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Quantum chemical calculations on the title compounds have been performed at different levels of theory in order to obtain a reasonable model capable of treating even larger systems with reliable results. The HF/6-31G*//PM3 model is shown to describe the cations reasonably well as compared to the corresponding ab initio results. The anions and neutral compounds yield reasonable results at the HF/ 6-31+G*//PM3 level. In the case of the oxygen compound the CH form is predicted to be the most stable tautomer at all levels of theory. The CH and the XH tautomer become energetically similar for the sulphur and selenium compounds, and the relative stability depends on the model used. All methods predict the acidity to increase in the order O&lt;S&lt;Se. Differences in basicity are not so pronounced. The most stable cation is always the N-protonated XH tautomer and is followed by the X-protonated CH tautomer. All other cations are calculated to be clearly higher in energy.

zeitschrift für naturforschung b

ab initio-Rechnungen zur Tautomerie, Acidität und Basizität von 1-MethyIpyrazolin- 2-on-5 und seinen Schwefel und Selen Analogen / ab initio Calculations of the Tautomerism, the Acidity and the Basicity of 1-Methyl- pyrazoline-2-one-5 and the Corresponding Sulphur and Selenium Analogs