THEORETICAL DENSITY-FUNCTIONAL STUDIES OF TAUTOMERIC AND CONFORMATIONAL FORMS OF N-4-HYDROXYCYTOSINE AND ITS 5-FLUORO AND 5-METHYL DERIVATIVES

Density functional (DFT) calculations at the BLYP/DNP and B3LYP/6-31G( d,p) levels were performed for amino and imino tautomers of N-4-hydrox ycytosine (HC), N-4-hydroxy-5-fluorocytosine (5F-HC) and N-4-hydroxy-5 -methylcytosine (5Me-HC). Full optimization of the molecular skeleton and zero-point vibrational corrections computed at the B3LYP level wer e included in the energy analysis. The results are compared with ab in itio MBPT(2)/6-31G(d,p)//HF/6-31G(d,p) calculations [1] performed for HC and 5F-HC. The DFT methods, and in particular the B3LYP approximati on, predict properly the higher stability of the imino tautomers, and with lower computational costs reproduce very well the energy differen ces obtained using the conventional ab initio methods.