Threshold Photoelectron Spectrum of Cyclobutadiene: Comparison with Time-Dependent Wavepacket Simulations

Lea Bosse; Barry P Mant; Domenik Schleier; Marius Gerlach; Ingo Fischer; Anke Krueger; Patrick Hemberger; Graham Worth

doi:10.1021/acs.jpclett.1c01848

Threshold Photoelectron Spectrum of Cyclobutadiene: Comparison with Time-Dependent Wavepacket Simulations

J Phys Chem Lett. 2021 Jul 29;12(29):6901-6906. doi: 10.1021/acs.jpclett.1c01848. Epub 2021 Jul 19.

Authors

Lea Bosse¹, Barry P Mant², Domenik Schleier¹, Marius Gerlach¹, Ingo Fischer¹, Anke Krueger³, Patrick Hemberger⁴, Graham Worth²

Affiliations

¹ Institute of Physical and Theoretical Chemistry, University of Würzburg, D-97074 Würzburg, Germany.
² Department of Chemistry, University College London, 20 Gordon St., London WC1H 0AJ, U.K.
³ Institute of Organic Chemistry, University of Würzburg, D-97074 Würzburg, Germany.
⁴ Laboratory for Femtochemistry and Synchrotron Radiation, Paul Scherrer Institut (PSI), CH-5232 Villigen, Switzerland.

PMID: 34279954
DOI: 10.1021/acs.jpclett.1c01848

Abstract

The C₄H₄ isomer cyclobutadiene (CBD) is the prime model for antiaromaticity and thus a molecule of considerable interest in chemistry. Because it is highly reactive, it can only be studied under isolated conditions. Its electronic structure is characterized by a pseudo-Jahn-Teller effect in the neutral and a E ⊗ β Jahn-Teller effect in the cation. As a result, recording photoelectron spectra as well as describing them theoretically has been challenging. Here we present the photoion mass-selected threshold photoelectron spectrum of cyclobutadiene together with a simulation based on time-dependent wavepacket dynamics that includes vibronic coupling in the ion, taking into account eight vibrational modes in the cation. Excellent agreement between theory and experiment is found, and the ionization energy is revised to 8.06 ± 0.02 eV.