dft study of kinetic and thermodynamic parameters of tautomerism in 4−acyl pyrazolone

in the present work, dft calculations are employed to obtain the optimized structures of 4-acyl pyrazolone tautomers (19 tautomers) using b3lyp/6-311++g** calculations. inaddition, molecular parameters, ir frequencies and relative energies are extracted for alltautomers. the existence of aromatic ring, keto tautomer (versus enol tautomer), n-h bond(versus c-h bond) and c=n double bond (versus n=n double bond) are stabilizing factors inrelative stabilities of tautomers. calculation of vibrational frequencies showed that, inaccordance with reported values, intramolecular hydrogen bond (existed in some tautomers)decreased the value of oh frequency. the solvent effects on relative stabilities of tautomersare calculated. the relative stabilities of all the tautomers in acetone, tetrahydrofurane andchloroform (in all solvents, except water) were relatively the same as those in the gas phase.in addition, a nearly good relationship is found between dipole moments of tautomers andtheir 7gsolv in chloroform. this relation shows that by increasing the dipole moment, theabsolute amount of 7gsolv in chloroform increases.