A density functional study of tautomerism of uracil and cytosine

The structure and the relative energies of all possible tautomeric forms of the uracil and cytosine molecules have been determined using both local and gradient-corrected density functional methods. The calculations have been performed with double-zeta plus polarization basis sets and the geometries optimized with analytic gradient techniques. The vibrational frequencies and the contribution of the zero-point energies have also been computed. In the uracil case, the dioxo form is predicted to be the most stable. In the cytosine case, three tautomers are found to be very close in energy with the oxo-amino form slightly more stable. The infrared absorption intensities and frequencies for the uracil and the two more stable tautomeric forms of the cytosine molecules are reported and compared with experimental spectra. The agreement with experiment and correlated ab-initio methods is good for geometries, energetics, and vibrational frequencies. © 1994 American Chemical Society.