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).
All matter is composed of atoms and molecules, but these fundamental components of nature are far too small to see with microscopes. However, to produce new materials, design new medicines, clean dirty surfaces, or construct microelectronic circuits, we must manipulate molecules. Unfortunately, it is very difficult to manipulate molecules or use them if we don't know what they look like. Because they are invisible, we must use special techniques to figure out what they look like. One such technique is spectroscopy, in which we shine light on molecules and record the pattern of colors that they absorb. Invisible infrared light is used to study the vibrational motions of molecules, in which the bonds connecting their atoms stretch and bend. If we can measure the infrared pattern, or "spectrum," of a molecule, we can use mathematical models from quantum mechanics to determine what the molecule looks like, i.e., its "structure." This determines how it interacts with its environment, whether or not it might dissolve in some liquid solvent, whether it might freeze or decompose at a certain temperature, and whether or not it might undergo a transformation, reacting with another kind of molecule to produce something new. Our research program studies special positively charged molecules known as "ions" that are formed when a proton (H+) is attached to an ordinary molecule. Protonated molecules are important in many chemical processes such as the function of batteries, electrochemistry (used for metal plating), synthetic organic chemistry (used to make new medicines) and in biological processes like photosynthesis, where protons are the main carrier of energy from place to place in plants. We would like to first understand how these processes work at the molecular level, and then we might be able to modify them to do something new and different. To understand ions and how they behave, we have to produce them in enough quantity, isolate them from the enormous excess of other normal molecules, and then design special experiments to be sure that we are measuring the specific properties of the ions. Our research group has been working for over 25 years designing special molecular beam machines to do this. These machines remove air and other impurities from a large steel chamber, inject small amounts of a desired molecule, and then react it with protons to make a protonated ion. We use a special instrument known as a mass spectrometer to detect the ions and to identify them based on their mass. This instrument then allows us to isolate and position the ions in front of a high intensity infrared laser beam. The laser excites the molecules, and we measure their pattern of infrared light absorption. Such infrared spectroscopy measurements on ions are extremely difficult because the number of ions that can be produced is so small. Ordinary molecules exist in such large numbers that the human mind can barely conceive of its magnitude (millions of billions of trillions), but only about a million ions can be produced at a time. Our experiments therefore have to be much more sensitive than normal infrared spectroscopy experiments. Fortunately, our machinery functions very well for this work, and we have been able to obtain the first infrared patterns for a number of small protonated ions, allowing their structures to be determined for the first time. We have also been able to measure the structures of these ions in "ion-molecule complexes," in which the ions attach to a normal neutral molecule such as water just as it would to a solvent or to a potential reaction partner. These are important first steps in the determination of ion structures, but they promise to provide important new understanding of significant chemistry. In addition to more practical matters, we are fascinated by the components of interstellar space, where many ions are expected. Infrared patterns can be measured from stars or interstellar gas clouds with special telescopes that record the light. If the patterns from space can be matched to those seen in the lab, specific ions can be identified in space, revealing what kind of chemistry happens there. Students working in our laboratory learn how to construct and operate some of the most exotic technical machines ever designed in a chemistry lab. They learn how to think about molecules, their motions and reactions, and how to analyze and manipulate them. They learn how to think critically about experiments and to imagine new ways to make and study molecules. These basic skills learned from fundamental science will equip them for unforeseeable challenges in the technology of the future.
