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

Abstract

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.

Giant negative linear compression positively coupled to massive thermal expansion in a metal–organic framework

“…3). The general empirical evidence reported so far mainly for minerals 10 , but also for framework materials 12,15 , MOFs 28 , as well as organic compounds 9,43,44 , show that in most compounds temperature and pressure cause the opposite strain, that is, the NLC exists in the direction of NTE or of the smallest PTE. This common behaviour is known as the inverse relationship rule 10 .…”

Effect of pressure on heterocyclic compounds: Pyrimidine and s-triazine

“…29 The transition of αto β-pyrazole at 0.45 GPa leads to disordering of H atoms. 30 Phenanthrene still displays some long range order despite it transforms to hydrogenated carbon above 20.0 GPa. 31 Overall, the purely pressure induced reaction is usually accompanied by many competitive reaction paths, and occurs in the compressed ground electronic state.…”

Multicomponent organic crystals at high pressure

“…Both the symmetrization and, oppositely, an increase of ionicity of the hydrogen bonds (OH … O or NH … N) has been reported in the literature [10,21,22,[87][88][89][90][91][92][93][94][95][96][97][98][99]. An increase in the ionicity of the intermolecular NH … O bonds between the terminal amino-and carboxygroups with increasing pressure has been reported for L-alanine [63,64].…”