Signatures of the Bromine Atom and Open-Shell Spin Coupling in the X-ray Spectrum of the Bromobenzene Cation

Michael Epshtein; Bruno Nunes Cabral Tenorio; Marta L Vidal; Valeriu Scutelnic; Zheyue Yang; Tian Xue; Anna I Krylov; Sonia Coriani; Stephen R Leone

doi:10.1021/jacs.2c12334

Signatures of the Bromine Atom and Open-Shell Spin Coupling in the X-ray Spectrum of the Bromobenzene Cation

J Am Chem Soc. 2023 Feb 15;145(6):3554-3560. doi: 10.1021/jacs.2c12334. Epub 2023 Feb 3.

Authors

Michael Epshtein^{1

2}, Bruno Nunes Cabral Tenorio³, Marta L Vidal³, Valeriu Scutelnic^{1

2}, Zheyue Yang¹, Tian Xue¹, Anna I Krylov⁴, Sonia Coriani³, Stephen R Leone^{1

2

5}

Affiliations

¹ Department of Chemistry, University of California, Berkeley, California 94720, United States.
² Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
³ DTU Chemistry─Department of Chemistry, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark.
⁴ Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States.
⁵ Department of Physics, University of California, Berkeley, California 94720, United States.

PMID: 36735829
DOI: 10.1021/jacs.2c12334

Abstract

Tabletop X-ray spectroscopy measurements at the carbon K-edge complemented by ab initio calculations are used to investigate the influence of the bromine atom on the carbon core-valence transitions in the bromobenzene cation (BrBz⁺). The electronic ground state of the cation is prepared by resonance-enhanced two-photon ionization of neutral bromobenzene (BrBz) and probed by X-rays produced by high-harmonic generation (HHG). Replacing one of the hydrogen atoms in benzene with a bromine atom shifts the transition from the 1s_C* orbital of the carbon atom (C*) bonded to bromine by ∼1 eV to higher energy in the X-ray spectrum compared to the other carbon atoms (C). Moreover, in BrBz⁺, the X-ray spectrum is dominated by two relatively intense transitions, 1s_C→π* and 1s_C*→σ*(C*-Br), where the second transition is enhanced relative to the neutral BrBz. In addition, a doublet peak shape for these two transitions is observed in the experiment. The 1s_C→π* doublet peak shape arises due to the spin coupling of the unpaired electron in the partially vacant π orbital (from ionization) with the two other unpaired electrons resulting from the transition from the 1s_C core orbital to the fully vacant π* orbitals. The 1s_C*→σ* doublet peak shape results from several transitions involving σ* and vibrational C*-Br mode activations following the UV ionization, which demonstrates the impact of the C*-Br bond length on the core-valence transition as well as on the relaxation geometry of BrBz⁺.