Experimental investigation of microscopic theory of X-ray Natural Circular Dichroism – XIMTEX

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.[1] 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, particularly at the metal K-edge. This issue can be best understood by comparison with X-ray Magnetochiral Dichroism (XMCD). For XMCD, it is well known that the signal is proportional to the orbital magnetic moment, and when the core-hole is split by spin-orbit coupling, XMCD is also roughly proportional to the spin magnetic moment. The two proportionalities stem from the magneto-optical sum rules. For XNCD, a general understanding is still lacking. The XNCD sum-rule 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.[2,3] The connection between the intensity of this tensor and other more common, physico-chemical properties of chiral systems (at large) 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.
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.

[1] J. Goulon, A. Rogalev, C. Brouder, in Comprehensive Chiroptical Spectroscopy, John Wiley & Sons, Ltd 2011.
[2] J. Goulon, A. Rogalev, F. Wilhelm, C. Goulon-Ginet, P. Carra, I. Marri, Ch. Brouder, J. Exp. Theor. Phys. 2003, 97 (2), 402–431. DOI: 10.1134/1.1609001.
[3] C. R. Natoli, C. Brouder, P. Sainctavit, J. Goulon, C. Goulon-Ginet, A. Rogalev, Eur. Phys. J. B. 1998, 4 (1), 1–11. DOI: 10.1007/s100510050344.