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

Abstract

Abstract The results of various 15 N solid-state NMR experiments performed on solid samples of doubly 15 N-labeled 3,5-dimethylpyrazole, 5-methyl-3-phenylpyrazole, (PMP), and 3,5-diphenylpyrazole are reported. In the solid state, these compounds form various hydrogen-bonded complexes. The principal values of the 15 N chemical-shift tensors (CST) of amine and imine nitrogen atoms are derived by lineshape analysis of the 15 N NMR spectra of the static powders obtained under the conditions of 1 H– 15 N cross polarization and 1 H decoupling. The orientations of the 15 N CST in the molecular principal axis system are obtained by taking into account the 15 N– 15 N dipolar interactions and the 15 N–D dipolar interaction after deuteration of the mobile proton sites. The relative orientations of the amine and imine CST in PMP are independently checked by one-dimensional off-magic axis sample spinning magnetization transfer experiments. The isotropic chemical shifts, the principal elements, and the orientations of the CST of both nitrogen atoms and the ND distances depend only slightly on the chemical structure and the associated hydrogen-bonded structure. The intramolecular structure of pyrazoles, therefore, does not vary substantially when different types of hydrogen-bonded complexes are formed.