1. Density Functional Theory, Cytosine, Tautomerism.

2. Keto - enol tautomerism is the most common tautomery phenomenon. Keto - enol themselves are the real chemical substance.

3. Tautomerism is the common existence in Organic Chemistry. In this paper, the process of Tautomerizing relationship is further analyzed.

4. Then, using another tautomer(2H-indazole form), all of the above processes were repeated to investigate the effect of tautomerism upon the QSAR modelling.

5. The states of active hydrogens, the hindered rotation within molecules and the kinetic phenomena of tautomerism have also been discussed.

6. The most usual problems encountered in the structure handling are tautomerism, alternating and aromatic systems.

7. This complex reaction is characterized by a keto - enol tautomerism equilibrium and ester exchange reaction.