Revealing excited state interactions by quantum-chemical modeling of vibronic activities: the R2PI spectrum of adenine.

We present a computational study encompassing quantum-chemical calculations of the ground and low-lying excited states of 9H-adenine and modeling of vibronic activities associated with the S(0) --> L(b) and S(0) --> n pi* transitions. Minima on the ground and excited states and the saddle point on the n pi* potential energy surface are determined with CASSCF calculations. Vibrational frequencies are computed at the same level of theory on ground and excited states while transition dipole moments and oscillator strengths are estimated, at the optimized geometries, with CASPT2//CASSCF calculations. Modeling of vibronic activities includes both Franck-Condon and Herzberg-Teller induced contributions. While the adopted harmonic approximation is acceptable for the S(0) --> L(b) transition and allows the assignment of several observed bands in the R2PI spectrum of adenine, the computed anharmonic potential along the puckering coordinate in the n pi* state requires a different treatment. To this end the vibronic levels and intensities associated with vibronic transitions in the puckering coordinate are evaluated by numerical solution of the 2D potential including the anharmonic puckering coordinate. All the remaining vibrational coordinates are treated as harmonic. On the basis of the modeling, the four major bands in the R2PI spectrum of adenine are assigned, along with a number of minor bands in the spectra.