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

In this paper we examine the Jahn–Teller (JT) and pseudo-Jahn–Teller (PJT) effects in the spiropentane radical cation (SP<SUP>+</SUP>) by an ab initio quantum dynamical method. Spiropentane (SP) possesses D<SUB>2d</SUB> symmetry at its equilibrium configuration. The two low-lying electronic states of SP<SUP>+</SUP> belong to X&#732;<SUP>2</SUP>B<SUB>2</SUB> and &#195;<SUP>2</SUP>E symmetry, respectively. SP<SUP>+</SUP> in the degenerate &#195;state is susceptible to JT distortions along the vibrational modes of b symmetry. The &#195; state of SP<SUP>+</SUP> is vertically ∼0.51 eV spaced from its X&#732; state. Symmetry rule allows a coupling of the X&#732; and &#195; states via the degenerate e vibrational modes. This is termed as the (B + E ⊗ b) ⊗ 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&#732;–&#195;bands of SP<SUP>+