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The Interplay of Open-Shell Spin-Coupling and Jahn-Teller Distortion in Benzene Radical Cation Probed by X-ray Spectroscopy

preprint
submitted on 13.08.2020 and posted on 13.08.2020 by Marta L. Vidal, Michael Epshtein, Valeriu Scutelnic, Zheyue Yang, Tian Xue, Stephen Leone, Anna I. Krylov, Sonia Coriani
We report a theoretical investigation and elucidation of the x-ray absorption spectra of neutral benzene and of the benzene cation. The generation of the cation by multiphoton ultraviolet (UV) ionization as well as the experimental measurement of
the carbon K-edge spectra of both species using a table-top high-harmonic generation (HHG) source have been described in a companion paper [M. Epshtein et al., J. Am.
Chem. Soc., submitted. Available on ChemRxiv]. We show that the 1sC -> pi transition serves as a sensitive signature of the transient cation formation, as it occurs outside of the spectral window of the parent neutral species. Moreover, the presence
of the unpaired (spectator) electron in the pi-subshell of the cation and the high symmetry of the system result in significant differences relative to neutral benzene in the spectral features associated with the 1sC ->pi* transitions. High-level calculations using equation-of-motion coupled-cluster theory provide the interpretation of the experimental spectra and insight into the electronic structure of benzene and its cation.
The prominent split structure of the 1sC -> pi* band of the cation is attributed to the interplay between the coupling of the core -> pi* excitation with the unpaired electron
in the pi-subshell and the Jahn-Teller distortion. The calculations attribute most of
the splitting (~1-1.2 eV) to the spin coupling, which is visible already at the Franck-Condon structure, and estimate the additional splitting due to structural relaxation to
be around ~0.1-0.2 eV. These results suggest that x-ray absorption with increased resolution might be able to disentangle electronic and structural aspects of the Jahn-Teller
effect in benzene cation.

Funding

U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (Contract No. DEAC02- 05CH11231)

Chemical Sciences Division of Lawrence Berkeley National Laboratory

Swiss National Science Foundation (P2ELP2 184414)

U.S. National Science Foundation (No. CHE-1856342)

Independent Research Fund Denmark-Natural Sciences, DFF-RP2 grant no. 7014-00258B

Simons Fellowship in Theoretical Physics

Mildred Dresselhaus Award from CFEL/DESY

European Union's Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie Grant Agreement No. 765739, 'COSINE- European Training Network on COmputational Spectroscopy In Natural sciences and Engineering.'

History

Email Address of Submitting Author

soco@kemi.dtu.dk

Institution

DTU Chemistry - Technical University of Denmark

Country

Denmark

ORCID For Submitting Author

0000-0002-4487-897X

Declaration of Conflict of Interest

Anna I. Krylov is the president and part-owner of Qchem, Inc.

Version Notes

version 0, to be submitted to J. Phys. Chem. Lett.

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