(B + E x b) x e Jahn-Teller and pseudo-Jahn-Teller effects in the spiropentane radical cation

Graphical abstract: Static and dynamic aspects of the Jahn-Teller and pseudo-Jahn-Teller effects in spiropentane radical cation of tetragonal equilibrium symmetry studied by ab initio quantum mechanical methods. - Abstract: In this paper we examine the Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) effects in the spiropentane radical cation (SP+) by an ab initio quantum dynamical method. Spiropentane (SP) possesses D2d symmetry at its equilibrium configuration. The two low-lying electronic states of SP+ belong to X∼2B2 and A∼2E symmetry, respectively. SP+ in the degenerate A∼ state is susceptible to JT distortions along the vibrational modes of b symmetry. The A∼ state of SP+ is vertically ∼0.51 eV spaced from its X∼ state. Symmetry rule allows a coupling of the X∼ and A∼ states via the degenerate e vibrational modes. This is termed as the (B + E x b) x e JT and PJT effects revealing the symmetry of the electronic states and the coupling vibrational modes. The theoretical findings establish significant impact of the JT and PJT coupling in the observed complex structure of the X∼-A∼ bands of SP+.