March 1, 2024

Probing chiral molecules with their own electrons

Physical 17, 26

A technique that can determine the chirality of a molecule using that molecule’s own electrons could allow researchers to investigate the dynamic behavior of chiral molecules on very short time scales.

Right-handed and left-handed versions of a chiral molecule preferentially diffract electrons in opposite directions, allowing researchers to determine a molecule’s chirality from an electron diffraction signal.

Whip up a batch of amino acids – the molecular building blocks of life – and only half of the molecules will turn out as planned. This is because each amino acid comes in two “chiral” forms, one that spins like a right-handed screw and another that spins like a left-handed screw. Any attempt to form one form always produces an equal amount of the other, unless some selective chiral substance is included in the mixture, as happens in nature: living creatures contain only left-handed amino acids; right-handed versions are not conducive to life.

Exactly why this asymmetry exists remains unanswered, as do other questions related to the behavior of chiral molecules. The problem is that chirality remains a difficult property to probe experimentally. Now, Debobrata Rajak and Yann Mairesse of France’s National Center for Scientific Research (CNRS) and their colleagues have developed a technique to determine the chirality of a molecule using the molecule’s own electrons. [1]. The technique could allow researchers to monitor the dynamics of chiral molecules on attosecond timescales, providing new insight into these sinuous objects.

There are methods for testing the chiral properties of a set of molecules in a concentrated form, such as a powder or liquid. But in these states, interactions between molecules can influence measurement results. Monitoring molecules in the gas phase solves this problem, and two decades ago theorists proposed a way to distinguish the chirality of such molecules using electrons. This proposal required aligning the orientations of the molecules with respect to the direction of the electron beam, a condition difficult to meet if the molecules were allowed to float. “There has been a lot of progress in developing methods to align gas molecules, but it remains something that is an experiment in itself,” says Mairesse.

Rajak, Mairesse and their colleagues solved this problem by imaging the molecules with their own electrons, ensuring alignment between the orientation of each molecule and the trajectory of the probe electron. The team studied left- and right-handed versions (enantiomers) of two chiral organic compounds: fenchone, found in wormwood and fennel, and 𝛼-pinene, which is present in eucalyptus leaves and orange peels. In separate tests, they placed an enantiomer in vapor form inside a vacuum chamber and then directed an intense elliptically polarized femtosecond pulsed laser at the system.

The strength of the pulses and their orientation relative to the molecules were such that each pulse expelled an electron from a subset of molecules. The elliptical shape of the pulses set the released electrons on a course that accelerated them away from their parent molecules and then pushed them back. But the electrons that returned did not return to the starting point. Instead, they were diffracted by electrostatic interactions with the ionized molecules and directed to a detector that recorded their properties.

The team repeated the ionization detection steps 72 times for 72 different orientations of the polarization of the laser pulse relative to the detector, allowing them to reconstruct the full three-dimensional electron diffraction pattern of the molecules, from which they recovered the structure of the molecules. They then removed all the molecules from the chamber and refilled it with a gas of the opposite enantiomer of that same compound and repeated the experiment.

Comparing the signals from the different experiments, Rajak, Mairesse and their colleagues found that the right- and left-handed versions of each compound preferentially diffracted electrons in opposite directions relative to the direction of the laser pulses. This asymmetry reached several percent and provided incontrovertible proof that the experiments were monitoring the chiral properties of the molecules, rather than an artifact, says Mairesse. The researchers were able to discover this signal in part thanks to shielding that prevented the entry of stray magnetic fields that can divert the paths of electrons after ionization, destroying chiral signals. “Even the Earth’s magnetic field can influence the result,” says Rajak. “The chiral signals we measured are very subtle.”

The team says another key to this successful demonstration was the femtosecond laser. It provided the strong electric field necessary to separate electrons from atoms and produce the effect of diffraction. The high repetition rate of the pulses also offers the possibility of observing the time-dependent chiral behavior of a molecule, for example during a photochemical process. The positions of a molecule’s nuclei relative to each other determine its chirality, and these positions can change on a time scale of a few tens to a few hundred femtoseconds during a chemical reaction. Our technique could certainly be used for such measurements, says Mairesse.

Molecular chirality plays a decisive role in determining the outcomes of chemical reactions, says Andrés Ordóñez, a theoretical physicist at Imperial College London who specializes in developing techniques for imaging and manipulating chiral molecules. For him, the team’s use of colliding electrons to produce important three-dimensional structural information about a molecule is an “impressive” advance that reveals a new chiral phenomenon. “This work will certainly motivate further fundamental research into using similar techniques to image the ultrafast dynamics of complex molecules.”

–Katherine Wright

Katherine Wright is deputy editor of Physics Magazine.

References

  1. D. Rajak and others.“Laser-induced electron diffraction in chiral molecules,” Physical. Rev. 14011015 (2024).

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