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

The title compound N-aminopyrazole (AMPZ) has been studied both theoretical ly and experimentally using ab initio methods, low-temperature X-ray crysta llography, differential scanning calorimetry, IR spectroscopy, liquid state H-1 NMR, broadband solid state H-2 NMR and high resolution solid state N-1 5 NMR spectroscopy, focussing on the structure and dynamics of the amino gr oup and the characteristics of hydrogen bond (HB) association. The complete amino group rotation-inversion surface of AMPZ and, for comparison, also o f the aniline monomer were calculated at the B3LYP/6-31G* level. The result s indicate that monomeric AMPZ exhibits an sp(3) hybridized pyramidal amino group where the HH-distance vector is located perpendicular to the pyrazol e 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 degrees rotation of the amino group lead to a me tastable structure, where the inversion barrier is much larger than the rot ational barrier. These results contrast with those obtained for aniline whe re the amino group is almost planar and subject to a degenerate 180 degrees rotation. Further calculations of the hydrogen bonded cyclic AMPZ dimer in dicate that hydrogen bond association does not substantially perturb the ge ometry of the amino group, where one of the amino group protons and the rin g nitrogen atoms are involved in hydrogen bonding. The X-ray structure was determined at -173 degrees C (100 K) on a monocrystal obtained by zone crys tallization. DSC shows that on cooling, neat liquid AMPZ exhibits a solid-s olid phase transition around -45 degrees C to a low-temperature phase. In t his 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 proton s is involved in hydrogen bonding, where the NH stretch of the hydrogen bon ded proton appears at 3197 cm(-1) and of the non-hydrogen bonded proton at 3314 cm(-1). Solid state H-2 NMR spectroscopy of [ND2]AMPZ shows low-temper ature structures with rigid ND vectors below and isotropically rotating vec tors above the solid-solid phase transition. Hydrogen bond association was further studied by solid state N-15 CPMAS (cross polarization magic angle s pinning) NMR and by low-temperature liquid state H-1 NMR of AMPZ specifical ly N-15 labelled in the amino group. In contrast to the solid state, hydrog en bond exchange rendering the two amino group protons equivalent was found to be fast in the NMR timescale even at 100 K.