Andrew L. Cooksy
Professor; Physical Chemistry.
(image by C. Cheng, instr. Spring 2006 CS689)
Reactive intermediates in combustion, interstellar chemistry, chemical synthesis, and biochemistry; investigated by laser spectroscopy and spectroscopic theory, and by computational quantum mechanics.
Molecular free radicals are crucial to the chemistry of combustion, the upper atmosphere, polymerization, and interstellar molecular clouds, and also figure in many biochemical electron transfer processes. We are interested in the physical and chemical properties of these molecules, particularly those containing conjugated π-electron systems, such as HC3O and C4H and the biochemical quinones, because the delocalized orbitals can confer surprising dynamic and reactive properties to these systems.
In our experimental work, we search for new spectra of small hydrocarbon or other first-row element free radicals in the visible or infrared regions of the spectrum in order to characterize these dynamic properties. Our mid-infrared diode laser spectrometer operates between 1800 and 2400 cm-1, and uses a 2-meter electric discharge cell as the sample chamber. With this system we search for strong stretching transitions in the free radicals, and then probe the isomerization coordinate by examining hot band and combination band spectra to obtain measurements at high resolution of the interesting vibrational dynamics of these molecules. These studies were originally supported by one of the first NSF CAREER awards, and is currently funded by the Army Research Office. Postdoc Erich Wolf is heading up this project, assisted by undergraduates Alex Colla, Thuy-Tien Pham, and Michael Baudé.
We are engaged in concurrent ab initio computational studies of these and larger molecules to investigate the relative stability of the competing structures, and their effect on the chemistry. These ab initio calculations guide the laboratory measurements of the energy level structure, geometry, and chemistry of these molecules, as well as offering information for the kinetic models of the highly complex chemical environments found in combustion and interstellar space.
Previous work in our group along these lines included studies of the mechanisms behind the elctrocyclic ring-closure of cyclopentadienyl radical (C5H5) and the vibrational dynamics of cyclooctatetraenyl (C8H7), including its effective isomerization from one structure to another under specific vibrational excitations. We are presently applying similar approaches to problems in computational biochemistry, elucidating the reaction mechanisms underlying vitamin E regeneration and enzyme-mediated metabolism of small compounds.
Recognizing the need for a general, easily mastered way to study these complex vibrational dynamics, we've recently published a protocol for the integration of the vibrational Schrodinger equation on an arbitrary potential energy surface. This work formed the doctoral work of Dong Xu, one of SDSU's first two PhD students in Computational Sciences, and now an assistant professor at Boise State University. PhD candidate Peter Zajac is extending that work. Sabbatical visitor Dra. Guadalupe Moreno has joined our group for 2011, to carry out electronic structure studies of high-spin radicals, which we can then analyze by our methods.
Current work with Prof. Doug Grotjahn strives to understand the activity of organometallic catalysts synthesized in his lab. To model the transformation of a π-complexed alkyne into vinylidene on one of these catalysts, we mapped the reaction surface of the complex in two dimensions, finding an unexpected parallel in our previous work on multiple minima on vibrational surfaces of free radicals. We are in the midst of an exhaustive analysis of a system that catalyzes the formation of aldehyde from alkyne and water, finding that Grotjahn's signature heterocyclic ligands play a major role in the dynamics by providing a basic chemical environment to stabilize the relocation of hydrogen atoms. Undergraduate Amy Arita has been hard at work on this project, which is funded by the National Science Foundation.
The complex interactions among the spin and orbital magnetic fields of unpaired electrons and the rotational, vibrational, and nuclear angular momenta in these molecules also means that we sometimes work at the limits of present spectroscopic theory. Work from a sabbatical with Prof. John M. Brown at Oxford University involved the first combined analysis of the lowest vibrational bending states in the NCO radical. NCO is a prototype example of the Renner-Teller effect, in this case the strong interaction between the two electronic states formed when the Π state symmetry of the linear is broken upon bending. To complete this analysis, we used third-order perturbation theory to derive additional contributions to the effective Hamiltonian, which is still growing after 70 years.
We gratefully acknowledge funding for past and current work from the Army Research Office, the National Science Foundation, the Petroleum Research Fund of the American Chemical Society, the Exxon Education Foundation, and the San Diego Foundation.