Comparison of the non-radiative decay mechanisms of 4-pyrimidinone and uracil: An ab initio study

We performed a comparative theoretical study of the relaxation mechanisms of the excited states of uracil and 4-pyrimidinone with the CASSCF, CASPT2, and CC2 ab initio methods. The calculated vertical excitation energies agree with the experimental UV absorption maxima of the two compounds. Three low-lying conical intersections between the S₀ and S₁ states (one for uracil, two for 4-pyrimidinone) are established. They are accessible from the Franck-Condon region of the ¹ππ* state through out-of-plane deformations related to CC (for uracil) or CN (for 4-pyrimidinone) torsions of the heterocyclic ring. These conical intersections mediate the radiationless deactivation of the compounds after excitation of the lowest ¹ππ* state. The relaxation of the ¹ππ * state of 4-pyrimidinone via CC twisting is hindered by a barrier. The relaxed scan of the CN double-bond twisting of 4-pyrimidinone indicates that the formation of the Dewar form may represent a photochemical channel in 4-pyrimidinone. This fact is detrimental for the photostability of 4-pyrimidinone, since the Dewar form is separated by a high potential-energy barrier from the canonical form of 4-pyrimidinone on the ground-state potential-energy surface, which prevents a thermal back-reaction. The investigation of the vertical excitation energies and the reaction paths shows that 4-pyrimidinone is less photostable than uracil.