DOI: 10.1140/epjd/e2002-00164-5 THE EUROPEAN PHYSICAL JOURNAL D On the mechanism of nonradiative decay of DNA bases: ab initio and TDDFT results for the excited states of 9H-adenine

Abstract. Minimum-energy reaction paths and corresponding potential-energy proles have been com-puted for the lowest excited states of the amino form of 9H-adenine. Complete-active- space self-consistent-eld (CASSCF) and density functional theory (DFT) methods have been employed. The potential-energy function of the lowest 1 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 potential-energy function inter-sects not only those of the 1 and 1n excited states, but also that of the electronic ground state. The 1{1 and 1{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 1 and 1n states by the 1 state and the conical intersection of the 1 state with the S0 state provide the mechanism for the ultrafast radiationless deactivation of the excited singlet states of adenine. PACS. 31.15.Ew Density-functional theory { 31.25.Qm Electron correlation calculations for polyatomic molecules { 31.50.Gh Surface crossings, non-adiabatic couplings