Abstract: A MINDO/3 MO method has been applied to study the tautomeric reaction of the N_1-H-4-NH_2-2-oxo-cytosine and 4-NH-2-oxo-cytosine. The geometries of the molecules involved in the tautomeric reaction are optimized by energy gradient method using MINDO/3 program. The results show that the amino-tautomer is more stable than imino-tautomer by △E=33.85 kJ mol~(-1).
The geometry of the transition state is optimized by Powell's method. The reaction path of the tautomeric reaction is presented, based on Fukui's IRC theory. The physical nature of the reaction is revealed by analysing the intramolecular charge transfer as the tautomeric reaction is occurring. The activation arrier of the forward reaction is 168.87 kJ mol~(-1), while it is 135.02 kJ mol~(-1) for the backward reaction.