Does tautomeric equilibrium exist in 4-nitroso-5-pyrazolones?

Abstract

The structure and relative stabilities of the tautomers and isomers of 4-nitroso-pyrazolones were investigated at HF, DFT and MPn ( n = 2, 4) quantum chemical levels. The effect of the medium on the relative stabilities of the different structures of the studied compounds was estimated using the polarizable continuum model. 1H, 13C and 15N NMR chemical shieldings and coupling constants were calculated using the GIAO approach at B3LYP/6-31+G(d,p) level and MP2/6-31+G(d,p) optimized geometries. The calculated relative energies of the tautomers and isomers of 3-methyl-4-nitroso-5-pyrazolone and 1-ethyl-3-methyl-4-nitroso-5-pyrazolone suggest that in solution the oxime tautomer is favoured. The second compound exists as a mixture of syn- and anti-oxime isomers. The coexistence of the oxime forms in solution corresponds to the available experimental NMR data.