Dan Gerrity attended Cornell College where he received a B.A. in Chemistry in 1977. From there he went to Harvard University where he earned a PhD in Physical Chemistry in 1982, working in Professor Veronica Vaida’s group. His research focused on the use of short-pulsed lasers to study the electronic structure and reactivity of small molecules. He then did a short (six month) postdoc at the University of Oregon, working with Professor Bruce Hudson, while he waited to get the necessary security clearance required to begin a Director’s Funded Postdoctoral Fellowship at Los Alamos National Lab. At Oregon, he used UV resonance Raman spectroscopy to study the exited electronic states of several small molecules, which included providing experimental evidence for the location and symmetry of the lowest energy forbidden excited state of butadiene. At Los Alamos, he worked with Dr. James Valentini to experimentally measure the product state distribution of one of the simplest bimolecular reactions, H + D2  ➡️  D + HD. These product state distributions provide the best test of quantum calculations of chemical reactions. He also studied the photodissociation of ozone, providing direct evidence for a mass-independent kinetic isotope effect. Dan has been teaching and doing research with undergraduates since 1984, first at Carroll College in Waukesha Wisconsin, and then at Reed College in Portland Oregon. He caught the teaching bug while in high school, when he was allowed to design and teach a science course for grade school students in the inner city of Milwaukee, WI.

Resonance Raman Excitation Profile of Cr(CO)₆ in the Region of the Lowest Energy UV Absorption Band: Support for Assignment to a Ligand Field Transition

Cr(CO)₆ has long been the archetypal compound for the study of transition-metal carbonyls and organometallics. These compounds are of practical interest due to their known catalytic and photochemical properties and also of theoretical interest due to their complex electronic structure. The energy of the lowest ligand field (LF) transition in Cr(CO)₆ has received particular attention in the past due to its importance in determining the extent of π-back-bonding and in understanding the mechanism of Cr(CO)₆ photolysis. Assignment of the lowest energy transition in the absorption spectrum to this LF transition was widely accepted until fairly recently, when it was challenged by the results of high-level quantum calculations. 

Clearly, experimental confirmation of the identity of the lowest energy electronic transition is highly desirable, both as a test of the reliability of our current quantum calculations in determining the relative energies of charge transfer (CT) and ligand field (LF) transitions, and because of the role the first excited singlet state likely plays in metal-ligand photodissociation. But this is an extremely challenging task, since almost all of the electronic transitions in this octahedral molecule are “dipole forbidden,” which means they result in very weak absorbances, and they show little or no vibrational substructure which could help determine the symmetry of each electronic transition.

This seminar presents the work of many Reed students over the past 20+ years. They were able to identify the inversion symmetry of the lowest energy excited electronic state of Cr(CO)₆ by using resonance Raman spectroscopy, a technique that can identify the vibrational promoting modes which allow “forbidden” electronic transitions to appear in the absorption spectrum. The seminar will include a discussion of what the symmetry of these vibrational modes in the Raman spectrum can tell you about the symmetry of the electronic states involved in the underlying forbidden transition.

The experimental results my students obtained do not support the ordering of electronic excited states suggested by the recent theoretical calculations (those calculations indicate that the transitions below the first dipole-allowed transition are all forbidden charge transfer (CT) transitions to excited states of ungerade symmetry). However, their results do support the assignment that had been widely accepted prior to these computational studies: the lowest energy band in the absorption spectrum corresponds to the ligand field (LF) transition, ¹A_1g → ¹T_1g.