HIGH-LEVEL ELECTRON CORRELATION CALCULATIONS ON SOME TAUTOMERS OF CYTOSINE

Five isomers of cytosine, representing three tautomers and rotamers fo r two of them, have been investigated by ab initio techniques. Complet e 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 ex citations to obtain energies at the MBPT(2) geometries. In the HF calc ulations basis sets up to 6-311G(2d,2p) were tested. The electron corr elation calculations applied a DZP basis set. In the geometries, bond lengths reflect the varying formal bond orders of the tautomers, speci fically 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 ene rgy. A qualitatively new result of the CCSD/DZP//MBPT(2)/DZP calculati ons is that the imino-oxo tautomer may be much more stable than though t previously, at only 0.2-0.3 kcal relative to the ground state. The o rder of energies in these high level calculations shows the pattern: a mino hydroxy < imino-oxo < amino-oxo. (C) 1997 Elsevier Science B.V.