Visualizing Solution-Phase Reaction Dynamics with Time-Resolved X-ray Liquidography

Abstract

Most chemical reactions occur in solution, and complex interactions between solute and solvent influence the rich chemistry of these processes. To track time-dependent processes in such reactions, researchers often use time-resolved spectroscopy. In these experiments, an optical pulse (pump) initiates a reaction, and another time-delayed optical pulse (probe) monitors the progress of the reaction. However, because of the wavelength range of the probe light used in these experiments, from infrared to ultraviolet, researchers cannot directly determine detailed structural information such as the bond lengths and bond angles of reaction intermediates. In addition, not all intermediates might be sensitive to the spectroscopic signal chosen for the experiment. This Account describes time-resolved X-ray liquidography (TRXL), a technique that overcomes these problems. In this technique, we replace the optical probe with the diffraction of hard X-ray pulses emitted from a synchrotron source. In TRXL, diffraction signals are sensitive to all chemical species simultaneously. In addition, each chemical species has a characteristic diffraction signal, a fingerprint, that we calculate from its three-dimensional atomic coordinates. Because, X-rays scatter from all atoms in the solution sample, including both the solute and the solvent, the analysis of TRXL data can track not only the reaction pathways of the solute molecules but also the solvent behavior and the solute-solvent arrangement, thus providing a global picture of the reactions. We have used TRXL to study structural dynamics and spatiotemporal kinetics of many molecular systems including diatomic molecules, haloalkanes, organometallic complexes, and protein molecules over timescales from picoseconds to milliseconds. We have observed that TRXL data adds to and, in some cases, contradicts results from time-resolved spectroscopy. For example, TRXL has shown that the reaction intermediates upon C-I bond dissociation in C(2)H(4)I(2), and C(2)F(4)I(2) have completely different structures and corresponding subsequent reaction pathways, underscoring the dramatic effect of the fluorine substitution. We have also used TRXL to identify a new reaction intermediate of the photolysis of Ru(3)(CO)(12) that has no bridging carbonyl groups. Though not detected by time-resolved infrared spectroscopy, this intermediate predominates based on the TRXL data. In looking at the quaternary conformational changes of hemoglobin, TRXL analysis suggests a faster transition than was suggested by optical spectroscopy. The time resolution of TRXL is currently limited by the X-ray pulse width available from synchrotron sources (similar to 100 ps). The resolution should improve to 100 fs or better with X-ray free electron lasers. With this higher resolution, real time observation of ultrafast chemical events such as bond-breaking and bond-making will be possible.