The Positive Outlooks of Researching Negatively-Charged Chiral Molecules

Forensic science, flavor/odor engineering, pharmaceutical firms, and other chemical industries depend on the capacity to discriminate between two chiral enantiomers.

When compared to their predecessors, a new generation of chiral optical techniques shown a considerable increase in chiral sensitivity, offering prospective analytical benefits for chiral discrimination.

One of these cutting-edge techniques has been combined with the study of gas-phase anions by researchers at the Fritz Haber Institute, enabling mass-selection and the use of a straightforward table-top laser for chiral effect detection. developing a robust analytical tool capable of chiral discrimination of diluted and complex chiral mixtures is thus one step closer.

Chirality and the characteristics of enantiomers that create these distinctions are recognized to be an essential component of biological processes, from the differences between the flavors of spearmint and fennel to the effects of a medicine that either treats morning sickness or results in birth defects.

Despite a pair of enantiomers’ unique biological function, their identical chemical structures make them difficult to detect by analytical or spectroscopic approaches. Finding chiral discrimination methods that can match nature’s capacity to distinguish between progressively smaller samples of two enantiomers has taken a lot of research and development.

A new generation of chiral discrimination methods has evolved in the last two decades, with chiral sensitives that are orders of magnitude higher than those of its forerunners. Due to the enhanced sensitivity, isolated chiral compounds can be studied in the gas phase even at very low initial sample concentrations.

One of these contemporary techniques, which holds significant analytical promise, is known as photoelectron circular dichroism (PECD) spectroscopy. PECD is a chiral optical effect that emerges in the photoemission of electrons from a sample of chiral molecules, when illuminated with chiral light (ie. circularly polarized light).

The effect is a difference in the average direction the electrons depart the molecular sample (either in the forward or backward direction). The enantiomer being investigated and the handedness of the light influence this orientation. This effect is extraordinarily observant to the chiral potential of a molecule since it is sensitive to various chiral molecular properties.

Although studies of PECD have historically been carried out on neutral molecules, researchers at the Fritz Haber Institute have explored this effect in anions. The use of anions for PECD spectroscopy has a few important analytical advantages: First, as anions are charged particles, ion optics can be used for mass selection.

Often, industrial chiral samples are known to be multi-component. Mass selective capabilities would make it possible to isolate target molecules prior to photoemission, simplifying the analysis procedure.

The loss of an electron from a chiral molecule causes this effect, hence lower energy for electron detachment are preferred since they enable the use of readily accessible table-top lasers.

Ionization of an electron from a closed-shell neutral molecule requires either high energy synchrotron radiation or a multiphoton process by visible or near-UV lasers. The smaller photon energies required for detachment of the extra electron in anions are accessible by common table-top lasers through single-photon detachment processes.

For the first time, the research group in the department of Molecular Physics has captured an energy-resolved PECD signal for a mass-selected anion. This serves as a crucial benchmark for the analytical potential of the approach and provides a window into the various electron dynamics involved in photoionization of neutrals and photodetachment of anions.

As the observation of this effect in anions has lagged two decades behind it’s observation in its neutral counterpart, comparisons of these photoemission processes could unlock comprehension of the universal dynamics that govern the PECD effect.

When taking into account traditional theoretical descriptions of electron photodetachment, the research team’s initial findings have shown a PECD effect that is surprisingly similar in magnitude to its neutral counterpart and an effect that persists at much higher electron kinetic energies than was to be expected. These findings highlight a knowledge gap on this effect that calls for additional research.

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