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Molecular chirality and parity violation

Molecular chirality was discovered over 150 years ago by Louis Pasteur. Jacobus van't Hoff and Joseph Le Bel independently developed the tetrahedral carbon model to explain the molecular chirality observed by Pasteur.

Since then, molecular chirality has led to the discovery of a whole range of new effects. For example, the first experiment in chemical kinetics was carried out by Ludwig Ferdinand Wilhelmy who investigated the cane sugar inversion with the aid of a polarimeter. The investigation of the temperature dependence of this reaction led Svante Arrhenius finally to the development of the Arrhenius relationship. Quantum mechanical tunneling was proposed in 1927 by Friedrich Hund using the example of chiral compounds for the first time (and thus before Georgi Gamov, who discussed this only two years later using the example of the alpha decay).

We are particularly interested in chiral systems, because there parity, the symmetry with respect to an inversion of all coordinates at the origin, is broken. This gives us excellent opportunities to study fundamental violations of this symmetry at the level of elementary particles.

In the past, it was assumed that the fundamental interactions - gravity, electromagnetic interaction, weak interaction and strong interaction - do not differentiate between right and left, therefore all interactions in a mirrored situation are exactly the same. Since 1957, however, we know that this is not the case for the weak interaction. This has been impressively demonstrated in the experiments proposed by the theoreticians and Nobel Prize winners Chen Ning Yang and Tsung-Dao Lee on the beta decay of the isotope cobalt-60. The weak interaction is therefore parity violating.

For the chemistry of compounds with stable nuclei this would seem at first glance to be of minor interest. But since the late 1960s the electromagnetic interaction was united with the weak interaction to the so-called electroweak interaction in the current standard model of elementary particles. From this unification of forces, exciting consequences in molecules with stable nuclei are expected to emerge. And for chiral molecules, the predicted effects are particularly impressive: the enantiomers of a chiral molecule are no longer energetically equivalent, but differ in a small amount of energy, the so-called parity violating energy difference. For nearly 50 years, attempts have been made to measure the energy difference spectroscopically, but so far this has not been successful. However, with our theoretical methods we can predict suitable molecules and advantageous experimental approaches to measure this effect for the first time.

On the way to this goal, we are also looking into the question of the absolute configuration of chiral molecules. Together with the team around Markus Schöffler, Reinhard Dörner and Horst Schmidt-Böcking from Frankfurt and other cooperation partners, we were the first to directly determine the absolute configuration of chiral molecules in the gas phase with Coulomb explosion imaging, a clever mass spectrometric experiment. And this even at the level of single molecules.