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The aggregation of molecules is usually associated with a specific type of interaction, which can be altered by thermodynamic conditions. Under normal conditions, the crystal structure of selenourea, SeC(NH2)2, phase α is trigonal, space group P31, Z = 27. Its large number of independent molecules (Zα′ = 9) can be associated with the formation of an NH...N hydrogen bond substituting one of 36 independent NH...Se hydrogen bonds, which prevail among intermolecular interactions. Phase α approximates the trigonal structure with a threefold smaller unit cell (Z = 9), which in turn approximates another still threefold smaller unit cell (Z = 3). The temperature‐induced transformations of selenourea have been characterized by calorimetry and by performing 21 single‐crystal X‐ray diffraction structural determinations as a function of temperature. At 381.0 K, phase α undergoes a first‐order displacive transition to phase γ, with space group P3121 and Z reduced to 9, when the NH...N bond is broken and an NH...Se bond is formed in its place. Previously, an analogous competition was observed between NH...N and NH...O hydrogen bonds in high‐pressure phase III of urea. The lattice vectors along the (001) plane in low‐ and high‐temperature phases of selenourea are related by a similarity rule, while the lattice dimensions along direction c are not affected. This similarity rule also applies to the structures of phase γ and hypothetical phase δ (Z = 3). The thermally controlled transition between enantiomorphic phases of selenourea contrasts with its high‐pressure transition at 0.21 GPa to a centrosymmetric phase β, where both the NH...Se and NH...N bonds are present. The compression and heating reduce the number of independent molecules from Z′ = 9 in phase α, to Z′ = 2 in phase β and to Z′ = 1.5 in phase γ. [ABSTRACT FROM AUTHOR]

The zinc aspartate (ZnAsp2) complex, a common dietary supplement, preferentially crystallizes as the dihydrate (ZnAsp2·2H2O) from aqueous solution. Under normal conditions the dihydrate easily transforms into the sesquihydrate (ZnAsp2·1.5H2O). The dihydrate crystal structure is triclinic, space group P1, and the sesquihydrate is monoclinic, space group C2/c. However, their structures are closely related and similarly consist of zinc aspartate ribbons parallel to pores accommodating water molecules. These porous structures can breathe water molecules in and out depending on the temperature and air humidity. High pressure above 50 MPa favours the sesquihydrate, as shown by recrystallizations under pressure and compressibility measured by single‐crystal X‐ray diffraction up to 4 GPa. This preference is explained by the reduced volume of the sesquihydrate and water compressed separately, compared with the dihydrate. The sesquihydrate undergoes an isostructural phase transition when the voids collapse at 0.8 GPa, whereas no phase transitions occur in the dihydrate, because its pores are supported by increased water content. [ABSTRACT FROM AUTHOR]

The structures and solid‐state dynamics of the supramolecular salts of the general formula [(12‐crown‐4)2⋅DABCOH2](X)2 (where DABCO=1,4‐diazabicyclo[2.2.2]octane, X=BF4, ClO4) have been investigated as a function of temperature (from 100 to 360 K) and pressure (up to 3.4 GPa), through the combination of variable‐temperature and variable‐pressure XRD techniques and variable‐temperature solid‐state NMR spectroscopy. The two salts are isomorphous and crystallize in the enantiomeric space groups P3221 and P3121. All building blocks composing the supramolecular complex display dynamic processes at ambient temperature and pressure. It has been demonstrated that the motion of the crown ethers is maintained on lowering the temperature (down to 100 K) or on increasing the pressure (up to 1.5 GPa) thanks to the correlation between neighboring molecules, which mesh and rotate in a concerted manner similar to spiral gears. Above 1.55 GPa, a collapse‐type transition to a lower‐symmetry ordered structure, not attainable at a temperature of 100 K, takes place, proving, thus, that the pressure acts as the means to couple and decouple the gears. The relationship between temperature and pressure effects on molecular motion in the solid state has also been discussed. [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]

Two phase transitions in compressed benzimidazole polymorphsreveala remarkable interplay of the H-site in NH···N hydrogen-bondedaggregates and the crystal structure. The ambient-pressure polar polymorphα, space group Pna21, at p1= 0.26 GPa transforms into centrosymmetricphase β, space group Pccn, and above p2= 2.26 GPa into another centrosymmetric polymorphγ, space group Pbca. Single crystals of formsα, β, and γ have been in situgrownin isothermal and isochoric conditions in a diamond-anvil cell, andtheir structures were determined by X-ray diffraction. Both at p1and p2the H-bonddistance N···N increases in the higher-pressure phase.However, the H-atom always assumes the site for which the H···Ndistance in the homoconjugated NH···N bond is shorter. [ABSTRACT FROM AUTHOR]

Abstract: Intermolecular interactions and the role of fluorine substitution have been investigated for a halogenated-ethane anesthetic. 2-Bromo-2-chloro-1,1,1-trifluoroethane, BrClCHCF3 (Halothane), has been in situ pressure frozen in a diamond anvil cell and its structure determined by single-crystal X-ray diffraction at 1.85(5)GPa/296K. Crystal is triclinic, space group . In this racemic structure the enantiomorphic molecules are substitutionally disordered at the same general positions in that way that bromine and chlorine atoms occupy the same site at the 50:50 ratio. Despite the fact that only the Br and Cl atoms are disordered, the crystal packing is dominated by halogen⋯halogen and halogen⋯hydrogen interactions. This X-ray diffraction study provides structural explanation of considerably increased vapor pressure of Halothane compared to its hydrogenated analogue. [Copyright &y& Elsevier]

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.

High pressure destabilizes the high Z' polymorph of 3-hydroxy-4,5-dimethyl-1-phenylpyridazin-6-one (1α); however, recrystallization is needed for obtaining a low Z', more dense polymorph. Three polymorphs α, β, and γ can be monotonically compressed to 2.0 GPa at least. At ambient pressure 1 crystallizes in space group C2/c, Z' = 4 as polymorph 1α, or as lower-density polymorph 1β, of space group P21/c and Z' = 1. Polymorph β is metastable, and after about one year, it transforms to phase α. The isochoric recrystallization above 0.40 GPa yields a new polymorph γ of space group P21/a and Z' reduced to 1. The γ polymorph retrieved to ambient conditions for months has showed no signs of transformations. The main motif of OH…O bonded chains is retained in all three phases, but high pressure enforced identical conformation of closely packed molecules and their identical crystal environment in phase γ. [ABSTRACT FROM AUTHOR]

The molecular structure of 1,2-di(3-hydroxymethylpyridinium)ethane dibromide ( 1 ) has been characterized by X-ray diffraction, B3LYP/6-311++G(d,p) calculations, FTIR, Raman and NMR spectra. The crystals are monoclinic, space group C2/c . 1,2-Di(3-hydroxymethylpyridinium)ethane dication and hydrogen-bonded bromide anions in crystals are located at the inversion center. The both CH 2 OH groups are engaged in two equal length hydrogen bonds with bromide anions. Two structures ( 2 ) and ( 3 ) were optimized at the B3LYP/6-311++G(d,p) level of theory. The optimized complex ( 2) resembles the crystal structure, while complex ( 3 ) is preferred energetically. The O⋯Br − hydrogen bonds distances are: 3.289(2) Å in crystals ( 1 ), but in the optimized structures ( 2 ) and ( 3 ) they are 3.303 Å and 3.461 Å, respectively. The investigated complex is additionally stabilized by the N + ⋯Br − electrostatic attractions. The potential energy distributions (PED) were used for the assignments of IR and Raman frequencies in the experimental and calculated spectra of the title compound. The FTIR spectrum of ( 1 ) is consistent with the X-ray results. Interpretation of the 1 H and 13 C NMR spectra in DMSO- d 6 has been based on 2D experiments. The calculated GIAO/B3LYP/6-311++G(d,p) magnetic shielding constants have been used to predict 1 H and 13 C chemical shifts for the optimized structures of ( 2 ) and ( 3 ). [ABSTRACT FROM AUTHOR]