The static and dynamic aspects of (E ⊗ e) – Jahn-Teller (JT) and (E + A) ⊗ e   Pseudo Jahn-Teller (PJT) interactions in the $\tilde{X}$ -$\tilde{A}$ -$\tilde{B}$ -$\tilde{C}$ electronic states of the barrelene radical cation (BL⁺) are investigated with the aid of an ab initio based quantum dynamical approach.

Nonadiabatic nuclear motion on the coupled manifold of $\tilde{X}$ -$\tilde{A}$ -$\tilde{B}$ -$\tilde{C}$ electronic states are simulated both by time-independent and time-dependent quantal methods.

The JT stabilization energy is ∼ 0.25 eV in the electronic state $\tilde{A}$ of BL⁺ and the PJT interactions between $\tilde{B}$ -$\tilde{C}$ coupled electronic states of BL⁺ is stronger.

The internal conversion rate for the $\tilde{A}$, $\tilde{B}$ and $\tilde{C}$ electronic states are ∼ 50 fs, ∼ 20 fs and 7 fs, respectively.

The theoretical results are found to be in very good accord with the experimental data.