College of Science & Engineering
Research in this group focuses on the chemical bonding between transition metals in dimers, trimers, and small clusters, and their reactivities with gas phase molecules. Experiments in this laboratory employ anion photoelectron spectroscopy, flow tube ion-molecule chemistry, and mass spectrometry to study negative ions and neutral molecules. To help assign the photoelectron spectra, the researchers compare the experimental results with those calculated using density functional (DFT) methods. The DFT studies can predict equilibrium geometries, vibrational frequencies, electronic state energies, and spin multiplicities of possible candidates for the ground and low lying excited states of the neutral and anionic systems. By simulating the Franck-Condon photodetachment spectra based upon the DFT results and directly comparing those predictions for possible electronic states and (for polyatomics) different isomers to the experimental spectra, it is often possible to assign the observed states, even for molecules and anions that have not previously been studied.
Current work includes studies of ligand-free diatomics and triatomics from Groups 5 and/or 6, which can exhibit very high multiple bond orders. For example, the researchers are investigating the Group 6 first transition series trimer of chromium, Cr3 and Cr3-, and the third transition series dimer of tungsten, W2 and W2-, as well as bimetallic dimers incorporating metals from both Groups 5 and 6, such as NbCr and NbW and their anions. Neutral W2 has been shown by high-level computational studies to have a bond order of 6, the highest value possible for homonuclear transition metal diatomics. In a recent paper on NbMo and NbMo-, published in the Journal of Physical Chemistry A, the researchers presented experimental evidence for a sextuple bond in this heteronuclear anion as well, and for a bond order of 5-1/2 in the neutral molecule. Their research also includes studies of organometallic complexes produced upon reaction of transition metals with simple hydrocarbons, such as ethylene, butadiene, and benzene. These results can enrich understanding of the relationships between the chemical reactivities of transition metal species and the properties of their ground and low-lying electronic states. In a broader context, they can help elucidate the mechanisms of catalytic processes mediated by transition metal systems. These experimental data can also provide useful benchmarks to aid in the further development, by computational chemists, of improved methods with which to treat the computationally-challenging ground and excited electronic state properties of bare transition metal clusters and coordinatively unsaturated organometallics.