Creation of a chiral vibrational wave packet in a planar molecule. Raman excitation (magenta arrows) of a superposition of the two lowest levels of the OOP vibration via an electronically excited prochiral state of vibrational level ∣v〉. In our carbonyl chlorofluoride (COFCl) example, ξ is the distance from the carbon atom to the OF-Cl plane, and the vertical dashed lines at ±〈0∣ξ∣1〉 indicate the maximum average displacement of the packet of vibrational waves. Credit: Scientists progress (2022). DOI: 10.1126/sciadv.ade0311
In a recent study, researchers from Freie Universität Berlin, DESY research center in Hamburg, Kiel University and Kansas State University showed how light can transform a flat molecule into a chiral with one particular hand, providing a solution to the long-standing problem of absolute asymmetric synthesis. This new process could be particularly useful in the chemical synthesis of compounds.
The study, conducted as part of the Collaborative Research Center 1319 Extreme Light for Sensing and Driving Molecular Chirality (ELCH), was published in Scientists progress December 7.
“Molecules made up of four or more atoms are often chiral, meaning that their atomic spatial arrangement is either left-handed or right-handed. Each chiral molecule has a twin molecule from a mirror world with the opposite laterality,” explains Professor Christiane Koch from the Department of Physics at Freie Universität Berlin.
The left-handed and right-handed versions of the molecule (also called enantiomers) are completely equivalent to each other, except that they interact very differently with their environment and with each other depending on laterality. It is just like when two hands come into contact in the world of human social interaction; if two right hands are extended, then the two participants will shake hands, whereas if a right hand and a left hand come together, the two participants will hold hands, which is a very different gesture.
Scientists are still discovering new information about how molecules Arrange in a left or right handed configuration. “Biologically relevant molecules are predominantly homochiral, which means that in the living cells, only one of the two possible enantiomers is present. Although they are composed of exactly the same atoms, the two mirror images of a molecule differ in many properties,” Koch explains.
For example, one enantiomer can smell like orange, while the other smells like lemon. “Or even worse, one enantiomer will be a cure for a disease, while the other will be a dangerous poison. Controlling the formation of a specific direction of chiral molecules is therefore an important goal of chemical synthesis,” adds Koch.
Usually, chemists rely on controlling chirality with chiral chemicals. However, there was an alternative that had not yet been realized: absolute asymmetric synthesis, which is the control of product chirality using only light fields. It was previously difficult to know exactly how this could be achieved.
In their work, Denis Tikhonov, Alexander Blech, Monika Leibscher, Loren Greenman, Melanie Schnell and Christiane Koch devised an absolute asymmetric synthesis procedure that can be tested in an experiment.
Starting from a gaseous set of the planar molecule COFCl as a non-chiral reactant, the carbon molecules are made chiral by exciting vibrational motion out of the molecular plane. This preparation step is called a pump. As the molecule vibrates back and forth from top to bottom of the plane, the molecular sensitivity changes. This change can be measured by ionizing the molecules and varying the delay between the pump and probe steps.
In order to be able to measure the time sensitivity of the molecular assembly, the pump stage must consist of three different electric fields which must be perpendicular to each other. Otherwise, molecules at a given orientation will start in a left-handed configuration while molecules at another orientation will start from a right, leaving no clear signal.
The combination of electric fields identified by the team ensures that all molecules, regardless of orientation, start with the same laterality. In more technical terms, the required combination of electric fields for the pump stage can be achieved by a left and right circularly polarized pulse, which together induce what is called a Raman transition, as well as a static electric field. The probe step consists in ionizing the molecules. The measurement of the directions in which the electrons are emitted makes it possible to deduce the laterality of the molecules.
The authors argue that the proposed experiment is feasible with already existing setups, for example, using circular polarization provided by FLASH at DESY.
Denis S. Tikhonov et al, Pump-probe spectroscopy of chiral vibrational dynamics, Scientists progress (2022). DOI: 10.1126/sciadv.ade0311
Provided by the Free University of Berlin
Quote: Light can be used to control molecular handedness (December 13, 2022) retrieved December 15, 2022 from https://phys.org/news/2022-12-molecular-handedness.html
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