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Real-time probing of chirality during a chemical reaction

November 13, 2019

Chiral molecules interact and react differently with other chiral objects, depending on their handedness. Therefore, it is essential to understand and ultimately control the evolution of molecular chirality during chemical reactions. Although highly sophisticated techniques for the controlled synthesis of chiral molecules have been developed, the observation of chirality on the natural femtosecond time scale of a chemical reaction has so far remained out of reach in the gas phase. Here, the authors demonstrate a general experimental technique, based on high-harmonic generation in tailored laser fields, and apply it to probe the time evolution of molecular chirality during the photodissociation of 2-iodobutane. These measurements show a change in sign and a pronounced increase in the magnitude of the chiral response over the first 100 fs, followed by its decay within less than 500 fs, revealing the photodissociation to achiral products. The observed time evolution is explained in terms of the variation of the electric and magnetic transition-dipole moments between the lowest electronic states of the cation as a function of the reaction coordinate. These results open the path to investigations of the chirality of molecular-reaction pathways, light-induced chirality in chemical processes, and the control of molecular chirality through tailored laser pulses.


Figure 1. Illustration of the investigated photochemical reaction. An initially chiral molecule is photoexcited by a femtosecond laser pulse centered at 266 nm. The ensuing photodissociation reaction leads to a time-dependent change in the chiral structure of the molecule. The leftmost image displays the equilibrium geometry of (R)-2-iodobutane (see SI Appendix for details). The structures in the other images have been obtained by varying the C–I bond distance and relaxing all remaining degrees of freedom.

This technique is sufficiently sensitive to be applicable to the gas phase, but is equally applicable to liquids (42) and solids (39) in future experiments. Its relatively large CD effects, comparable in magnitude to PECD, combined with its all-optical nature make it a powerful chiral-sensitive method for detecting ultrafast changes in molecular chirality. The ability to probe chiral photochemical processes on such time scales opens up a variety of possibilities for investigating chiral-recognition phenomena, such as the processes that determine the outcome of enantioselective chemical reactions. Paired with recent developments in condensed-phase high-harmonic spectroscopy, the new technique will rapidly be applied to all phases of matter, where it will unlock the study of a range of chiral phenomena on ultrashort time scales.
 
Reference: Baykusheva, D., Zindel, D., Svoboda, V., Bommeli, E., Ochsner, M., Tehlar, A., and Wörner, H.J. (2019). Real-time probing of chirality during a chemical reaction. Proceedings of the National Academy of Sciences, 201907189. (www.pnas.org/content/early/2019/11/12/1907189116)

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