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 (1)pisigma* 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 (1)pisigma* potential-energy function intersects not only those of the (1)pipi* and (1)npi* excited states, but also that of the electronic ground state. The (1)pipi*-(1)pisigma* and (1)pisigma*-S-0 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 (1)pipi* and (1)npi* states by the (1)pisigma* state and the conical intersection of the (1)pisigma* state with the S-0 state provide the mechanism for the ultrafast radiationless deactivation of the excited singlet states of adenine.