Our research investigates the fundamental inter- and intramolecular forces that drive the course of chemical reactions. To experimentally probe the detailed molecular dynamics, both nuclear and electronic, during a chemical reaction, we use a combination of molecular beam reactive scattering and laser spectroscopic techniques. Traditionally, predicting rate constants and microscopic dynamics has relied on statistical transition state theories or, in smaller systems, quantum scattering calculations on a single adiabatic potential energy surface that provides the barriers to each reaction. However, a reaction evolves on a single potential energy surface only if the Born Oppenheimer separation of nuclear and electronic motion is valid. Much of our recent work investigates classes of important chemical reactions where the breakdown of the Born-Oppenheimer approximation (the inability of the electronic wavefunction to readjust rapidly enough during the nuclear dynamics) near the transition state alters the dynamics and markedly reduces the reaction rate. The studies test the predictions of emerging quantum theories on nonadiabatic reaction dynamics in small systems and develop an intuitive framework for understanding chemical reaction dynamics in more complex organic and inorganic reactions not yet accessible to precise quantum calculations.
As an example, our recent work on nitric acid and other important atmospheric species seeks to understand from first principles quantum mechanics why some chemical products are produced and not others. In the photodissociation of nitric acid two chemical bonds may break, producing OH+NO2 and HONO+O respectively. The ability of the electronic wavefunction to change along the reaction coordinate, particularly the orientation of the radical OH p electron's orbital, plays a critical role in determining what products are formed. Our early molecular beam experiments showed that nonadiabatic recrossing of the transition state plays a dominant role in determining the branching between chemical bond fission channels, reversing the expected branching between C-Br and C-Cl fission in the Br(CH2)nCOCl. Suppressing rapid intramolecular electronic energy transfer allows you to preferentially cleave a selected chemical bond. Our experiments and supporting ab initio calculations elucidate the intramolecular distance and conformation dependence of nonadiabatic recrossing of the reaction barriers in the competing reaction channels.
Other experiments use molecular photodissociation to directly access both the upper and lower adiabatic potential energy surfaces, respectively, near the transition state region of a excited state bimolecular reaction to probe the influence of nonadiabatic coupling in chemical reaction dynamics. Our molecular beam photofragmentation and emission spectroscopy experiments and collaborative theoretical work on CH3SH investigated how accessing different regions of the CH3S + H → CH3+SH reactive potential energy surfaces changes the branching between the S-H and C-S bond fission channels and how nonadiabatic coupling influences the dynamics. In this system and in H2S, we used the technique of emission spectroscopy of dissociating molecules to investigate the dynamics which occurs during the subpicosecond dissociation event, providing a key link between the absorption spectrum and the final product quantum states.
We have recently introduced a method for investigating the competing unimolecular dissociation channels of isomerically-selected radicals as a function of internal energy in the radical. Radical intermediates play a key role in a wide range of chemical processes, yet many key isomeric radical intermediates elude direct experimental probes. Our experiments photolytically produce from an appropriate precursor a selected radical isomer and disperse the radicals by their neutral velocity imparted in the photolysis, thus dispersing them by internal energy. For the unstable radicals, they then measure the branching between C-C and C-H fission products via tunable VUV photoionization of products dispersed by their velocity. This offers the unprecedented ability to measure the branching between isomeric product channels as a function of internal energy in the dissociating radical isomer on the ground state potential energy surface.
Massachusetts Institute of Technology
B.S.
1981
University of California, Berkeley
Ph.D.
1985
University of Wisconsin
Postdoctoral Research Associate
1986
The University of Chicago
Professor
Present
Dissociation Dynamics of the Methylsulfonyl Radical and its Photolytic Precursor, J. Chem. Phys., 131, 044305 (2009).
Investigation of the O + allyl addition/elimination reaction pathways from the OCH2CHCH2 radical intermediate, J. Chem. Phys., 129, 084301 (2008).
Determination of the Barrier Height for Acetyl Radical Dissociation from Acetyl Chloride Photodissociation at 235 nm Using Velocity Map Imaging, J. Phys. Chem. B, 112, 16050 (2008).
Unimolecular Dissociation of the CH3OCO radical: An Intermediate in the CH3O + CO Reaction, J. Chem. Phys., 110, 1625 (2006) and Fig. 9 in J. Phys. Chem. A, 111, 1762 (2007).
Chemical Reaction Dynamics Beyond the Born-Oppenheimer Approximation, Annu. Rev. Phys. Chem., 49, 125-171 (1998).
The Influence of Local Electronic Character and Nonadiabaticity in the Photodissociation of Nitric Acid at 193 nm, J. Chem. Phys., 107, 5361 (1997).
Photodissociation Dynamics, J. Phys. Chem., 100, 12801 (1996).
Fellow, American Physical Society
2002
Llewellyn John and Harriet Manchester Quantrell Award for Excellence in Undergraduate Teaching, The University of Chicago
1993
Alfred P. Sloan Fellow
1992
Camille and Henry Dreyfus Foundation Teacher-Scholar
1989
National Science Foundation Presidential Young Investigator
1988
Office of Naval Research Young Investigator
1987
Camille and Henry Dreyfus Distinguished New Faculty in Chemistry Grant
1986