HIGH-RESOLUTION SOLID-STATE C-13 AND N-15 NMR-SPECTROSCOPY OF PYRAZOLE AND 3,5-DIMETHYLPYRAZOLE ADSORBED ON ALUMINA AND SILICA

High-resolution solid state C-13 and N-15 CPMAS NMR experiments (CP = cross polarization, MAS = magic angle spinning) have been performed on mixtures of pyrazole (1) and of 3,5-dimethylpyrazole (2) with alumina and silica, prepared both by mixing and mechanical grinding of the co mponents and by solvent-assisted adsorption. A comparison of the spect ra obtained with those of the bulk solids shows a strong dependence of the NH N proton tautomerism on the environment. Whereas prototropy is suppressed in crystalline 1 and moderately fast in crystalline 2, a f ast degenerate proton tautomerism is observed for adsorbed 1 and 2, si milar to the liquid solution. Because of the different proton dynamics the line contributions of adsorbed and bulk pyrazoles are easily dist inguished. By analysis of the environment-dependent H-1-N-15 cross-pol arization, efficiency quantitative information about the distribution of 2 between the bulk crystalline phase and the silica surface was obt ained as a function of both types of sample preparation. The results a re compatible with a monomolecular coverage of the silica surface by 2 . Whereas the state of the adsorbate is not dependent of the type of s ample preparation, the surface area accessible for 2 is smaller in the case of the mechanically grinded components as compared to the case o f solvent-assisted loading. This result can be modeled in terms of the fractal dimension of the silica surface and slow diffusion of the ads orbate in the latter during the time of grinding. The nature of the pr oton transfer of 2 adsorbed on silica was studied by low-temperature N -15 CPMAS NMR spectroscopy. The slow proton exchange regime is reached below 250 K. The spectral changes and their comparison with the spect ra of the corresponding hydrochloride reveal that (i) 1 and 2 are adso rbed as the neutral species (ii) that there are different adsorption s ites where 1 and 2 experience not only different chemical shifts but a lso different rate constants of proton exchange. At room temperature, site exchange, rotational diffusion, and proton exchange are faster th an the Larmor frequency difference between the protonated and nonproto nated N-15 atoms. Since the proton exchange in the cyclic pyrazole hom oassociates is much slower than in the adsorbed state, it follows that it is catalyzed by OH groups on the alumina and silica surface.