Synchrotron radiation studies for chiral recognition of molecules: the case of chiral oxiranes and thiiranes

Chiral recognition and studying of chiroptical properties of molecular systems play a role of utmost importance in many and diverse fields in Chemistry, from both fundamental and applied science points of view. Since 1976, it was theoretically recognized that light-matter interaction, specifically chiral photon-chiral molecule interaction, could allow investigating intrinsic properties of chiral free molecules in the ionization regime [1]. However, the first proof-of-principle experimental measurement of this new dichroic observable was only performed in 2001 [2], when bromocamphor enantiomers in the vapour phase were ionized by circularly polarized synchrotron radiation (CP-SR). This circular dichroism spectroscopy, PECD (PhotoElectron Circular Dichroism), involves angle resolved photoelectron detection, where emission is induced by CP radiation. The high photon flux and degree of polarization, typically provided by third generation CP-SR photon sources, proved to be crucial for PECD studies. The sign of the photoelectron dichroism coefficient, Di(h), depends on the specific enantiomer, while its magnitude is a function of the ionizing photon energy. The combination of a high accuracy theoretical spectral simulation of the Di(h) coefficients, and their accurate and detailed experimental measurement by CP-SR ARPES (Angle Resolved PhotoElectron Spectroscopy), provides a powerful method for chiral recognition. The perspectives offered by recent improvements in the ARPES technique [3], and the developments of much more accurate state-of-the-art theoretical methods to calculate continuum wavefunctions, among them the Di(h) coefficients [4,5], will be described in the case of the recent studies of epichlorohydrin [6,7] and chiral thiiranes. [1] B. Ritchie. Theory of the angular distribution of photoelectrons ejected from optically active molecules and molecular negative ions. Phys. Rev. A 13, 1411 (1976). [2] N. Bӧwering, T. Lischke, B. Schmidtke, N. Müller, T. Khalil, and U. Heinzmann. Asymmetry in photoelectron emission from chiral molecules induced by circularly polarized light. Phys. Rev. Lett. 86,1187 (2001). [3] L.Schio, M. Alagia, D. Toffoli, P. Decleva, R. Richter, O. Schalk, R. D. Thomas, M. Mucke, F. Salvador, P. Bertoch, D. Benedetti, C. Dri, G. Cautero, R. Sergo, L. Stebel, D. Vivoda, S. Stranges. Photoionization Dynamics of the Tetraoxo Complexes OsO4 and RuO4. Inorg. Chem. 59, 7274 (2020) and Article Supporting Information; Elettra Highlights 2019 – 2020, Low Density Matter, 62 – 63. [4] T. Moitra, A. Ponzi, H. Koch, S. Coriani, P. Decleva. Accurate Description of Photoionization Dynamical Parameters. J. Phys. Chem. Lett. 11, 5330 (2020). [5] B. Nunes Cabral Tenorio, A. Ponzi, S. Coriani, P. Decleva. Photoionization Observables from Multi-Reference Dyson Orbitals Coupled to B-Spline DFT and TD-DFT Continuum. Molecules 27, 1203 (2022). [6] S. Stranges, M. Alagia, P. Decleva, M. Stener, G. Fronzoni, D. Toffoli, M. Speranza, D. Catone, S. Turchini, T. Prosperi, N. Zema, G. Contini, Y. Keheyan. The valence electronic structure and conformational flexibility of epichlorohydrin. Phys. Chem. Chem. Phys. 13, 12517 (2011). [7] L. Schio. Studies of molecular photoionization of simple systems by advanced photon sources. Ph.D. Thesis, Department of Basic and Applied Sciences for Engineering, Sapienza University of Rome (2020).