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Ambient‐pressure trigonal phase α of selenourea SeC(NH2)2 is noncentrosymmetric, with high Z′ = 9. Under high pressure it undergoes several intriguing transformations, depending on the pressure‐transmitting medium and the compression or recrystallization process. In glycerine or oil, α‐SeC(NH2)2 transforms into phase β at 0.21 GPa; however in water, phase α initially increases its volume and can be compressed to 0.30 GPa due to the formation of α‐SeC(NH2)2·xH2O. The single crystals of α‐SeC(NH2)2 and of its partial hydrate α‐SeC(NH2)2·xH2O are shattered by pressure‐induced transitions. Single crystals of phase β‐SeC(NH2)2 were in situ grown in a diamond‐anvil cell and studied by X‐ray diffraction. The monoclinic phase β is centrosymmetric, with Z′ = 2. It is stable to 3.20 GPa at least, but it cannot be recovered at ambient conditions due to strongly strained NH...Se hydrogen bonds. No hydrogen‐bonding motifs present in the urea structures have been found in selenourea phases α and β. [ABSTRACT FROM AUTHOR]

High‐nitrogen‐content compounds 6‐azido‐1,2,4‐triazolo[4,3‐b]pyridazine (C5H3N7) and its 3‐methyl derivative (C6H5N7) have been in situ crystallized in a diamond‐anvil cell and their structures determined by single‐crystal X‐ray diffraction. Under ambient and high‐pressure conditions the crystallizations yield the same phases: the C5H3N7 anhydrate and C6H5N7 hydrated clathrate. In both the structures there are clearly distinguished regions of short CH...N and N...N intermolecular contacts, the latter involving exclusively the azide groups. High pressure initially increases the contents of water in the channel pores of the clathrate. [ABSTRACT FROM AUTHOR]

Abstract: X‐ray diffraction structure of two new bis [1‐methyl‐3‐(methoxycarbonylmethyl) benzimidazolium]2+ [CuBr4]2− and [ZnBr4]2− compounds has been determined. Zn‐crystal is monoclinic with perfect Td symmetry of the ZnBr4 tetrahedron, whereas the Cu‐compound is orthorhombic with D2d symmetry of CuBr4 tetrahedron. The difference in these structures is described as due to the static Jahn‐Teller effect. Vibrational spectroscopy (1800–3200 cm−1) identified hydrogen bond network. NIR‐UV‐Vis spectra (5000–50000 cm−1) are dominated by ligand‐to‐metal bands with d‐d transition contribution. EPR results (Q‐band) analyzed in terms of the MO‐theory showed a very strong delocalization of unpaired electron density on ligands accompanied by the Cu(II) 4pz orbital contribution. This effect is partially compensated by charge transfer transitions to the EPR g‐factor. [ABSTRACT FROM AUTHOR]

1,4-Dibromobenzene melts at a considerably higher temperature than the 1,2- and 1,3-isomers. This melting-point difference is consistent with the molecular symmetry, as described by Carnelley's rule, and with the frequency of Br...Br halogen bonds. The lowest melting point of 1,3-dibromobenzene correlates with its two symmetry-independent molecules, indicating their inability to pack closely. Single crystals of 1,2- and 1,3-dibromobenzene have been grown under isochoric conditions in a diamond-anvil cell and at isobaric conditions in a glass capillary. Their structures have been determined in situ by X-ray diffraction. At 295 K 1,2-dibromobenzene crystallizes at 0.2 GPa as orthorhombic, space group Pbca, Z′ = 1, and 1,3-dibromobenzene at 0.3 GPa as orthorhombic, space group P212121, Z′ = 2. The same crystal phases are formed at ambient pressure by freezing these liquids below 256.15 and 248.45 K, respectively. The third isomer, 1,4-dibromobenzene, is a solid at room temperature and crystallizes as monoclinic, space group P21/ a. Striking relations between the structures and melting points of the corresponding dibromobenzene and dichlorobenzene isomers have been discussed. [ABSTRACT FROM AUTHOR]

Reports on the third polymorph (MH3) of maleic hydrazide, studied by X-ray diffraction and shown to be monoclinic, space group P2[sub 1]/n. Finding that the polymorph MH3 is the prevailing form along with the rare triclinic polymorph MH1; Accommodation of the supramolecular conformations to the requirements of close pacing of the aggregates in crystal lattices.

The half-chair conformation of the dynamically disordered molecular ring of 2-methyl-1-3-cyclohexanedione. C7H10O2 transforms to a sofa below Tc = 244K, when the crystal undergoes a continuous phase transition induced by the onset of halting large-amplitude vibration of methylene groups C(4)H2. The temperature dependence of the crystal structure has been investigated by X-ray diffraction. The Ibam symmetry of the crystal reduces below T.. to space group Pccn. the mechanism of the phase transition and of the conversion of the ring conformation is discussed. [ABSTRACT FROM AUTHOR]