Anionic and neutral complexes of uracil and water

Several isomeric structures of the uracil-water complex and its covalent-bo und anion were calculated ab initio with second-order, many-body, perturbat ion theory and the 6-311++G** basis set. In all neutral complexes, water fo rms two hydrogen bonds with uracil. In each of the conventional anionic for ms, a single, but stronger and shorter, hydrogen bond is found. All complex es are nonplanar, but ring-puckering is less pronounced in neutrals than in anions. Several isomers of the anionic uracil-water complex have positive adiabatic electron-detachment energies. The existence of multiple anionic i somers with vertical electron-detachment energies between 0.30 and 0.90 eV accounts for the broad photoelectron spectrum. The lowest unoccupied molecu lar orbital of the neutral complex at the geometry of the anionic complex p rovides a simple explanation for the structural and energetic consequences of electron attachment.