Internal Dynamics in Molecular Systems
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What is "chirality"?
Chirality occurs when the mirror image of an object cannot be made to coincide with the original by rotation. In nature, this is evident, for example, in the left and right hands or in left- and right-handed snail shells.
Molecules can also be chiral; while their mirror images (enantiomers) behave identically in physical terms—with the exception of optical activity—they can react very differently chemically. A well-known example of this is thalidomide (brand name “Contergan”); see Fig. 2.
Parity Violation in Chiral Molecules
It was originally assumed that, for reasons of symmetry, enantiomers have exactly the same energies. The underlying principle is called “parity conservation” in physics: A physical process remains the same even when the parity operation is performed, i.e., when the space-reflected process is considered. This makes sense to the human mind, since in our macroscopic world we are directly confronted only with natural forces to which parity conservation applies, such as gravity and electromagnetism.
It was only when it was discovered that the weak interaction—one of the four fundamental forces of physics—violates parity that scientists subsequently realized there must be a small energy difference between the mirror images of a molecule. According to the latest theoretical models, this energy—or frequency—difference should be on the order of millihertz. The invention of the frequency comb—a high-precision frequency standard—opened up new possibilities around the turn of the millennium for experimentally detecting parity-violation effects in molecules. In this context, various experiments have been proposed, and initial feasibility studies have already been conducted. However, a measurement of molecular parity violation has not yet been achieved and remains the subject of current research.
Project on Chirality by the Laboratory Astrophysics Division
The Laboratory Astrophysics Group focuses, among other things, on investigating the internal dynamics of chiral molecules using high-precision spectra in the infrared and terahertz ranges. The molecules currently under investigation are the chalcogen hydrides HSSH, HSOH, HSSOH, HSSSH, as well as hydrogen peroxide (HOOH) and their deuterated forms. Some of these compounds are unstable and must be continuously regenerated during the measurement process, e.g., via gas discharges (see Fig. 3).
With their short, twisted chain structure, these molecules are among the simplest examples of chiral molecules, which minimizes the computational effort required for theoretical predictions. According to the latest findings, they also exhibit relatively large parity-violation effects. Furthermore, these compounds are excellent for studying the internal dynamics of molecular systems, since the enantiomers are separated by a relatively low energy barrier, making it easy to excite their internal rotation and convert them into one another (see Fig. 4). All of this makes them ideal subjects of study for our purposes.
We are particularly interested in the following questions:
- Is it possible to distinguish between right- and left-handed molecules solely based on their interaction with IR and THz radiation? (Chiral detection)
- Is it possible to use monochromatic radiation to specifically convert a molecule into its enantiomer? (Chiral “switching”)
- Can the theoretically predicted parity violation in molecules be measured using infrared experiments?