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The basic physical chemistry question that drives research in my group is the following: what is the role of the environment in chemical reactions, and how much of a gas-phase picture can we keep when we consider reactions in solution? Virtually all chemical processes in condensed phases address this central question, including photochemistry, isomerization, charge transfer, proton translocation, protein conformational changes, catalysis etc. We have spent the last few years advancing the methods of equilibrium and transient two-dimensional infrared spectroscopy, while applying these techniques to various kinds reactions in solution.

A. Mn2(CO)10: Watching a Photoproduct Cool with a Novel Orientational Thermometer Dimanganese decacarbonyl (DMDC) has ten strongly coupled carbonyl oscillators that exhibit three absorption bands in the carbonyl stretching region of the IR spectrum near 2000 cm-1. Previously we have studied this system at equilibrium and analyzed in detail the marked vibrational quantum beating phenomenon observable directly as cross-peak modulations in a series of 2DIR spectra.[Nee et al. J. Chem. Phys. 129 (2008) 084503] These modulations are the vibrational analogues of similar coherent phenomena observed recently in proteins and protein complexes related to photosynthesis by the Fleming group. Vibrational systems can actually model these more complicated electronic problems since in the vibrational case high level quantum calculations enable ab initio determination of the anharmonic eigenstates and the system-bath interactions can be tuned by altering the solvent. In collaboration with the Geva group, we have developed the machinery to compute 2DIR spectra from quantum chemical output, such as Gaussian or Q-Chem, using our own implementation of vibrational perturbation theory. This collaboration has examined the “reactant” DMDC as well as two of the photoproducts that can be generated following UV excitation.[Baiz et al. Acc. Chem. Res. 42 (2009) 1395 and Baiz et al. J. Phys. Chem. A 113 (2009) 9617] Following excitation with 400-nm light (Fig. 3), the Mn–Mn bond is cleaved giving two Mn(CO)5 fragments, and given the energy difference between the absorbed light and the bond dissociation, each fragment retains roughly 6000 cm-1 of thermal energy. Each recorded 2D spectrum corresponds to two independent time delays: the UV-IR delay and the waiting time delay within the 2DIR pulse sequence. This work was the first to systematically vary both the phototrigger-2DIR delay and the delay within the 2DIR probe, and with that innovation (enabled by our optics advances) came a new perspective on reaction dynamics. By integrating the induced 2DIR spectrum of the photoproduct, we found that the waiting time dependent 2DIR signal decayed more slowly with increased phototrigger delay. The vibrational lifetimes of metal carbonyl complexes in nonpolar solvents (cyclohexane in this case) are typically >100 ps, but the decay time constants found in the measurements ranged from 2 ps at early phototrigger delay to 12 ps at 300 ps, which is effectively equilibrium. These timescales correspond to orientational diffusion, and via the Debye-Stokes-Einstein theory the orientational correlation time has a simple inverse temperature dependence. Thus, in principle orientational relaxation is a proxy for temperature. One of the origins for the 2DIR signal to decay is the loss of orientational correlation. In conventional pump-probe spectroscopy this decay is measured using transient absorption anisotropy, and in our case, the transient anisotropy is measured as the photoproduct cools. Most experiment and theory for orientational relaxation is based on equilibrium descriptions, which is sensible given the use of “temperature” in the first place. Using both equilibrium and nonequilibrium molecular dynamics simulations with an ab-initio derived force field for the metal carbonyl complexes, we found that the orientational relaxation of the parent dimer agreed essentially perfectly with IR-IR transient anisotropy, and for the monomer, the 300-ps phototrigger 2DIR relaxation time constant of 12 ps corresponded to the MD simulation's orientational correlation time scale. Thus, we determined that the transient 2DIR experiment measured the changing rate of orientational relaxation as the photoproduct was cooling. The rate constant for cooling measured using the decay of the orientational decay constant was found to be 70 ps.

B. Equilibrium Isomerization of Co2(CO)8: Separation of Static and Dynamic Barriers One of the most powerful uses for 2DIR spectroscopy is to study ultrafast chemical exchange, where the excitation/detection correlation links molecules that undergo reaction at equilibrium. In an equilibrium ensemble of two interconverting species, no overall changes in populations are evident, which results in the condition of detailed balance. The equilibrium constant defines the ratio of product and reactant species, and that ratio is maintained throughout the experiment. The initial excitation vibrationally tags molecules of both reactants and products, and as the waiting time delay is increased, the number of molecules initially excited as reactants that have become products increases, and vice versa. Thus cross peaks in the 2D spectrum reflect the degree of chemical exchange, and their waiting time dependent amplitudes yield the reaction kinetics even though the system remains at equilibrium (except for the vibrational excitation) throughout. 2DIR chemical exchange has been applied to complexation, C–C bond rotation, hydrogen bond making/breaking and to fluxional motion. We have applied 2DIR chemical exchange to dicobalt octacarbonyl (DCO), a flexible complex that exists in three spectroscopically distinct isomers at room temperature. Our work first showed that such a complex mixture could be dissected despite congested spectra, and that the barrier to isomerization between the two most stable isomers could be determined using an Arrhenius-type analysis of the temperature-dependent exchange kinetics.[Anna et al. J. Phys. Chem. A 113 (2009) 6544]