Structure, vibrational frequencies, ionization energies, and photoelectron spectrum of the para-benzyne radical anion

Equilibrium structure, vibrational frequencies, and ionization energies of the para-benzyne radical anion are characterized by coupled-cluster and equation-of-motion methods. Vibronic interactions with the low-lying excited state result in a flat potential energy surface along the coupling mode and even in a lower-symmetry C-2v structures. Additional complications arise due to Hartree-Fock instabilities and near-instabilities. The magnitude of vibronic interactions was characterized by geometrical parameters, charge localization patterns and energy differences between the D-2h and C-2v structures. The observed trends suggest that the C-2v minimum predicted by several theoretical methods is an artifact of incomplete correlation treatment. The comparison between the calculated and experimental spectrum confirmed D-2h structure of the anion, as well as accuracy of the coupled-cluster and spin-flip structures, frequencies and normal modes of the anion and the diradical. Density functional calculations (B3LYP) yielded only a D-2h minimum, however, the quality of the structure and vibrational frequencies is poor, as follows from the comparison to high- level wave function calculations and the calculated spectrum. The analysis of charge localization patterns and the performance of different functionals revealed that B3LYP underestimates the magnitude of vibronic interactions due to self-interaction error.