Electron delocalization in conjugated sulfur heterocycles probed by resonant Auger spectroscopy

In a collaboration with Marc Simon’s experimental group from the Sorbonne Université, we propose a novel approach for an indirect probing of conjugation and hyperconjugation in core-excited molecules using Resonant Auger Spectroscopy. Our work demonstrates that the changes in the electronic structure of thiophene (C₄H₄S) and thiazole (C₃H₃NS) occur in the resonant sulfur K-shell excitation process and Auger decay, affect the stabilization energy resulting from π-conjugation and hyperconjugation. The variations in the stabilization energy manifest themselves in the resonant S KL2,3L2,3 Auger spectra of thiophene and thiazole. The results were compared for the conjugated molecules, and thiolane (C₄H₈S), the saturated analog of thiophene, was performed. The experimental observations are interpreted using high-level quantum-mechanical calculations and the Natural Bond Orbital analysis.

To help the comprehension of the RAS phenomena, we applied multiconfigurational CASSCF functions to describe the core excited and final Auger states. Using this approach, we obtained accurate transition and Auger kinetic energies, which enables the assignment of the unexpected shift between the core excitation to the low-lying π* and σ* orbitals. Additionally, to express the RAS spectroscopy in terms of chemical concepts, we applied the Natural Bond Orbitals analysis, using the stabilization energy of these orbitals to rationalize the excited states’ behavior.

Martins, J. B., de Moura, C. E. V., Goldsztejn, G., Travnikova, O., Guillemin, R., Ismail, I., Journel, L., Koulentianos, D., Barbatti, M., Lago, A. F., Céolin, D., Rocco, M. L. M., Püttner, R., Piancastelli, M. N., Simon, M., Marchenko, T. (2022) Electron Delocalisation in Conjugated Sulfur Heterocycles Probed by Resonant Auger Spectroscopy, Physical Chemistry Chemical Physics, 24 (14), 8477–8487. [Link]