The structure of N-aminopyrazole in the solid state and in solution: an experimental and computational study

Abstract

The title compound N-aminopyrazole (AMPZ) has been studied both theoretically and experimentally using abinitio methods, low-temperature X-ray crystallography, differential scanning calorimetry, IR spectroscopy, liquid state ¹H NMR, broadband solid state ²H NMR and high resolution solid state ¹⁵N NMR spectroscopy, focussing on the structure and dynamics of the amino group and the characteristics of hydrogen bond (HB) association. The complete amino group rotation–inversion surface of AMPZ and, for comparison, also of the aniline monomer were calculated at the B3LYP/6-31G* level. The results indicate that monomeric AMPZ exhibits an s hybridized pyramidal amino group where the HH-distance vector is located perpendicular to the pyrazole ring and the hydrogen atoms bent towards the pyrazole ring nitrogen atom, i.e. with the lone electron pair anti to the latter. Both inversion at the nitrogen atom and the 180° rotation of the amino group lead to a metastable structure, where the inversion barrier is much larger than the rotational barrier. These results contrast with those obtained for aniline where the amino group is almost planar and subject to a degenerate 180° rotation. Further calculations of the hydrogen bonded cyclic AMPZ dimer indicate that hydrogen bond association does not substantially perturb the geometry of the amino group, where one of the amino group protons and the ring nitrogen atoms are involved in hydrogen bonding. The X-ray structure was determined at -173°C (100 K) on a monocrystal obtained by zone crystallization. DSC shows that on cooling, neat liquid AMPZ exhibits a solid–solid phase transition around -45°C to a low-temperature phase. In this phase AMPZ forms a weakly hydrogen bonded chain where the structure is similar to the one calculated for the monomer. Only one of the amino protons is involved in hydrogen bonding, where the NH stretch of the hydrogen bonded proton appears at 3197 cm^-1 and of the non-hydrogen bonded proton at 3314 cm^-1. Solid state ²H NMR spectroscopy of [ND₂]AMPZ shows low-temperature structures with rigid ND vectors below and isotropically rotating vectors above the solid–solid phase transition. Hydrogen bond association was further studied by solid state ¹⁵N CPMAS (cross polarization magic angle spinning) NMR and by low-temperature liquid state ¹H NMR of AMPZ specifically ¹⁵N labelled in the amino group. In contrast to the solid state, hydrogen bond exchange rendering the two amino group protons equivalent was found to be fast in the NMR timescale even at 100 K.

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