Research
Chemical reactions span a wide range of timescales with elementary steps occurring in the subpicosecond range and slower reactions, such as large structural changes, in the micro to millisecond range. New ultrafast techniques employ short light pulses, mainly in the infrared and visible regions, to probe these ultrafast events and to provide a microscopic picture of the atomic motions that compose the reactions as well as a molecular-level interpretation of the forces that guide the reactions.
A non-equilibrium two-dimensional infrared (2DIR) spectrum of dimanganese decacarbonyl. The red features are associated with transient molecules undergoing vibrational and orientational relaxation. This is an example of using non-equilibrium 2DIR to study photochemical reactions. See the full paper.
My current research in the Kubarych Group at the University of Michigan consists of studying the chemical dynamics of small molecules, such as metal carbonyls, in the condensed phase using (Non-Equilibrium) Multidimensional Infrared Spectroscopy (MDIR) and other non-linear techniques. MDIR uses the vibrational modes of the molecules to provide dynamical information with femtosecond time resolution including molecular structure, obtained by mapping the connections between different modes, and chemical dynamics such as energy transfer pathways. In addition, MDIR can be extended to the study of phototriggered reactions, usually done by optically perturbing the system before the measurement, which enables us to understand how the dynamics of the molecules change when subject to non-equilibrium conditions. To obtain further insight, along with the experiment we perform quantum computational modeling as well as non-equilibrium molecular dynamics simulations of our experiments. Comparison between computational results and experimental measurements yields information related to the quantum mechanical behavior of the molecules as well as the subtle interactions between the molecules and their solvent environment. In addition, electronic-structure models are able to accurately predict configuration-dependent properties such as vibrational frequencies, transition dipoles and reaction barriers which are in turn used to assist in the interpretation of the measured spectra.
The 7-azaindole dimer is a widely studied model for excited-state double hydrogen-atom transfer reacitions
Hydrogen-transfer reactions are one of the simplest and most important reactions in all of chemistry and biology. Much of the challenge in studying these reactions arises from the quantum nature of the hydrogen atom where tunneling and other effects become important. I am also involved in a theory project aimed at computing reaction barriers and thermodynamics of excited-state hydrogen-atom transfer reactions in DNA analogs such as 7-azaindole dimers as well as other biologically relevant molecules with particular interest in the role of the extended conjugation in modulating the tautomerization barriers.
A more recent project in collaboration with the Geva theory group consists of using first-principles electronic structure techniques to predict two-dimensional infrared spectra of the molecules we study in the lab (see publications). Modeling the system with electronic structure theory makes it possible to gain specific insight into the molecular structure beyond the experimental measurements. The modeling is done mainly by computing the full potential energy surface of the molecule along the normal modes to capture the true anharmonic behavior of the system. In a similar project, anharmonic potentials, combined with molecular dynamics simulations are used to predict other quantum mechanical effects observed in 2DIR such as coherence transfer among vibrational modes of similar energy.