Jahn–Teller Effect of the Benzene Radical Cation: A Direct ab Initio Molecular Dynamics Study

Abstract

The benzene radical cation (Bz) is a typical model molecule of the Jahn-Teller (J-T) active species. Bz has two structural forms due to the J-T effect. These are the compressed and elongated forms, expressed as Bz(comp) and Bz(elong), respectively. In Bz(comp), the hexagonal structure of the benzene ring is compressed up and down, and in Bz(elong), it is pulled up and down. From electron spin resonance experiments, it was found that Bz takes a compressed form in low-temperature Freon matrices (CFCl and CFClCFCl), whereas the elongated form was found in argon matrices. However, the selectivity of these structural forms is still unclear. In this study, the ionization dynamics of isolated benzene (Bz) and benzene-M complexes (where M denotes counter-molecules, M = NH, HO, CFCl, CH, CHOH, Ar, SH, ammonia dimer, or water dimer) have been investigated by means of the direct ab initio molecular dynamics (AIMD) method in order to shed light on the Bz formation mechanism. The static ab initio calculations showed that Bz(comp) is slightly more energetically stable than Bz(elong), although the energy difference was only 0.1 kcal/mol at the CCSD/6-311++G(d,p) level. The direct AIMD calculations indicated that Bz(comp) was formed from the Bz-M complexes when M was NH, CFCl, or an ammonia dimer, whereas the ionization of Bz-M when M was HO, CH, CHOH, SH or a water dimer formed Bz(elong). In the case of complexes with an argon dimer, Bz(Ar), both forms were obtained from a slight orientation change of Ar on Bz. A selective rule is discussed on the basis of the calculated results.

How to stay out of trouble in RIXS calculations within equation-of-motion coupled-cluster damped response theory? Safe hitchhiking in the excitation manifold by means of core–valence separation

“…We considered the optimized geometry of the 2 B 3g state, which is the lowest Jahn-Teller state (this structure can be described as "elongated", in terms of Ref. 82 or "acute D 2h ", in terms of Ref. 83).…”

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

“…Lowest ionized (and some of the excited) states of benzene are subject to JT distortions, which have been investigated in numerous studies. [45][46][47][49][50][51][52][53][54][55][56][57] Here we present theoretical characterization of the electronic structure of the benzene cation produced by valence photoionization of the parent molecule and probed via x-ray absorption.…”

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

“…The ionization creates two degenerate states ( 2 E 1g or, using the largest Abelian subgroup notations, 2 B 3g and 2 B 2g ), 77 which undergo strong JT distortion. 51,55,58 Figure 3: Molecular orbital diagram and electronic configurations of the two lowest electronic states of the cation illustrating JT effect in benzene cation. At the FC (symmetric) structure, the π-type HOMO of benzene (and, consequently, the ground state of the cation) are doubly degenerate.…”

“…sets of three-fold degenerate minima corresponding to the elongated and compressed structures. 55,57,58 The motion connecting these minima is called pseudorotation. Following the standard formalism, 79,80 the JT displacement is described by polar coordinates: the amplitude ρ, characterizing the magnitude of the displacement from the symmetric structure, and the pseudorotation phase φ.…”

“…High-level electronic structure calculations using equation-of-motion coupled-cluster (EOM-CC) theory [65][66][67] adapted to describe core-level states 10,68,69 facilitate the interpretation and assignment of the experimental spectra and provide additional insight into the nature of the spin coupling and the JT effect in benzene cation. In contrast to many previous studies, 48,51,55,57,58 which focused on understanding the JT effect in the occupied orbital domain, our work investigates the JT effect in both the occupied and the virtual orbital domain, highlighting additional complexity arising due to the presence of the three unpaired electrons. In addition, this contribution is the first quantitative characterization of the core-level transitions in benzene cation.…”