Tautomerism and polarizability in uracil: coupled cluster and density-functional theory study

Abstract Geometries, relative stabilities, static, dynamic and vibrational polarizabilities of the six uracil tautomers were calculated by coupled cluster (CCSD, CCSD(T)) and DFT B97-1 and PBE1PBE functionals in vacuo and in solution. The dioxo structure is invariably the neutral ground state of uracil, with the lowest energy keto-enol form about 11 kcal/mol less stable. The polarizability behaviour of the uracil tautomers has been characterized by computing static and dynamic values, vibrational contributions and solvent effects. Our best static gas-phase electronic CCSD(T)/aug-cc-pVDZ dipole polarizability of uracil is reported to be 70.22 a.u. Dynamic ( ω =0.10 a.u.) and vibrational contributions are significant, amounting to ca. 10% and 20%, respectively, of the static 〈 α e 〉 value. Solvent effects, investigated with the polarizable continuum model, show that both the static electronic and vibrational polarizabilities increase noticeably on passing from gas phase to solution, the α v / α e ratio being higher in solution than in vacuo. The throughout excellent agreement between CC and DFT results is worth noting.