~~ 3(5)-Phenyl-and 5(3)-methyl-3(5)-phenylpyrazole have been studied using multinuclear NMR spectroscopy at low temperature to determine the tautomeric equilibrium constants in the slow proton exchange regime by simple signal integration. In order to compare the results in solution with those in the solid state, the X-ray structure of a derivative of the first, namely 4-bromo-3-phenylpyrazole was * Lists of thermal components, hydrogen parameters and bond distances and angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC). For details of the deposition scheme see 'Instructions for Authors,' J. Chem. Soc., Perkin Trans. 2, 1992, issue 1. * 3-216//3-216 calculations on fully optimized geometries 36 also favours 3a over 3b by 4.2 kJ mol-'.
structure structure (organic substances) K 9000
-063Electronic Absorption Spectra of Pyrazoles. Part 3. Semiempirical Calculations and the Problem of Tautomerism of 3(5)-Phenylpyrazole.-The structures of pyrazole, 3-phenylpyrazole, its 5-phenyl tautomer, 4phenylpyrazole and 3,5-diphenylpyrazole adopted in solution are investigated by UV and low temp. 13C NMR spectroscopy and semiempirical calculations. The NMR spectra reveal that the 3-phenyl tautomer is the major component in the tautomeric couple with its 5-phenyl tautomer. A mathematical model is presented for the comparison of the experimental and calculated electronic spectra. -(CATIVIELA, C.; GARCIA LAUREIRO, J. I.; ELGUERO, J.; ELGUERO, E.; Gazz.
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