Structure and Dynamics of 3,5-Di-tert-butylpyrazole Probed by Combined X-ray Crystallography and 15N Solid State NMR

The crystal structure of the title compound was determined by X-ray crystallography: CIIHZON~, Pbca, 2 = 8, a = 11.4779(3), b = 21.1004(1), c = 9.9801(2) A, D, = 0.99 g ~m-~. The results show that crystals of 3,5-di-tert-butylpyrazole are formed by three types of dimers AA, AB or BA, and BB differing in the conformation of the tert-butyl groups. Variable temperature 15N-CPMAS-NMR experiments which were carried out on a solid I5N-1abeled sample indicate a conformational interconversion of the different dimers via tert-butyl group rotation as well as a nondegenerate intradimer double proton transfer. The rate and equilibrium constants of the tautomerism were obtained by l5N-CPMAS-NMR line-shape analysis. These constants represent averages of the different conformational states. The analysis of the data suggests that the tautomerism takes place only in the symmetric conformer AA where the tautomerism is degenerate. The observed nondegeneracy is then not the consequence of solid state effects, but of the intrinsic conformational exchange.