Pressure-Controlled Neutral–Ionic Transition and Disordering of NH···N Hydrogen Bonds in Pyrazole

Pyrazole exhibits exceptional effects of temperature and pressure for H-disorder in hydrogen bonds NH···N. Under normal conditions, α-pyrazole is polar (orthorhombic space group Pna2₁), but the structure is pseudo-centrosymmetric. Two symmetry-independent molecules differently favor the H-sites at their amine nitrogen atoms. The asymmetric disorder of protons in hydrogen bonds NH···N leads to the disproportionation favoring the cations at one site and anions at the other. High temperature increases distortions of the structure from the centrosymmetric symmetry, activates the disproportionation defects, and stabilizes the averaged ionic disparity of symmetry-independent molecules at the ±0.1 e̅ level. Critical pressure P_c = 0.45 GPa induces a transition to centrosymmetric phase β, space group Pnab. This transition involves the 50:50 H-disordering in NH···N hydrogen bonds and eliminates molecular displacements as well as the favored H-sites. The NH···N bonds are hardly compressed at P_c in pyrazole, and they are by over 0.4 Å longer than the OH···O^– bond in KH₂PO₄ (KDP). The main structural transformations are reorientations of pyrazole molecules, which suggests the primarily displacive character of this pressure-induced phase transition.