Gas-phase tautomerism in 2,4-dihydropyrazol-3-ones: an ultraviolet photoelectron spectroscopic study and MNDO molecular orbital calculations

Abstract

The He¹ photoelectron spectra of 2,5-dimethyl-, 5-methyl-2-phenyl-, 4,4,5-trimethyl-2-phenyl-, and 5-amino-2-phenyl-2,4-dihydropyrazol-3-one and 1,5-dimethyl-2-phenyl-1,2-dihydropyrazol-3-one have been recorded. Comparison of the spectra where three tautomeric forms may be present with those of the fixed tautomeric forms and with MNDO calculations indicates that the 2,4-dihydropyrazol-3-one is the predominant tautomeric form in the gas phase. The low ionization energy bands are assigned on the basis of MNDO calculations. The HOMO in 2,5-dimethyl-2,4-dihydropyrazol-3-one has n_N–2–π_CN character and the oxygen lone pair is the second outermost occupied MO. In the 2-phenyl derivatives, these MOs are in reverse order. The calculated relative stability of the tautomeric forms increases in the order 1,2-dihydropyrazol-3-one, 5-hydroxy-1H-pyrazole, 2,4-dihydropyrazol-3-one.