Electronic spectra of azaindole and its excited state mixing: A symmetry-adapted cluster configuration interaction study

Electronic structures of azaindole were studied using symmetry-adapted cluster configuration interaction theory utilizing Dunning's cc-pVTZ basis set augmented with appropriate Rydberg spd functions on carbon and nitrogen atoms. The results obtained in the present study show good agreement with the available experimental values. Importantly, and contrary to previous theoretical studies, the excitation energy calculated for the important n-π^â- state agrees well with the experimental value. A recent study by Pratt and co-workers concluded that significant mixing of π-π^â- and n-π^â- states leads to major change in the magnitude and direction of the dipole moment of the upper state vibrational level in the 0,0 + 280 cm^-1 band in the S₁â†S₀ transition when compared to that of the zero-point level of the S₁ state. The present study, however, shows that all the four lowest lying excited states, ¹L_b π-π^â-, ¹L_a π-π^â-, n-π^â-, and π-σ^â-, cross each other in one way or another, and hence, significant state mixing between them is likely. The upper state vibrational level in the 0,0 + 280 cm^-1 band in the S₁â†S₀ transition benefits from this four-state mixing and this can explain the change in magnitude and direction of the dipole moment of the S₁ excited vibrational level. This multistate mixing, and especially the involvement of π-σ^â- state in mixing, could also provide a route for hydrogen atom detachment reactions. The electronic spectra of benzimidazole, a closely related system, were also investigated in the present study.