Department of Chemistry
Associate Professor of Chemistry
Ph.D., Yale University, 1989
We use mass spectrometry and laser spectroscopy to study molecular-level interactions between multiply-charged transition metal ions and neutral solvent molecules in gas-phase clusters. Gas-phase clusters consisting of an ioncore surrounded by neutral atoms or molecules behaving as solvent are ideally suited to dissecting solution phenomena because they allow us to eliminate bulk solvent effects, which are inherent in condensed phase systems and often obscure local ion-solvent interactions. We generate clusters containing multiply-charged transition metal ions by electrospray ionization, a process which transfers ions to the gas phase with a portion of the solvent shell intact. Mass spectrometry allows us to isolate specific gas-phase "microsolution" environments defined by the number of solvent molecules, which are subsequently probed by monitoring laser-induced fragmentation. Laser photofragmentation spectra are a first step in understanding how the direct interaction between transition metal ions and solvent influence chemical reactivity and dynamics.
My group is currently investigating the influence of sequential solvation on the energetics of photoinitiated charge transfer in transition metal ion complexes. Theory to describe charge transfer in the condensed phase generally treats the solvent as a dielectric continuum, neglecting the influence of molecules in the first solvent shell on this process. Our work using gas-phase clusters provides a first glimpse at the role of individual solvent molecules. Systems currently under study include Ru(II) and Fe(II) polypyridine complexes clustered with a variety of organic solvents. We have observed the metal-to-ligand charge transfer transition for these complexes in gas-phase clusters (Figure 1), demonstrating that electrospray ionization does indeed prepare analogs of solution species with the transition metal ion oxidation state maintained. We are also interested in probing solvent effects on photoinitiated intervalence electron transfer in mixed valence transition metal compounds. Mixed valence metal complexes embedded in proteins act as redox catalysts, and some mixed valence compounds have interesting nonlinear optical properties. In a second project, we are exploring the spectroscopy of probe ligands, such as s-tetrazine and pyridazine, clustered with the first row transition metal ions Fe (II), Co (II), and Ni (II) (Figure 2). The electronic absorption spectra of these neutral probe molecules reveal the electronic and structural perturbations which accompany coordination or electrostatic binding of neutral molecules to a doubly-charged transition metal ion. The evolution of ligand spectra as the environment surrounding the transition metal is constructed by stepwise addition of ligands provides a molecular perspective on the fundamental changes which accompany transfer of a transition metal ion from the gas phase to condensed phase.
Cyr, D. M.; Posey, L. A.; Bishea, G. A.; Han, C.-C.; Johnson, M. A. J. Am. Chem. Soc. 1991, 113, 9697-9699. "Collisional Activation of Captured Intermediates in the Gas Phase SN2 Reaction Cl- + CH3Br * Br- + CH3Cl."
Posey, L. A.; Guettler, R. D.; Zare, R. N. J. Chem. Phys. 1994, 101, 3772-3786. "Influence of Vibrational Excitation and Collision Energy on the Ion-Molecule Reaction NH3+(*2) + ND3."
Burns, T. D.; Spence, T. G.; Mooney, M. A.; Posey, L. A. Chem. Phys. Lett. 1996, 258, 669?679. "Electrospray Ionization of Divalent Transition Metal Ion Polypyridine Complexes: Spectroscopic Evidence for Preparation of Solution Analogs in the Gas Phase."
Spence, T. G. ; Burns, T. D.; Posey, L. A. Submitted to J. Phys. Chem. "Controlled Synthesis of Transition Metal Ion Complex/Solvent Clusters by Electrospray Ionization."
Spence, T. G. ; Burns, T. D.; Guckenberger, G. B. ; Posey, L. A. Submitted to J. Phys. Chem. "Wavelength-Dependent Photodissociation of [Fe(bipyridyl)3]2+*(CH3OH)n Clusters, n=2-6, Triggered by Excitation of the Metal-to-Ligand Charge Transfer Transition."
|Figure 1. Percent depletion of [Fe(terpyridyl)2* (solvent)4]2+ clusters normalized to 1 W laser power plotted as a function of photon energy for the solvents DMSO = dimethylsulfoxide, DMF = dimethylformamide, acetone, and MeOH = methanol. The smooth curves are Gaussian fits to the experimental data points for each solvent.||Figure 2. Electrospray ionization mass spectrum of 1.5x10-4 M FeCl2 in methanol containing 5% pyridazine (pyrd) by volume.|
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