Hesam Sotoodeh - Department of Chemistry, Computational Quantum Chemistry Laboratory, University of Sistan and Baluchestan,Zahedan, Iran
Najmeh Mostafavi - Department of Chemistry, Computational Quantum Chemistry Laboratory, University of Sistan and Baluchestan,Zahedan, Iran
Ali Ebrahimi - Department of Chemistry, Computational Quantum Chemistry Laboratory, University of Sistan and Baluchestan,Zahedan, Iran

Phenytoin (Pht) has anticonvulsive, antiepileptic, and antiarrhythmic effects in the human organism(see Fig. 1). Tautomerismcan affect the chemical and biological activities of Pht. Inthe present work, the tautomerism in Pht has been investigated. The geometries of compoundswere optimized at the B3LYP/6-31G (d,p) level of theory using the Gaussian 09 program [2].As can be seen in Table 1, Pht with two H atoms on the N atoms of hydantoin ring is morestable than other tautomers with respect to the relative energy values (ΔEg). The stabilitygreatly decreases when two H atoms are located on the O atoms, (Pht4). The steric effectsseem to be very effective in the stability of tautomers.