Several isomeric structures of the uracil−water complex and its covalent-bound anion were calculated ab initio with second-order, many-body, perturbation theory and the 6-311++G** basis set. In all neutral complexes, water forms two hydrogen bonds with uracil. In each of the conventional anionic forms, a single, but stronger and shorter, hydrogen bond is found. All complexes 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 isomers with vertical electron-detachment energies between 0.30 and 0.90 eV accounts for the broad photoelectron spectrum. The lowest unoccupied molecular orbital of the neutral complex at the geometry of the anionic complex provides a simple explanation for the structural and energetic consequences of electron attachment.

Electron propagator methods are applied to the ionization energies of the five most stable tautomers of guanine. Excellent agreement with gas-phase photoelectron spectra is obtained for the amino-oxo form of 7H-guanine. According to ionization energy assignments, both 9H-guanine and its amino-oxy tautomers also may be present in the gas phase. The presence of amino-oxy 7H-guanine, however, is less certain, due to its higher total energy. In all cases, the lowest ionization occurs from a π level. There are strong correlation effects for higher cationic states. Energy orderings of π and σ hole states are different for each of the isomers.

Diffuse-bound and valence-bound anions of cytosine have been examined with correlated, ab initio calculations employing large basis sets. Five structures of the cytosine anion have been considered for both kinds of anions. Adiabatic electron affinities of neutrals and vertical electron detachment energies were determined with perturbative, coupled-cluster and electron propagator methods. Basis sets with many diffuse, atom-centered functions were used. Among the five tautomeric cytosine structures, only the 1H-amino-oxo isomer is capable of forming both diffuse and valence-bound anions. Two imino-oxo isomers produce valence-bound anions with positive vertical electron detachment energies. The most stable neutrals, the amino-oxy tautomers, have adiabatic electron affinities that are negative with respect to both dipole-bound and valence-bound anionic forms. Valence-bound, amino-oxy anions are not even vertically bound with respect to electron loss. Diffuse-bound anions of the amino-oxy and imino-oxo isomers have negative adiabatic electron detachment energies. Close agreement with the lowest peak in an anion photoelectrum spectrum obtains for the vertical electron detachment energy of the diffuse-bound anion of 1H-amino-oxo cytosine. A peak of higher energy is assigned to the valence-bound anions of 1H-amino-oxo and imino-oxo tautomers.

Double-Rydberg anions consist of a closed-shell cationic core and two electrons in diffuse orbitals. In addition to the already characterized tetrahedral NH 4and pyramidal OH 3 -, the list of stable double-Rydberg anions includes members of the NH 3 Rand OH 2 Rsets (where R ) F, OH, NH 2 , and CH 3 ) considered here. Double-Rydberg isomers of NH 3 OH -, NH 3 NH 2 -, NH 3 CH 3 -, and OH 2 CH 3are stable minima in their potential energy surfaces and have positive, vertical electron detachment energies. Similar structures have been found for the corresponding neutrals, which all have adiabatic electron affinities that are greater than zero. Dyson orbitals pertaining to the least bound electrons of the NH 3 Rand OH 2 Ranions are distributed chiefly outside the N-H and O-H bonds of the tetrahedral nitrogen and pyramidal oxygen atoms.