Associate Professor Doreen Leopold

This research focuses on the chemical bonding between transition metal atoms in dimers, trimers, and small clusters, and their reactivities with gas phase organic molecules. Experiments in this lab use anion photoelectron spectroscopy, flow tube ion-molecule chemistry, and mass spectrometry to study neutral clusters and the corresponding singly-charged negative ions. To help assign the photoelectron spectra, spectroscopic results are compared with the predictions of density functional calculations. 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 the experimental spectra to the simulations for possible electronic transitions and (for polyatomics) different isomers, 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 , 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 paper on NbMo and NbMo^- in the Journal of Physical Chemistry A, the researched 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. The group's 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 the 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.