High-Resolution Solid-State 13C and 15N NMR Spectroscopy of Pyrazole and 3,5-Dimethylpyrazole Adsorbed on Alumina and Silica

Abstract

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 components and by solvent-assisted adsorption. A comparison of the spectra 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 fast degenerate proton tautomerism is observed for adsorbed 1 and 2, similar to the liquid solution. Because of the different proton dynamics the line contributionns of adsorbed and bulk pyrazoles are easily distinguished. By analysis of the environment-dependent * H-l SN cross-polarization, efficiency quantitative information about the distribution of 2 between the bulk crystalline phase and the silica surface was obtained as a function of both types of sample preparation. The results are compatible with a monomolecular coverage of the silica surface by 2. Whereas the state of the adsorbate is not dependent of the type of sample preparation, the surface area accessible for 2 is smaller in the case of the mechanically grinded components as compared to the case of solvent-assisted loading. This result can be modeled in terms of the fractal dimension of the silica surface and slow diffusion of the adsorbate in the latter during the time of grinding. The nature of the proton transfer of 2 adsorbed on silica was studied by low-temperature lSN CPMAS NMR spectroscopy. The slow proton exchange regime is reached below 250 K. The spectral changes and their comparison with the spectra of the corresponding hydrochloride reveal that (i) 1 and 2 are adsorbed as the neutral species (ii) that there are different adsorption sites where 1 and 2 experience not only different chemical shifts but also different rate constants of proton exchange. At room temperature, site exchange, rotational diffusion, and proton exchange are faster than the Larmor frequency difference between the protonated and nonprotonated lSN atoms. Since the proton exchange in the cyclic pyrazole homoassociates is much slower than in the adsorbed state, it follows that it is catalyzed by OH groups on the alumina and silica surface. cross polarization, MAS