Diradicals, antiaromaticity, and the pseudo-Jahn-Teller effect: Electronic and rovibronic structures of the cyclopentadienyl cation

The electronic and rovibronic structures of the cyclopentadienyl cation (C₅H₅⁺) and its fully deuterated isotopomer (C₅D₅⁺) have been investigated by pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy and ab initio calculations. The vibronic structure in the two lowest electronic states of the cation has been determined using single-photon ionization from the X∼E1″2 ground neutral state and 1+1^' resonant two-photon ionization via several vibrational levels of the ÃA2″2 excited state. The cyclopentadienyl cation possesses a triplet ground electronic state (X^∼+A2'3) of D_5h equilibrium geometry and a first excited singlet state (a^∼+E2'1) distorted by a pseudo-Jahn-Teller effect. A complete analysis of the E ⊗e Jahn-Teller effect and of the (A+E)⊗e pseudo-Jahn-Teller effect in the a^∼+E2'1 state has been performed. This state is subject to a very weak linear Jahn-Teller effect and to an unusually strong pseudo-Jahn-Teller effect. Vibronic calculations have enabled us to partially assign the vibronic structure and determine the adiabatic singlet-triplet interval (1534±6cm^-1). The experimental spectra, a group-theoretical analysis of the vibronic coupling mechanisms, and ab initio calculations were used to establish the topology of the singlet potential energy surfaces and to characterize the pseudorotational motion of the cation on the lowest singlet potential energy surface. The analysis of the rovibronic photoionization dynamics in rotationally resolved spectra and the study of the variation of the intensity distribution with the intermediate vibrational level show that a Herzberg-Teller mechanism is responsible for the observation of the forbidden a^∼+E2'1←ÃA2″2 photoionizing transition.