on the ions’ bond strengths, spectroscopic constants, and dissociation kinetics and dynamics. The study of TiO+(CO2) spectroscopy (Chapter 3) was a result of study of CO2 bond activation by Ti+ that went on to provide a wealth of information on the spectroscopy and dissociation kinetics of this molecule. An electronic transition of the TiO+ chromophore was observed, 2Π←2Δ, revealing new information about the excited state and the effect of TiO + electronic state on the metal-CO2 ligand interaction. The photodissociation spectrum of this molecule is well resolved and shows progressions in the covalent Ti-O stretch and metal-ligand stretch and rock. The lifetime of electronically excited TiO+(CO 2) was measured, and depends strongly on vibrational energy. Calculations on TiO+ and TiO+(CO2) were combined with experimental results on TiO+(CO2) to predict spectroscopic transitions of TiO+, an astrophysically interesting molecule. The photodissociation dynamics of M2+(CH3CN)n(H 2O)m where M = Co and Ni, (Chapter 4) is important in understanding the gas phase microsolvation of metal dications. The coordination number and type of solvent affect the dissociation pathways. M2+(CH 3CN)n (n\u3e2) primarily lose a solvent molecule. Electron transfer is a minor channel for n=3 and is the only channel observed for n=2. Mixed clusters M2+(CH3CN)n(H2O)m preferentially lose water. Loss of acetonitrile is a minor channel, as is proton transfer. Water is the proton donor. Replacing acetonitrile with water increases the proton transfer channel. Nickel and cobalt complexes show similar dissociation dynamics, with proton transfer more likely for nickel complexes. Methane activation by transition metal catalysts is industrially important as it can be used to produce gasoline from natural gas. We studied the products and intermediates of the reaction of laser-ablated platinum atoms with methane (Chapter 5). Photoionization efficiency curves were measured for PtCH 2 and the [H-Pt-CH3] insertion intermediate using tunable vacuum ultraviolet light. The resulting ionization energies were combined with bond strengths for the cations to derive bond strengths for the neutrals. These were used to construct a potential energy surface for methane activation by platinum atoms