An ab initio Study of Decay Mechanism of Adenine: the Facile Path of the Amino NH Bond Cleavage

A comprehensive study of the radiationless decay processes of the lowest excited singlet states in the isolated 9H‐Adenine has been performed at the CASPT2//CASSCF level. The minimum energy paths of the La, Lb and nπ* singlet states along different skeletal distortions have been computed and the Conical Intersections (CIs) involving these states have been determined. The fast deactivation path of La along a skeletal deformation, which leads to a S0/La CI, as previously discussed, is confirmed. Moreover, low‐lying CIs between S0 and πσ* singlet states have been characterized, where σ* is the antibonding orbital localized on a N‐H bond of the amino (πσNH2*) or of the azine group (πσN9H*). We have found that the repulsive πσNH2* state associated with an amino N‐H bond can be populated through a barrierless way. Therefore, the decay path shows a bifurcation leading to two possible ways of radiationless deactivation: on one hand a non‐photochemical decay through the S0/La or S0/nπ* CIs and on the other hand a photochemical process via the possible access to the S0/πσNH2* CI that produces N‐H cleavage. In this way, we can explain the H atom loss found upon UV excitation. We have considered also the decay of higher energy bright states. We have found that these states can decay also by converting to the repulsive πσN9H* state associated with the azine NH bond. This new channel suggests an increase of H‐atom photoproduction yield by excitating Adenine with lower wavelength radiations. The study of the decay processes of an Adenine molecule in the double strand d(A)10⋅d(T)10 in water solvent is currently underway: Adenine is treated by the Quantum Mechanical (QM) approach and the remaining molecules are described at the Molecular Mechanics (MM) level. We use the COBRAMM program that is a tunable QM/MM approach to complex molecular architectures developed by our research group.