title: Comparison of the non-radiative decay mechanisms of 4-pyrimidinone  and uracil: an ab initio study.
abstract: 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 S(0) and S(1) states (one for uracil, two for 4-pyrimidinone) are established. They are accessible from the Franck-Condon region of the 1pipi* state through out-of-plane deformations related to C=C (for uracil) or C=N (for 4-pyrimidinone) torsions of the heterocyclic ring. These conical intersections mediate the radiationless deactivation of the compounds after excitation of the lowest 1pipi* state. The relaxation of the 1pipi* state of 4-pyrimidinone via C=C twisting is hindered by a barrier. The relaxed scan of the C=N 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.
authors.affiliation: Department of Chemistry, Technische Universität München, D-85747 Garching, Germany. vdelchev@uni-plovdiv.bg
authors.forename: Vassil B
authors.initials: VB
authors.lastname: Delchev
authors.forename: Andrzej L
authors.initials: AL
authors.lastname: Sobolewski
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Domcke
mesh_terms: D011744:Pyrimidinones / Q000737:chemistry*; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics; D014498:Uracil / Q000737:chemistry*
chemical_list: D011744:Pyrimidinones; D014498:Uracil