High-level electron correlation calculations on some tautomers of cytosine

Five isomers of cytosine, representing three tautomers and rotamers for two of them, have been investigated by ab initio techniques. Complete geometry optimizations were carried out at the Hartree-Fock (HF) and second-order perturbation theory, MBPT(2), levels. Our highest-level calculations used the coupled cluster method with single and double excitations to obtain energies at the MBPT(2) geometries. In the HF calculations basis sets up to 6-311G(2d,2p) were tested. The electron correlation calculations applied a DZP basis set. In the geometries, bond lengths reflect the varying formal bond orders of the tautomers, specifically the aromatic character of the pyrimidine ring in the amino-oxo tautomer. This latter form is found to be the most stable one in all calculations, in accordance with recent related literature results. It is found that its rotamer pair lies only about 0.8 kcal higher in energy. A qualitatively new result of the CCSD/DZP//MBPT(2)/DZP calculations is that the imino-oxo tautomer may be much more stable than thought previously, at only 0.2-0.3 kcal relative to the ground state. The order of energies in these high level calculations shows the pattern: amino hydroxy < imino-oxo < amino-oxo. (C) 1997 Elsevier Science B.V.