SOLVENT EFFECTS IN DENSITY-FUNCTIONAL CALCULATIONS OF URACIL AND CYTOSINE TAUTOMERISM

The effects of the solvent on the tautomeric equilibria of cytosine an d uracil are studied using Onsager's reaction field model in the frame work of density functional theory. In this model, the solute molecule is placed in a spherical cavity of radius a(0) immersed in a continuou s medium with a dielectric constant epsilon, treating the solute-solve nt electrostatic interactions at the dipole level. The cavity radius i s evaluated by determining the molecular volume with a quantum mechani cal approach. The solvent effect brings significant changes in the geo metrical parameters of some cytosine tautomers, but only very small ch anges to those of the uracil tautomers. Our results are in good agreem ent with available experimental results and confirm that the polarizat ion of the solute by the continuum has important effects on the absolu te and relative solvation energies. Frequency shifts and intensity var iations in the infrared spectra due to the presence of the solvent are also presented. (C) 1995 John Wiley & Sons, Inc.