Eitan Geva of the University of Michigan is supported by an award from the Theoretical and Computational Chemistry program to carry out research on the development of theoretical methods to model vibrational energy flow in complex systems using mixed quantum-classical methods. Included in this study are systems with extensive hydrogen bonding. An additional project is to develop methods to simulate 2DIR spectroscopic studies and this study is being carried in collaboration with Kevin Kubarych and Barry Dunietz, colleagues in Geva's department.
The work is having a broader impact through the training of students and the development of new methods that will have application in a wide area of science.
Molecules are aggregates of atoms held together by chemical bonds. Molecules are also the basic components of most substances. Although the atoms in a molecule are held together by chemical bonds, the molecular structure is not frozen, and the atoms engage in vibrational motion around the equilibrium molecular structure. Understanding these vibrational motions is important for several reasons. First, the flow of vibrational energy between atoms dictates chemical reactivity. Second, infrared spectroscopy, which provides a powerful tool for probing molecular structure and dynamics, is based on exciting these vibrational motions. Understanding molecular vibrations in complex systems such as liquid solutions and biological molecules also represents an ongoing challenge for theoretical chemistry. This is because of the huge number of possible inter-connected vibrational motions and their relatively high frequency (for example, the bending vibration of a water molecule occurs at a frequency of 56 billion (56,000,000,000) oscillations per second!). These high frequencies imply that one needs to be able to describe vibrational motion on an extremely short time scale and use quantum, instead of classical, mechanics, which increases the difficulty of the calculations by orders of magnitude. Recent advances in spectroscopic techniques have made it possible to use infrared spectroscopy in order to probe vibrational motion in complex molecular systems on extremely short times scales comparable to the actual vibrational period. In many cases, the interpretation of these measurements requires the development of sophisticated theoretical and computational methods for molecular modeling. Our research program aims at advancing developing such methods and applying them to molecules studied by our experimental collaborators. The novel methods whose development was funded by this grant employ semi-classical and mixed quantum-classical techniques. These techniques are based on restricting the costly quantum calculation to high frequency motions while treating other types of motion classically. The usefulness of these techniques was demonstrated by a series of successful applications to a variety of complex systems, including hydrogen-bonded liquids (known to play a central role in chemistry and biology), metal-carbonyl complexes (suggested as probes of the structure and dynamics of complex liquids and biomolecules) and molecular liquids. 7 individuals have contributed to research projects supported by this grant, including 5 graduate students (2 minority and 3 female) and 2 postdocs. The research funded by this grant was reported in 14 papers in peer-reviewed journals and 18 invited talks in domestic and international conferences.
