Quantum-Chemical Study of the Structure and Reactivity of Pyrazol-5-ones and Their Thio and Seleno Analogs: VI. Tautomeric and Acid-Base Properties of 3-Methyl-1-phenyl-4,5-dihydropyrazol-5-one, 3-Methyl- 1-phenyl-4,5-dihydropyrazole-5-thione, and 3-Methyl-1-phenyl-4,5-dihydropyrazole-5-selenone

Abstract

The relative stabilities of tautomeric forms of 3-methyl-1-phenylpyrazol-5-one and its 5-thioxo and 5-selenoxo analogs, as well as their acid-base properties in the gas phase, were estimated in terms of nonempirical calculations and density functional theory. According to the results of both calculation methods, the CH tautomer of 3-methyl-1-phenylpyrazol-5-one is the most stable. The stabilities of the XH and CH forms of its heteroanalogs (X = S, Se) are comparable; the relative stability of the SeH (SH) tautomers increases when thermal corrections, zero-point energy, and electron correlation effects are taken into account. The two methods indicate increase in the gas-phase acidity of the title compounds on variation of the heteroatom in the series O <S < Se and fairly similar basicities of the oxygen and sulfur analogs.