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Techniques & Tools Spectroscopy

The Key to Chirality

Researchers from the University of Alberta have developed a new form of chiral Raman spectroscopy: eCP-Raman (1). They claim that the technique, which combines electronic circular dichroism (ECD) and circularly polarized Raman (CP-Raman), can identify chirality with greater sensitivity than current methods.

There are a number of techniques researchers use to study chirality, but they all have limitations. X-ray crystallography requires a single crystal; NMR and HPLC require specific chiral shift agents/stationary phases; chiral tag-molecular rotational resonance (MRR) is only for small molecules; and ECD and optical rotatory dispersion (ORD) may not be sensitive or unique to all stereogenic centers – and the theoretical modeling needed to extract chirality information can lack accuracy. 

Vibrational circular dichroism (VCD) and Raman optical activity (ROA) are both highly sensitive to chirality and can be done in solution directly, but their signals can be weak and often require long acquisition times to obtain a good signal-to-noise ratio. The Alberta researchers showed that eCP-Raman can detect chirality with a sensitivity often much greater than regular ROA and comparable to ECD. eCP-Raman can also be measured using a regular ROA instrument.

“Until now, researchers had not recognized the existence of eCP-Raman in ROA experiments, and our discoveries provide a new mechanism for understanding this chiral phenomenon, and may help with the design of new chirality sensors,” says Yunjie Xu, a professor in the Department of Chemistry at Alberta and corresponding author of the study. 

In a study demonstrating the technique, the team untangled different chirality transfer mechanisms reported previously using ROA measurements. According to Xu, there have been a good number of reports about amplification of ROA features in complicated systems, including those with nanostructures. She says, “Whether such amplification is  caused by the specific inter/intramolecular bonding interactions with the resonating chiral solute or not could be difficult to evaluate and we show that such specific interactions are not always needed for the large amplification observed.” 

Xu and her colleagues also showed that resonance ROA signals are generally contaminated by eCP-Raman signals. “The current work not only points out this crucial issue, but also offers ways to remove such contamination,” says Xu. “So the discovery should greatly aid the current theoretical development of resonance ROA.”

What’s next for eCP-Raman? Though the technique is new, some recent applications have already been reported; for example, using it for the molecular structure information of a series of atropisomeric naphthalene diimides (2). Xu adds, “We are currently focusing on extracting resonance ROA of the Ni complex and related systems, and working together with theorists to evaluate/further develop theoretical models for resonance ROA.”

Introducing eCP-Raman 
 

By Yunjie Xu

eCP-Raman is a combination of ECD and CP-Raman and can be measured using a regular ROA instrument. This phenomenon was first discovered in the measurements of Raman and chiral Raman of a resonating Ni complex. This Ni complex has very large magnetic dipole moments and, therefore, a large dissymmetry factor g (the intensity ratio of ECD over UV-vis). Nowadays, the majority of ROA measurements are done using a back scattered circular polarized design. In the case of Ni, a strong ECD absorption causes a noticeable imbalance of the right versus left circularly polarized light (RCPL vs LCPL). This imbalance leads to CP-Raman. Such CP-Raman can happen for both chiral or achiral molecules, and is collected in the same manner as ROA, IRCPL - ILCPL.

This is an important discovery because researchers have not recognized the existence of eCP-Raman in ROA experiments. It provides a new mechanism for chirality transfer and a new way of monitoring chirality.

Hero & Teaser Image Credit: by fdecomite / CC BY

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  1. Guojie et al., Chemistry–A European Journal (2022). DOI: 10.1002/chem.202104302
  2. Machalska et al., Chemical Communications (2022). DOI: 10.1039/d1cc06974h
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