INTRODUCTION Tautomerism interconversions 1,2 have been investigated by chemists during last decades. Recently, study of tautomerism received renewed attention due to its importance on the determination of compounds properties and their area of applications. The importance of tautomerism is revealed more since in recent years the investigation about tautomerism has been the major topic in theoretical chemistry. For example, tautomerism in keto-enol 3,4 , imine–enamine 5,6 , purine 7 , pirimidine 8 and many other systems 9 . have ORIENTAL JOURNAL OF CHEMISTRY www.orientjchem.org Est. 1984 An International Open Free Access, Peer Reviewed Research Journal ISSN: 0970-020 X CODEN: OJCHEG 2013, Vol. 29, No. (1): Pg. 69-74 A DFT Study of Solvation Effects on Tautomerism of 6-oxo Purine by Polarisable Continuum Method (PCM) ZABIALAH HEIDARNEZHAD 1 *, FATEMEH HEIDARNEZHAD 1 , FATEMEH HEYDARI 2 and ELHAM BAHRAMIAN 3 1 Andimeshk Branch, Islamic Azad University, Andimeshk, Iran. 2 Department of Physics, Kermanshah Science and Research, Islamic Azad University, Kermanshah, Iran. 3 Shushtar Branch, Islamic Azad University, Shushtar, Iran. *Corresponding author E-mail:Z.heidarnezhad@gmail.com (Received: November 11, 2012; Accepted: December 30, 2012) ABSTRACT Computational calculations at B3LYP/CC-PVDZ level were employed in the study of tautomers of 6-ox purine (MH) in the gas phase and selected solvents such as benzene, tetrahydrofuran (THF), methanol and water using PCM model. All tautomers are optimized at this level. In addition, stability of the tautomers in different solvents shows interesting results. In the gas phase and benzene (non-polarisable solvent) OP2 form is more stable than the other forms but in polarisable solvents (THF, methanol and water) OP1 is the most stabilized form. Variation of dipole moments and NBO charges on atoms in the solvents were studied. Key words: NBO charge, DFT Studies, PCM model, 6-oxo purine, Tautomerism. been studied during the past decades. Thereupon, compounds containing different tautomers can be the subject of interest by theoretical chemists 10 . Purine bases and their thio derivatives are found in many biological systems and they are very important from clinical point of view. For example, 6-thiopurine (THIOP) is one of the purine analogues most commonly used in the treatment of the acute human leukaemia 11 . It is also known that 6- thiopurine derivatives, among them 2-amino-6- thiopurine has interesting chemotherapy effects, including powerful antitumoral activity 12 . The oxopurines are of biological importance, because