Protonation and proton transfer studied with cluster-ion infrared spectroscopy

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In this award, funded by the Chemical Structure, Dynamics and Mechanisms Program of the Chemistry Division, and the Office of International Science and Engineering, Professor Duncan frm the University of Georgia will study protonated molecular ion clusters produced in the gas phase as model systems for proton binding and proton transport processes. These processes are central in electrochemistry, in photosynthesis and in the operation of hydrogen fuel cells. The structures of protons attached to key molecules and the bridging structures that are intermediates in proton transfer are the focus of this project. Complexes containing a proton bound to one or more small molecules are produced at low temperature in a molecular beam environment with a pulsed-discharge supersonic nozzle source, so that the intrinsic structures of these systems unperturbed by their environment may be studied. Protonated complexes are size-selected with a mass spectrometer and their structures are investigated with infrared laser spectroscopy. This work focuses on small molecular carbonyls, amino acids and carbocation species, investigating the monomer structures resulting from protonation, their symmetric and asymmetric proton-bound dimer units, and their behavior upon stepwise solvation with water. Using the expanded frequency coverage of new infrared lasers, characteristic vibrational patterns are measured for each molecular unit present, including the critical low frequency region of the spectrum where proton vibrations occur. The shift in vibrational bands for protonated versus free molecules provides a diagnostic for charge-induction effects, while the proton stretching and bending vibrations provide direct access to the proton-transfer potential energy surface. These experiments provide specific new chemical insights about selected proton transfer reactions. They also produce benchmark data for comparison to the results of computational quantum chemistry which attempts to calculate proton transfer dynamics. Vibrational signatures measured for protonated organic molecules known as carbocations may be useful for infrared astronomy of interstellar gas clouds. Equally important is the training of undergraduates, graduate students and postdoctoral fellows in the methods, techniques and instrumentation of modern physical chemistry. Ongoing collaborations include a non-Ph.D. granting institution (Kennesaw State University) and two international labs (Chemistry Dept., Nottingham, UK and Fritz Haber Institute, Berlin).