Marsha Lester of the University of Pennsylvania is supported by the Experimental Physical Chemistry Program to carry out a series of fundamental experimental studies of chemical reaction dynamics, many of which are based on chemical systems of significance in the atmosphere. The experiments utilize state-of-the-art spectroscopic and dynamical methods, but with the unique approach of initiating the studies from stabilized intermediates that lie along the reaction coordinate. Specifically, the planned studies focus on hydrogen trioxide, Cl-HCl, methyl trioxy, peroxymethyl nitrite, and HO-nitric acid intermediates. In each of these studies, fundamental or overtone excitation of a hydride stretch of the intermediate will provide sufficient energy to initiate dynamics on a reactive potential energy surface. Experimental observables include spectroscopic parameters that characterize the reaction intermediates in high detail, for example molecular structures, vibrational frequencies, and thresholds for dissociation and/or reaction. In addition, dynamical information will be obtained on intramolecular vibrational energy redistribution, dissociation and/or reaction dynamics, bond energies, and and enthalpies of formation of the intermediates. Experimental outcomes will be compared with theoretical calculations of the analogous properties derived from ab initio potential energy surfaces.
In this project, young scientists at the undergraduate, graduate, and postdoctoral levels will participate in forefront scientific research, developing skills, experience, and confidence required to move into the scientific and technical workforce. The participants -- many of whom are women -- use technologically advanced laser equipment as they work on basic research projects, which are also relevant to broader issues in atmospheric chemistry.
The HOOO radical has long been postulated to be an important intermediate in atmospherically relevant reactions and could potentially be a significant sink for OH radicals in the lower atmosphere. There has also been much debate in the literature as to whether HOOO is stable or metastable with respect to the OH + O2 limit, as well as the relative stability of its cis and trans conformers. With NSF funding, this laboratory has utilized infrared action spectroscopy to characterize the geometric structure, vibrational frequencies, and stability of the cis and trans conformers of HOOO and its deuterated analog. In particular, by measuring the OH product state distribution following IR excitation of HOOO, this group directly determined a rigorous upper limit for the stability of trans-HOOO (D0 ≤ 5.3 kcal mol-1). As a result, one predicts that HOOO may act as temporary sink for OH radicals and may be present in measurable concentrations in the Earth’s atmosphere. Subsequent studies derived a torsional potential from the spectroscopic data to obtain the relative stability of the cis and trans conformers and isomerization barrier, which are critical for atmospheric modeling of HOOO. The OH + HONO2 reaction also plays an important role in the chemistry of the lower atmosphere by converting nitric acid into highly reactive NOx species. Again, using a direct spectroscopic approach, infrared action spectroscopy, this laboratory made the first experimental determination of the structure and stability of the doubly hydrogen-bonded OH-HONO2 intermediate in this reaction. The identification of this intermediate is an important experimental validation of key steps in the mechanism, which had been only inferred previously from its unusual kinetic behavior and theoretical predictions. The NSF-sponsored research in this laboratory has provided opportunities for young scientists at the undergraduate, graduate, and postdoctoral level to participate in forefront scientific research, as they develop the skills, experience, and confidence required to move into the scientific and technical workforce. The participants – many of whom are women – use technologically advanced laser equipment as they work on basic research projects with relevance to atmospheric chemistry. The results of the research are disseminated at local, national and international scientific meetings, and seminars at research universities and undergraduate institutions.
