15N Chemical-Shift Tensors and Hydrogen-Bond Geometries of 3,5-Substituted Pyrazoles and Their Orientation in the Molecular Frame (original) (raw)

1996, Journal of Magnetic Resonance, Series A

The results of various 15 N solid-state NMR experiments perin the molecular principal axis system (PAS) is necessary formed on solid samples of doubly 15 N-labeled 3,5-dimethylpyrafor the analysis of several solid-state NMR experiments, e.g., zole, 5-methyl-3-phenylpyrazole, (PMP), and 3,5-diphenylpyrafor distance measurements by dipolar couplings in static zole are reported. In the solid state, these compounds form various powder spectra (4), by rotational resonance experiments (5) hydrogen-bonded complexes. The principal values of the 15 N or multipulse techniques (6), for the evaluation of structures chemical-shift tensors (CST) of amine and imine nitrogen atoms by CST correlation (7), for the detection of molecular moare derived by lineshape analysis of the 15 N NMR spectra of the tions (8), or for the analysis of relaxation phenomena (9). static powders obtained under the conditions of 1 H-15 N cross po-The size of the three principal elements of the CST can larization and 1 H decoupling. The orientations of the 15 N CST in provide additional information, which is empirically interthe molecular principal axis system are obtained by taking into preted and becomes more important with an increasing numaccount the 15 N-15 N dipolar interactions and the 15 N-D dipolar interaction after deuteration of the mobile proton sites. The rela-ber of data available (10). tive orientations of the amine and imine CST in PMP are indepen-In the present paper, we report our investigation of 15 N dently checked by one-dimensional off-magic axis sample spinning CST in 3,5-dimethylpyrazole (DMP), b-5-methyl-3-phemagnetization transfer experiments. The isotropic chemical shifts, nylpyrazole (b-PMP), and 3,5-diphenylpyrazole (DPP) the principal elements, and the orientations of the CST of both (Fig. 1). Our interest in these compounds actually stems nitrogen atoms and the ND distances depend only slightly on the from the study of their solid-state hydrogen bonding and chemical structure and the associated hydrogen-bonded structure. proton tautomerism. The chemical structure variations of The intramolecular structure of pyrazoles, therefore, does not vary these compounds leads to interesting differences in the solidsubstantially when different types of hydrogen-bonded complexes state hydrogen-bonded structure: DMP forms cyclic trimers are formed. ᭧ 1996 Academic Press, Inc. and DPP cyclic tetramers in which degenerate triple and quadruple proton transfers take place as shown by 15 N CPMAS experiments (MAS Å magic-angle sample spin-46