On the mechanism of nonradiative decay of DNA bases: Ab initio and TDDFT results for the excited states of 9H-adenine

Minimum-energy reaction paths and corresponding potential-energy profiles have been computed for the lowest excited states of the amino form of 9H-adenine. Complete-active-space self-consistent-field (CASSCF) and density functional theory (DFT) methods have been employed. The potential-energy function of the lowest ¹πσ* state, nominally a 3s Rydberg state, is found to be dissociative with respect to the stretching of the NH bond length of the azine group. The ¹πσ* potential-energy function intersects not only those of the ¹ππ* and ¹nπ* excited states, but also that of the electronic ground state. The ¹ππ* - ¹πσ* and ¹πσ* -S₀ intersections are converted into conical intersections when the out-of-plane motion of the active hydrogen atom is taken into account. It is argued that the predissociation of the ¹ππ* and ¹nπ* states by the ¹πσ* state and the conical intersection of the ¹πσ* state with the S₀ state provide the mechanism for the ultrafast radiationless deactivation of the excited singlet states of adenine.