Photo-Ionization Spectrum of 1,2-Azaborine: A Theoretical Study

Multidimensional quantum mechanical investigations are carried out to understand the multistate and multimode vibronic interactions in the first seven low-lying, viz., ${\tilde{X}}^{2}A″$ , ${\tilde{A}}^{2}A\text{'}\text{'}$ , ${\tilde{B}}^{2}A\text{'}$ , ${\tilde{C}}^{2}A\text{'}$ , ${\tilde{D}}^{2}A\text{'}\text{'}$ , ${\tilde{E}}^{2}A\text{'}$ , and ${\tilde{F}}^{2}A\text{'}$ , electronic states of azaborine radical cation. Vibronic coupling theory is employed in coordination with symmetry selection rules to develop the theoretical framework in a diabatic electronic basis and symmetry selection rules. A model 7 × 7 diabatic electronic Hamiltonian is constructed. The parameters of the diabatic Hamiltonian are estimated by performing extensive calculations of adibatic electronic energies ab initio employing equation-of-motion ionization-potential coupled-cluster singles and doubles (EOM-IP-CCSD) method. The nuclear dynamics calculations are performed by both time-independent and time-dependent quantum mechanical methods. Theoretically calculated vibronic structures of seven electronic states are found to be in excellent agreement with the available experimental findings. Vibrational progressions estimated from the calculated theoretical spectrum are assigned. It is found that extremely strong vibronic interactions among the ${\tilde{A}}^2{A}^{\text{'}\text{'}}$ - ${\tilde{B}}^2{A}^{\text{'}}$ - ${\tilde{C}}^2{A}^{\text{'}}$ - ${\tilde{D}}^2{A}^{\text{'}\text{'}}$ - ${\tilde{E}}^2{A}^{\text{'}}$ and ${\tilde{F}}^2{A}^{\text{'}}$ electronic states result into highly overlapping vibronic bands due to multiple multi-state conical intersections. The impact of associated nonadiabatic effects on the vibronic structure and dynamics of these electronic states are examined at length.