My research group is exploring a variety of topics that range from the basic understanding of chemical reaction dynamics to the nature of the chemical contents of single cells.
Under thermal conditions nature seems to hide the details of how elementary reactions occur through a series of averages over reagent velocity, internal energy, impact parameter, and orientation. To discover the effects of these variables on reactivity, it is necessary to carry out studies of chemical reactions far from equilibrium in which the states of the reactants are more sharply restricted and can be varied in a controlled manner. My research group is attempting to meet this tough experimental challenge through a number of laser techniques that prepare reactants in specific quantum states and probe the quantum state distributions of the resulting products. It is our belief that such state-to-state information gives the deepest insight into the forces that operate in the breaking of old bonds and the making of new ones.
Space does not permit a full description of these projects, and I earnestly invite correspondence. The following examples are representative:
The simplest of all neutral bimolecular reactions is the exchange reaction H H2 --> H2 H. We are studying this system and various isotopic cousins using a tunable UV laser pulse to photodissociate HBr (DBr) and hence create fast H (D) atoms of known translational energy in the presence of H2 and/or D2 and using a laser multiphoton ionization time-of-flight mass spectrometer to detect the nascent molecular products in a quantum-state-specific manner by means of an imaging technique. It is expected that these product state distributions will provide a key test of the adequacy of various advanced theoretical schemes for modeling this reaction.
Analytical efforts involve the use of capillary zone electrophoresis, two-step laser desorption laser multiphoton ionization mass spectrometry, cavity ring-down spectroscopy, and Hadamard transform time-of-flight mass spectrometry. We believe these methods can revolutionize trace analysis, particularly of biomolecules in cells.
1) "Vibrational Excitation Through Tug-of-War Inelastic Collisions," S.J. Greaves, E. Wrede, N.T. Goldberg, J. Zhang, D.J. Miller, and R.N. Zare, Nature,454, 88-91 (2008).
2) "Counting Low-Copy-Number Proteins in a Single Cell," B. Huang, H. Wu, D. Bhaya, A. Grossman, S. Granier, B.K. Kobilka, and R.N. Zare, Science, 315, 81-84 (2007).
3) "Reaction Products with Internal Energy beyond the Kinematic Limit Result from Trajectories Far from the Minimum Energy Path: an Example from H HBr -> H2 Br," A.E. Pomerantz, J.P. Camden, A.S. Chiou, F. Ausfelder, N. Chawla, W.L. Hase, and R.N. Zare, J. Am. Chem. Soc., 127, 16368-16369 (2005).
4) "A Reinterpretation of the Mechanism of the Simplest Reaction at an sp3-Hybridized Carbon Atom: H CD4 -> CD3 HD," J.P. Camden, H.A. Bechtel, D.J.A. Brown, M.R. Martin, R.N. Zare, W.Hu, G.Lendvay, D.Troya, and G.C. Schatz,J. Am. Chem. Soc., 127, 11898-11899 (2005).
5) "High-Precision Optical Measurements of 13C/12C Isotope Ratios in Organic Compounds at Natural Abundance," R.N. Zare, D.S. Kuramoto, C. Haase, S.M. Tan, E.R. Crosson, and N.M.R. Saad, Proc. Natl. Acad. Sci. (US), 106, 10928-10932 (2009).