| Abstract
| - Geometry optimizations on complexes composed of an Al atom, a guanine molecule, and an ammonia moleculehave been performed with density functional methods. In the most stable structures, the ammonia moleculeforms hydrogen bonds with previously studied Al−guanine complexes. The two lowest structures correspondto an unusual tautomer of guanine in which both N atoms of the five-membered ring, N7 and N9, are protonated.Within 3 kcal/mol in energy lie two additional structures in which a proton is shifted from N9 to N3. Ionizationenergies calculated with ab initio, electron-propagator methods for the two latter structures are in closeagreement with the experimentally observed ionization threshold. Higher ionization energies are obtained forthe two lowest structures. Dyson orbitals for the lowest ionization energies are guanine π* functions. Theorder of isomers in the cationic species is different from that of the neutrals. Energies of ammonia dissociationare approximately equal for all forms of the Al−guanine−NH3 complex, except for a structure with an Al−NH3 dative bond.
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