Within a range of 0.1 Å, the H atom in the O−H−N hydrogen bond of the adduct 4‐methylpyridine⋅pentachlorophenol could be shifted by a simple adjustment of temperature (see scheme). At approximately 90 K the H atom is exactly centered between the O and the N atoms, as could be shown by stepwise monitoring by using variable‐temperature single‐crystal neutron diffraction.

Flexible organic materials possessing useful electrical properties, such as ferroelectricity, are of crucial importance in the engineering of electronic devices. Up until now, however, only ferroelectric polymers have intrinsically met this flexibility requirement, leaving small-molecule organic ferroelectrics with room for improvement. Since both flexibility and ferroelectricity are rare properties on their own, combining them in one crystalline organic material is challenging. Herein, we report that trisubstituted haloimidazoles not only display ferroelectricity and piezoelectricity—the properties that originate from their non-centrosymmetric crystal lattice—but also lend their crystalline mechanical properties to fine-tuning in a controllable manner by disrupting the weak halogen bonds between the molecules. This element of control makes it possible to deliver another unique and highly desirable property, namely crystal flexibility. Moreover, the electrical properties are maintained in the flexible crystals.

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In the neutron diffraction study of the 1+1 adduct 2-methylpyridine-pentachlorophenol, a very short O-H…N hydrogen bond is found with a sharply defined proton position, and distances of O-H = 1.068(7), H…N = 1.535(7) and O…N = 2.588(3) Å; the bond order of H…N is about 0.24 valence units.

AIM analysis was applied to study the changes in such topological parameters as the electron density at critical points of all the bonds of the molecule during the so-called nonadiabatic proton transfer in intramolecular hydrogen bonding in o-hydroxy aryl Schiff bases. Proton transfer is presented by a stepwise elongation and fixing of the hydroxyl bond with complete optimization of the rest of the parameters of the molecule by the B3LYP/6-311++G(d,p) method. A more detailed study of electron density changes at the critical points of the chelate and phenol rings in the stepwise proton-transfer process is presented. It was shown that the dependency of the electron density at the critical point of the chelate ring on tautomeric equilibrium is of a complicated character, whereas it is linear for the phenol ring. A complex study of the changes in the total electron density at the hydrogen bond, the quasi-aromatic ring, and in the whole molecule has been accomlished. The calculations of the intramolecular hydrogen bond by means of conformational and topological methods are discussed.