2D IR spectroscopy of the C-D stretching vibration of the deuterated formic acid dimer.

We report transient grating and 2D IR spectra of the C-D stretching vibration of deuterated formic acid dimer. The C-D stretching transition is perturbed by an accidental Fermi resonance interaction that gives rise to a second transition. The transient grating results show that the population lifetime of these states, which are in rapid equilibrium, is 11 ps. 2D IR spectroscopy reveals the energies of the eigenstates in the regions of one quantum and two quanta of C-D stretching excitation. Using these eigenstate energies, we construct a simplified model for the zeroth-order states that we then use to simulate the 2D IR spectrum. The results of this simulation suggest that the model captures the essential features of the vibrational spectroscopy in the region of the C-D stretching transition and compares well with previous gas-phase spectroscopy of the C-D stretch of deuterated formic acid dimer.

Two-dimensional infrared study of 3-azidopyridine as a potential spectroscopic reporter of protonation state.

The lack of general spectroscopic probes that can be used in a range of systems to probe kinetics and dynamics is a major obstacle to the widespread application of two-dimensional infrared (2D IR) spectroscopy. We have studied 3-azidopyridine to characterize its potential as a probe of the protonation state of the pyridine ring. We find that the azido-stretching vibration is split by accidental Fermi resonance interactions with one or more overtones and combination states. Using 2D IR spectroscopy, we determine the state structure of the resulting eigenstates for complexes of 3-azidopyridine with formic acid and trifluoroacetic acid in which the pyridine ring is unprotonated and protonated, respectively. Based on the measurements, we develop a two-oscillator depurturbation model to determine the energies and couplings of the zeroth-order azido-stretching state and the perturbing dark state that couples to it. Based on these results, we conclude that the azido-stretching vibration is, in fact, sensitive to the protonation state of the pyridine shifting up in frequency by 8 cm(-1) in the complex with trifluoroacetic acid relative to the formic acid complex. These results suggest that, although 3-azidopyridine is not suitable as a spectroscopic probe, the approach of employing an organic azide as a remote probe of protonation state holds significant promise.

Characterizing the dynamics of functionally relevant complexes of formate dehydrogenase.

The potential for femtosecond to picosecond time-scale motions to influence the rate of the intrinsic chemical step in enzyme-catalyzed reactions is a source of significant controversy. Among the central challenges in resolving this controversy is the difficulty of experimentally characterizing thermally activated motions at this time scale in functionally relevant enzyme complexes. We report a series of measurements to address this problem using two-dimensional infrared spectroscopy to characterize the time scales of active-site motions in complexes of formate dehydrogenase with the transition-state-analog inhibitor azide (N(3)(-)). We observe that the frequency-frequency time correlation functions (FFCF) for the ternary complexes with NAD(+) and NADH decay completely with slow time constants of 3.2 ps and 4.6 ps, respectively. This result suggests that in the vicinity of the transition state, the active-site enzyme structure samples a narrow and relatively rigid conformational distribution indicating that the transition-state structure is well organized for the reaction. In contrast, for the binary complex, we observe a significant static contribution to the FFCF similar to what is seen in other enzymes, indicating the presence of the slow motions that occur on time scales longer than our measurement window.

Vibrational relaxation of C-D stretching vibrations in CDCl3, CDBr3, and CDI3.

We present time-resolved transient grating measurements of the vibrational relaxation rates of the C-D stretching vibrations of deuterated haloforms in benzene and acetone. We compare our results with previous measurements of excited C-H stretches in the same solvents to obtain insight into the solvent effect on the vibrational relaxation. In deuterated molecules, there are more low-order-coupled states and the states are closer in energy to the C-D stretch than in the unlabeled isotopologs. Therefore, the relaxation is faster for the deuterated molecules. The relaxation also shows a significant solvent dependence. Bromoform and iodoform form charge-transfer complexes with both benzene and acetone which enhance the relaxation rate. For chloroform, hydrogen bonding to acetone is expected to be a more favorable interaction. Surprisingly, however, the vibrational relaxation of CDCl(3) is slower in acetone than in benzene.