H. Tavakol, Study of tautomerism in azirine and related systems: DFT calculations in the gas phase and solvent, ICYC 2010, Penang, Malaysia, 23-25 June 2010

Abstract:

Tautomerism interconversions have been mostly investigated by chemists during last centuries. Recently, study of tautomerism received renewed attention due to its importance on determination of compounds' properties and area of applications. The importance of tautomerism is revealed more since in recent years the investigation about tautomerism has been one of the major topics in theoretical chemistry. In the course of recent studies on the tautomerism of organic compounds, we focused our new research on azirine, dihydroazete, dihydropyrrole and tetrahydropyridine. All of these compounds are N-containing heterocycles with the existence of two tautomeric forms, one is imine form (I) and the other is enamine form (E). In this research, DFT calculations were applied to optimize the structures of azirine, dihydroazete, dihydropyrrole and tetrahydropyridine tautomers and transition states at B3LYP/6-311++G** and B3LYP/AUG-CC-PVTZ levels of theory. Moreover, solvation effects were calculated in three different solvent (cyclohexane, chloroform and DMSO). From these calculations, important molecular parameters, IR frequencies, and energetic results in the gas phase and solvents were extracted. The results show that, in 3 to 5-membered rings, the imine tautomer is more stable than enamine tautomer and this difference decreases by increasing ring size, but in 6-membered rind, enamine is more stable than imine tautomer. As well as, the barrier energies of tautomerisms are very high and various solvents have a little effect of on kinetic and thermodynamic data. Moreover, frequencies show that by decreasing ring size from 6 to 4-membered ring, the order of endocyclic double bonds decreases, but in 3-membered ring, substantial increasing was observed.