A Theoretical Survey of Tautomerism Stability of 4-hydroxyquinoline Considering Solvent Effect by PCM Model

Computational calculations at B3LYP/6-311++G (d,p) level were employed in the study of the predominant tautomeric of 4-hydroxyquinoline derivatives (5-H, 5-NO2, 5-Cl, 5-OH, 5- CH3, 6-NO2, 6-Cl, 6-OH, 6-CH3, 7-NO2, 7-Cl, 7-OH, 7-CH3, 8-NO2, 8-Cl, 8-OH, 8-CH3, ) in the gas phase and selected solvents (benzene (non-polar solvent), tetrahydrofuran (THF) (polar aprotic solvent) and water (protic solvent)). The tautomers were also optimized in solvents according to the polarisable continuum method (PCM). For electron withdrawing and releasing derivatives the order of stability is X1 >X2 >X3, and X1 form is a more stable and dominant form. An exception in the gas phase is X2 isomer of (8-OH) which has two forms. In one, the hydrogen bond between hydroxyl substituted with nitrogen isn’t formed, but in another it is formed. If the hydrogen bond is formed, X2 isomer is more stable than X1. In addition variation of dipole moments and charges on atoms in the solvents are studied.