Transformations of NH•••N hydrogen bonds can be employed in new classes of electronic materials. Imidazole, a prototypic NH•••N bonded crystal, remains centrosymmetric when compressed to 2.7 GPa. However, its recrystallization at 1.2 GPa leads to a new polar phase which in turn can be decompressed to 0.5 GPa at least. This transformation and calculated potential-energy function of the H-atom correlate with the transforming dimensions of the hydrogen bond and reveal high polarity of nonionic NH•••N bonded compounds.

Ferrocene exhibits a most intriguing transformation reflected in highly anomalous compression of the crystal, involving a discontinuity in unit-cell parameter c and reversed sign of compression coefficient β b = 1/b•∂b/∂p. The origin of this unexpected property is connected to the rotational and vibrational states of bis(cyclopentadienyl)iron(II) molecules in the crystal environment transformed under the high-pressure conditions. At ambient pressure, the cyclopentadienyl rings are conformationally disordered in the double-well potential energy E p function in the structure of ferrocene phase I. Above 3.24 GPa, the single-well E p function prevails due to intermolecular interactions enhanced by pressure, which leads to the unprecedented isostructural phase transition to the ordered ferrocene phase I′. Combined laboratory and synchrotron diffraction as well as Raman spectroscopy have been applied for describing structural transformations associated with this unique phase transition.

sp3)C7-H7AC•••C3' 2.789(3) 2.726(2) 2.667(2) 2.668(6) x-1, y, z (sp2)C6-H6•••C5' 2.706(2) 2.661(2) 2.642(2) 2.632(5) x-0.5, y, 0.5-z

The ultimately thin single-strand gold filaments, of Au(+)···Au(+) bonded gold(I) diethyldithiocarbamate polymer, AuEt2DTC, can be transformed depending on pressure and solvate contents. When synthesized in the presence of CH2Cl2, it crystallizes into a tetragonal AuEt2DTC·xCH2Cl2 phase α with ligand-supported and unsupported Au(+)···Au(+) bonded filaments modulated into molecular Au8-pitch helices. Low contents of CH2Cl2 favors the β phase of significantly reduced volume and orthorhombic space group Fddd. The α-AuEt2DTC·xCH2Cl2 crystal exhibits a highly unusual negative-area compressibility, due to the spring-like compression of helices. Above 0.05 GPa, the crystal transforms to phase β, where the Au16-pitch helices partly unwind their turns, which relaxes the tension generated by external pressure between neighboring helices of the opposite handedness. This is a unique observation of atomic-scale helical filaments transformation, which otherwise is a universal process analogous to the helix reversal between DNA forms B and Z, and in macroscopic world it is similar to nonperiodic unwind kinks in grapevine tendrils and telephone cords. Pressure also reduces the differences between the ligand-supported and unsupported Au(+)···Au(+) bonds.