The benzene radical anion: A computationally demanding prototype for aromatic anions

Abstract

The benzene radical anion is studied with ab initio coupled-cluster theory in large basis sets. Unlike the usual assumption, we find that, at the level of theory investigated, the minimum energy geometry is non-planar with tetrahedral distortion at two opposite carbon atoms. The anion is well known for its instability to auto-ionization which poses computational challenges to determine its properties. Despite the importance of the benzene radical anion, the considerable attention it has received in the literature so far has failed to address the details of its structure and shape-resonance character at a high level of theory. Here, we examine the dynamic Jahn-Teller effect and its impact on the anion potential energy surface. We find that a minimum energy geometry of C2 symmetry is located below one D2h stationary point on a C2h pseudo-rotation surface. The applicability of standard wave function methods to an unbound anion is assessed with the stabilization method. The isotropic hyperfine splitting constants (Aiso) are computed and compared to data obtained from experimental electron spin resonance experiments. Satisfactory agreement with experiment is obtained with coupled-cluster theory and large basis sets such as cc-pCVQZ.

Characterizing metastable states beyond energies and lifetimes: Dyson orbitals and transition dipole moments

“…Our values for the position and width of the 2 E 2u resonance of the benzene anion (E R = 1.64 eV, Γ = 0.04 eV) also show reasonable agreement with experimental results 91 (E R = 1.12 eV, Γ ≈ 0.12 eV) and EOM-EA-CCSD values obtained with the stabilization method. 85,92 However, for the higher-lying 2 B 2g resonance of the benzene anion, we observe a substantial discrepancy between our results (E R = 6.75 eV, Γ = 0.35 eV), EOM-EA-CCSD stabilization calculations 92 (E R = 5.93 eV), and experiment 91 (E R = 4.82 eV). A possible reason could be that the 2 B 2g resonance is not fully stabilized in our calculations and is affected by the artificial stabilization of a lower-lying pseudocontinuum state.…”

Description of Two-Particle One-Hole Electronic Resonances Using Orbital Stabilization Methods

“…50 Another orbital stabilization study using DFT gave similar positions. 61 Our results for the position of the shape 2 B 2g resonance at the EOM-EA-CCSD and MR-CISD levels are in good agreement with the previous study at the EOM-EA-CCSD level, 50 even though we used smaller basis sets. They are however still 1 eV off when compared to the experiment,…”

“…64,67 Orbital stabilization method using EOM-EA-CCSD on the other hand gave a value of 5.9 eV and a width of 1.4 eV. 50 Another orbital stabilization study using DFT gave similar positions. 61 Our results for the position of the shape 2 B 2g resonance at the EOM-EA-CCSD and MR-CISD levels are in good agreement with the previous study at the EOM-EA-CCSD level, 50 even though we used smaller basis sets.…”

“…The resulting ground state in the anion has 2 E 2u symmetry and is subject to the Jahn-Teller effect. 50 The next shape reso- (e 1g → e 2u in D 6h symmetry) excitations, 3 B 1u , 3 E 1u and 3 B 2u . [51][52][53][54][55] The first singlet state 1 B 2u is also in this energy range while the other singlet states are higher in energy.…”

Coupling of electronic and nuclear motion in a negative ion resonance: Experimental and theoretical study of benzene

“…39) or even C 2 (Ref. 40) ground state of the benzene anion. The process of nuclear wavepacket relaxation appears to be accompanied by intramolecular vibrational redistribution (IVR) whereby the initially vibrationally selective electron attachment at higher energies within the resonance is converted to vibrational states with many overtones and combinations excited.…”