Dr. Armentrout's research is intended to provide a detailed understanding of the thermochemistry, kinetics, and dynamics of simple and complex chemical reactions. Of particular interest is elucidation of the intrinsic reactivity and thermochemistry of transition metals, metal-ligand complexes, metal-clusters, solvated ions, metallated macrocycles and metallated molecules of biological and environmental relevance. His group seeks to understand from a fundamental viewpoint reactions involved in catalysis, surface chemistry, organometallic chemistry, and plasma chemistry. Measurement of integral cross sections, product branching ratios, and the dependence of these quantities on the translational, internal, and electronic energies of the reactants provide a uniquely detailed look at the chemistry of interest. Spectroscopic experiments augment these studies by providing even more microscopic information. Techniques involved include mass spectrometry, ion beams, molecular beams, and laser spectroscopy. Specific areas of interest include :
Chemistry of state-selected atomic metal ions
Transition metals have an abundance of low-lying electronic states due to the near degeneracy of s and d orbitals. We have shown that different electronic states can have very different reactivity and that studies of these effects lend considerable insight into metal chemistry. Previous studies have involved mostly first row transition metal ions, however, we are developing a novel laser-based source that should permit such studies to be extended to second and third row transition metals where spin-orbit interactions become much more important.
Chemistry of unsaturated organometallic complexes
In order to understand condensed phase organometallic chemistry and homogeneous catalysis, systematic studies of the reactivity of ligated metals are underway. By varying the number and types of ligands in these complexes, we can study the periodic trends, the influence of ligand substitution, and the effects of metal oxidation state on the chosen reaction. Such studies hold promise of providing quantitative thermodynamic information and qualitative electronic data on reactive and transient unsaturated organometallic complexes, the key intermediates in homogeneous catalysis.
Thermochemistry of metal ions interacting with biological molecules
We are now in a position to study the interactions of metals ions with molecules of biological relevance. Early studies include some of the first quantitative measurements of the binding energies of Li+, Na+, and K+ with the DNA bases and amino acids.
In a collaborative study with theoreticians and experimentalists at the Pacific Northwest National Laboratories, we are conducting studies to examine the thermochemistry and dissociation dynamics of host-guest complexes of interest in environmental clean up. We seek to answer a key question with respect to engineering ion-selective ligands: namely, how to arrange donor atoms to interact favorably with a selected target ion and at the same time unfavorably with other ions. Previous work has concentrated on exploring the interactions of alkali metal ions with crown ethers and related molecules. Plans for continued work expand the range of metals and ligands under consideration.
Chemistry of metal cluster ions
By using laser vaporization, supersonic expansion techniques, we are able to generate cold transition metal cluster ions. Mass spectrometry then allows the size-specific reactivity of these clusters to be investigated easily. Our experiments enable us to measure the thermodynamic stabilities of these clusters and their reactivity with a variety of molecules. In essence, these studies constitute a gas-phase approach to providing quantitative data conerning surface chemistry and heterogeneous catalysis.
Chemistry of solvated ions
Gas phase solvated ions are important species in the atmosphere and in aerosols. Further, solvated ions can provide a bridge between phenomena in condensed phases and the gas phase. By performing detailed experiments on such species, we hope to obtain quantitative information that cannot be obtained easily in condensed phase media.
In collaboration with Prof. M. D. Morse, we are using resonantly enhanced multi-photon (REMP) laser spectroscopy coupled with pulsed field ionization (PFI/ZEKE) spectroscopy to study small transition metal cluster ions and ligated metal ions. Such work should provide detailed molecular information about the neutrals and ions.
Threshold behavior and non-adiabatic effects: theory and experiments
Much of the information gleaned from our work relies on accurately measuring the kinetic energy dependence of a reaction. Theoretical understanding of the variation in cross sections with energy is not well established. We are interested in developing theoretical models for such behavior by careful experimental characterization of the effects of translational, rotational, and vibrational energy on reactions, with a recent emphasis on collision-induced dissociation. The analysis includes detailed application of statistical phase space theories, reaction dynamics, and consideration of non-adiabatic effects.