A SEMIEMPIRICAL AND ABINITIO MO STUDY OF THE TAUTOMERS OF N-UNSUBSTITUTED PYRAZOLONES (HYDROXY PYRAZOLES)

Semiempirical [MNDOC, MINDO/3, AM1, and MNDO] and ab initio (STO-3G and 4-31G basis sets) calculations on the relative stabilities, structures, and dipole moments of the 8 theoretically possible tautomeric forms of pyrazolone are reported. It is shown that MNDO + Cl and MINDO/3 predict that 5-hydroxy pyrazole, 3-hydroxy pyrazole, and 2-pyrazolin-5-on are the most stable. These results correspond to the known experimental data. Of all used quantum chemical methods, the MINDO/3 results for the dipole moments of the investigated tautomers are in best agreement with the known experimental data. The electronic excitation energies were calculated using the CNDO/S-Cl method. The results are in good agreement with the experimental UV spectra.