In this project, funded by the Experimental Physical Chemistry Program of the Chemistry Division, Prof. Helen O. Leung and Prof. Mark D. Marshall of Amherst College and their undergraduate research students will continue spectroscopic studies of gas-phase van der Waals complexes formed in a pulsed jet. These experiments include studies of fluoro- and chlorofluoroethylenes complexed with protic acids in order to determine the effect of charge distribution changes on the free-space geometries of the resulting complexes. A laser ablation source will also be used to create complexes of metal atoms that can not be produced via the usual helium-nanodroplet method, in order to explore the role of charge transfer between the metal atom and its partner. The PIs will also construct a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer that will allow complete rotational spectra to be acquired in a small fraction of the time taken by standard FTMW spectrometers, thereby increasing the productivity of the undergraduate researchers.
Besides the potential broader scientific impact of the proposed research on a fundamental understanding of molecular interactions, Prof. Leung and Prof. Marshall will provide their undergraduate research students with a challenging opportunity to perform difficult research, the results of which they will present at national professional meetings and publish in the scientific literature.
Since the early days of modern organic chemistry and continuing to the present, the addition of electron withdrawing or electron donating groups to hydrocarbons has been recognized to influence both the site of reaction and the identity of the products in chemical processes. Through this NSF-supported research, the effect of electron withdrawing groups on the nature of the first encounters between molecules is examined by investigating weakly associated complexes formed between two molecules. Specifically, structures are determined for a series of complexes formed between the simplest unsaturated hydrocarbon, ethylene, in which one or more hydrogen atoms has been replaced with either a fluorine or chlorine atom on the one hand and a small protic acid, such as HF or HCCH, on the other. This works shows that these structures can be related to the elementary chemical ideas of comparative nucleophilicity and electrophilicity, and that it is possible to separate the influence of the attraction of oppositely charged portions of the two molecules upon each other (electrostatic effects) from the geometric "fit" of one molecule with the other (steric effects). Additionally, the characterization of complexes formed between various halogenated ethylenes and the argon atom, obtained simultaneously with the protic acid species, allows the electronic distribution of the ethylene away from its molecular plane to be probed. With NSF support, two new experimental techniques were added to the laboratory. A laser ablation source allows the formation of complexes containing metal atoms that in turn will provide basic chemical information regarding the interaction of molecules with metal surfaces and investigation of processes involving heterogeneous catalysis. This source has already been used to characterize the small coupling between nuclear and electronic motion in the tin sulfide (SnS) molecule. More significantly, a chirped-pulse Fourier transform microwave spectrometer has been constructed. Using state-of-the art electronic technology for high speed generation and capture of microwave signals, this instrument has greatly enhanced the capabilities of the laboratory, has provided greater efficiency in collecting data, and has led to collaborations with researchers at Oxford University (UK) and the University of Minnesota. This NSF grant has provided 20 separate research opportunities to 16 undergraduate students. Through active research, these students become familiar and comfortable with the increasingly complex technological world. The integration of theory and practice in a research lab is the essence of science and must be experienced first-hand by a student. In addition to the specific skills learned in microwave spectroscopy and in quantum chemistry calculations, participation in research provides training in critical thinking and allows students to explore their options for a career in science. The research experience is especially valuable during the students’ formative years as young scientists, as they learn to integrate and apply material learned in the classroom and through textbooks. The hands-on approach of a realistic, challenging, and relevant research project using multi-component, state-of-the-art equipment is a necessary part of a complete education. With NSF support, many of these students have attended the International Symposium on Molecular Spectroscopy at the Ohio State University where some have had the additional opportunity to present their results in a contributed talk. Connecting in this manner with the greater scientific community provides further exposure for their work while allowing them to place it in a larger context as they broaden their knowledge. In addition to oral and poster presentations at this and other local, regional, and national meetings, results are published in peer-reviewed journals. Student work is also displayed prominently on campus where it serves to further encourage involvement with research. The opportunities afforded by this grant raise the scientific literacy and sharpen the thoughts of our students as we prepare future scientists and leaders.
