JOURNAL OF MAGNETIC RESONANCE, Series A 120, 46–55 (1996) ARTICLE NO. 0098 15N Chemical-Shift Tensors and Hydrogen-Bond Geometries of 3,5- Substituted Pyrazoles and Their Orientation in the Molecular Frame

The results of various 15N solid-state NMR experiments per- in the molecular principal axis system (PAS) is necessary formed on solid samples of doubly 15N-labeled 3,5-dimethylpyra- for the analysis of several solid-state NMR experiments, e.g., zole, 5-methyl-3-phenylpyrazole, (PMP), and 3,5-diphenylpyra- for 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 15N 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 mo-are derived by lineshape analysis of the 15N NMR spectra of the tions (8) , or for the analysis of relaxation phenomena (9).static powders obtained under the conditions of 1H ± 15N cross po-The size of the three principal elements of the CST canlarization and 1H decoupling. The orientations of the 15N CST in provide additional information, which is empirically inter-the molecular principal axis system are obtained by taking into preted and becomes more important with an increasing num-account the 15N ± 15N dipolar interactions and the 15N±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 15N dently checked by one-dimensional off-magic axis sample spinning CST in 3,5-dimethylpyrazole (DMP), b-5-methyl-3-phe-magnetization transfer experiments. The isotropic chemical shifts, nylpyrazole (b-PMP), and 3,5-diphenylpyrazole (DPP