Chirality, or the handedness of chemical substances, has fascinated scientists since Pasteur first noticed that tartaric acid extracted from wine yeast rotated polarized light, while the same molecule synthesized in the lab had no such effect. The structural properties of chiral molecules play an important role in pharmaceutical, separations and biotech industries, as the preferential interaction of enantiomers with chiral biomolecules can have a major impact on human health. Chiral molecules also demonstrate preferential interactions with circularly polarized light, which allows their absolute configuration determination, and have applications in sensors, lasers, polarizers, and advanced materials such as multiferroics and metamaterials. For this reason, the stereochemical determination of molecules is a major preoccupation of academics and industrials alike. Such determination is most often performed using circularly polarized radiation (CPR) in the UV-visible range, and more recently, in the infra-red range. Less studied is the interaction of chiral molecules with CPR in the X-ray range, even if the differential absorption, or X-ray Natural Circular Dichroism (XNCD) presents many unique and powerful features.
A fascinating manifestation of the light-matter interaction, XNCD allows element-selective spectroscopy and evaluation of the local and crystal symmetry of different ordered, non-centrosymmetric materials. However, its interpretation remains essentially phenomenological and qualitative, as the underlying microscopic theory has not been quantitatively verified by theoretical calculations combined with experimental data. XNCD theory states that the integrated intensity of XNCD is proportional to a rather complicated tensor, the pseudo-deviator, yielding at K-edges the degree of mixing between empty levels with “p” and “d” symmetry. The connection between the intensity of this tensor and other more common, physico-chemical properties of chiral systems has not yet been made. This step is absolutely essential to offer to the scientific community the theoretical and experimental tools necessary to provide information about chiral centers that is presently unattainable.
In addition to a comprehensive study of XNCD, we will also explore a related phenomenon, Resonant Inelastic X-ray scattering-Natural Circular Dichroism (RIXS-NCD). This is a photon-in/photon-out spectroscopy, which allows the simultaneous probe filled and empty orbitals. RIXS-NCD has not yet been observed experimentally.
Additional experiments include the study of the ligand donor ligands at their K-edge as well as the metal L-edge, here using soft X-rays. Very few examples of NCD of these types of transitions have been reported.
The XIMTEX project assembles chemists and physicists from the ICMCB UMR5026 and the IMPMC UMR 7590, as well as from the SOLEIL and ESRF synchrotrons to extract the meaningful information present in the XNCD signals so that this rather new spectroscopy can become a useful tool for physical chemists. This will be done by the selection and fabrication of model compounds, the collection of benchmark XNCD data, the use of these data to identify the important parameters in the simulation of the spectra, leading to software that can be used by the broader scientific community working in X-ray optical activity.
What is XNCD?
Natural circular dichroism (NCD) − the differential absorption of left and right circularly polarized radiation by a chiral material − was first observed in 1895 by A. Cotton, and remains one of the most powerful tools for obtaining stereochemical information. It is mainly exploited by using UV-vis radiation in Electronic Circular Dichroism (ECD) and infrared radiation Vibrational Circular Dichroism (VCD).
NCD in the X-ray range (XNCD) has been much less explored. Important advantages of XNCD with respect to ECD or VCD is its element-specificity, and the fact that one can probe the chiral molecular environment around the absorbing atom. However, XNCD can only be observed in oriented samples, more specifically for crystals for which belong to one of 13 noncentrosymmetric point groups. Furthermore, XNCD can only be observed using 3rd generation synchrotron sources, such as the ID12 beamline at the ESRF in Grenoble…. READ MORE ABOUT ANR XIMTEX