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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]

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]

Easy crystallization distinguishes xylitol from other sugars, which usually condense into a syrup from aqueous solution. Although two polymorphs, i.e. metastable monoclinic and high‐density orthorhombic, have been reported for xylitol, only the latter is in practical use. Under high pressure, the same orthorhombic phase has been obtained by both isothermal and isochoric recrystallization. The stability of the orthorhombic xylitol phase to 5.0 GPa has been correlated with a uniform compression of all hydrogen bonds and some flexibility of the molecular conformation, which cushion the pressure‐induced local strains. The anisotropic compressibility of xylitol and its thermal expansion are consistent with the rule of inverse effects of pressure and temperature. This inverse strain relationship has been correlated with the dimensions and orientation of xylitol molecules in the crystal structure. [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 diamond‐anvil cell (DAC) was invented 60 years ago, ushering in a new era for material sciences, extending research into the dimension of pressure. Most structural determinations and chemical research have been conducted at ambient pressure, i.e. the atmospheric pressure on Earth. However, modern experimental techniques are capable of generating pressure and temperature higher than those at the centre of Earth. Such extreme conditions can be used for obtaining unprecedented chemical compounds, but, most importantly, all fundamental phenomena can be viewed and understood from a broader perspective. This knowledge, in turn, is necessary for designing new generations of materials and applications, for example in the pharmaceutical industry or for obtaining super‐hard materials. The high‐pressure chambers in the DAC are already used for a considerable variety of experiments, such as chemical reactions, crystallizations, measurements of electric, dielectric and magnetic properties, transformations of biological materials as well as experiments on living tissue. Undoubtedly, more applications involving elevated pressure will follow. High‐pressure methods become increasingly attractive, because they can reduce the sample volume and compress the intermolecular contacts to values unattainable by other methods, many times stronger than at low temperature. The compressed materials reveal new information about intermolecular interactions and new phases of single‐ and multi‐component compounds can be obtained. At the same time, high‐pressure techniques, and particularly those of X‐ray diffraction using the DAC, have been considerably improved and many innovative developments implemented. Increasingly more equipment of in‐house laboratories, as well as the instrumentation of beamlines at synchrotrons and thermal neutron sources are dedicated to high‐pressure research. [ABSTRACT FROM AUTHOR]

The survey of the shortest contacts in structures deposited in the Cambridge Structural Database shows that chalcogen...halogen, halogen...halogen and chalcogen...chalcogen interactions can compete as cohesion forces in molecular crystals. The smallest parameter δ (defined as the interatomic distance minus the sum of relevant van der Waals radii) for Ch...X contacts between chalcogens (Ch: S, Se) and halogens (X: F, Cl, Br, I) is present only in 0.86% out of 30 766 deposited structures containing these atoms. Thus, in less than 1% of these structures can the Ch...X forces be considered as the main type of cohesion forces responsible for the molecular arrangement. Among the 263 structures with the shortest Ch...X contact, there are four crystals where no contacts shorter than the sums of van der Waals radii are present (so‐called loose crystals). The smallest δ criterion has been used for distinguishing between the bonding (covalent bond) and non‐bonding contacts and for validating the structural models of crystals. [ABSTRACT FROM AUTHOR]

A strongly anisotropic architecture of stacked metal–organic grids, with pores arranged through their eyes, has been revealed for the new coordination polymer Ni(bipy)(hip)(H2O)2·DMF·CH3OH·H2O, where bipy is 4,4′-bipyridine and hip is 5-hydroxyisophthalic acid. This porous crystal, when submerged in various nonpenetrating oils used as the hydrostatic media, displays a negative volume compressibility. This counterintuitive effect results in the zone amorphization injecting the contents of collapsing pores into the retained crystalline portions of the sample. This additional contents of the pores supports the crystalline parts and prevents their amorphization. [ABSTRACT FROM AUTHOR]

A negative linear temperature expansion and a negative linear compressibility were observed for imidazolium benzoate salt. Its strongly anisotropic strain induced by the temperature and pressure changes has been explained by the mechanism of H-bonded helices deformed in the structure. X-ray diffraction and vibrational spectroscopy were used to analyze interactions in the crystal. The Quantum Theory of Atoms in Molecules (QTAiM) approach was applied to analyze the hydrogen bonds and other interactions. In the salt under study, the interactions within the helix are substantially higher in energy than between helices. With decreasing temperature and increasing pressure, the value of the helix pitch increases while the value of the semi-major axis decreases, which results in the negative linear expansion and negative linear compression, respectively. [ABSTRACT FROM AUTHOR]

Pressure efficiently changes the crystallization preferences of three polymorphs and three solvates of 4,4'-bipyridinium perchlorate, [C10H9N2]+ClO4-. At ambient pressure a triclinic dihydrate [C10H9N2]+ClO4-·2H2O is formed of the solutions in water and in the water/methanol mixture, while the methanol solution in open vials yields concomitant monoclinic quarterhydrate 4([C10H9N2]+ClO4-)·H2O and triclinic anhydrate [C10H9N2]+ClO4-. At 0.30 GPa an orthorhombic anhydrate is formed, stable to 0.60 GPa, when it collapses into a monoclinic phase. The transition changes the conformation of cations from planar to twisted, differently than the analogue phase transition in [C10H9N2]+Br·H2O. Recrystallizations from the methanol solution above 0.55 GPa leads to methanol solvate [C10H9N2]+ClO4-·CH3OH. In the solvates, chains of the cations are aggregated through either NH+···N bonds (like in the neat polymorphs) or NH+···OH···N bonds, or both these bonds in one crystal. The pyridinium protons are disordered in all these structures, which implies disproportionation between neutral molecules, C10H8N2, monocations [C10H9N2]+ and dications [C10H10N2]2+. In quarterhydrate 4([C10H9N2]+ClO4-)·H2O the disorder of protons propagates through the hydrogen bonds mediated by disordered water molecules, also involved in weak H-bonds to perchlorate anions. [ABSTRACT FROM AUTHOR]

The cation of 3-(trimethylammonium)-benzoic acid exhibits a considerable conformational flexibility connected to the orientation of carboxyl group, coupled to the proton position, and rotations of the trimethylammonium group. This conformational flexibility considerably affects the aggregation of ions and molecules of 3-(trimethylammonium)-benzoic acid iodide hydrate, which crystallizes in a low-symmetry structure with four symmetry-independent formula units (monoclinic space group Pc, Z' = 4, model I). However this structure approximates the symmetry of space group P2/ c in the same unit cell ( Z' = 2, model II), and also even a still more highly symmetric structure of space group P2/ c in a half-smaller unit cell ( Z' = 1, model III). The independent formula units are differentiated by the conformations of trimethylammonium groups (ordered in model I, but disordered in models II and III), conformations of the carboxyl groups and positions of OH···O bonded water molecules (ordered in molecules I and II, and disordered in model III), while the bulks of the cations and iodide anions are consistent for all models. While the calorimetric study and varied-temperature structural determinations exclude phase transitions of the highly pseudosymmetric model I. This pseudosymmetry is independent of temperature between 100 K and the m.p. at 463 K, and illustrates subtle structural effects leading complex molecular aggregation in the crystal. Theoretical computations confirm the low energy differences between the conformers. [ABSTRACT FROM AUTHOR]

At high pressure the urea crystal abruptly collapses in two stages. When transforming from phase I to III at 0.48 GPa, the crystal volume is abruptly compressed by 7.3%. At this transition a huge abrupt linear compression to 65% along a and to 95% along c is partly compensated by an unprecedented abrupt negative linear compression (i.e., expansion) to 148% along b. At another discontinuous transformation, at 2.8 GPa to phase IV, the crystal still displays an abrupt negative linear compression to 106% along c. The intermediate phase III between 0.48 and 2.80 GPa increases the contribution of the zwitterionic mesomeric structure of urea molecules and the formation of weak NHN bonds has been evidenced in phase III. The formation of phase II, above 373 K and above 0.6 GPa reported by Bridgman, has not been confirmed. [ABSTRACT FROM AUTHOR]

High pressure and temperature trigger symmetric chemical reactions of 4,4′-bipyridine hydrobromide monohydrate (44′biPyHBr·H2O) in methanol solution. Above 0.1 GPa and 423 K, the 4,4′-bipyridinium dibromide salt (44′biPy2HBr) precipitates, while the 4,4′-bipyridine free base remains dissolved in the methanol–water mixture. At 0.35 GPa and 473 K, both pyridine moieties are N-methylated and N,N-dimethylbipyridinium dibromide (44′biPy2CH3Br) is formed. None of the high-pressure compounds of 44′biPy2HBr and 44′bPy2CH3Br are solvated, which contrasts with the strong preference of analogous 1,4-diazabicyclo[2.2.2]octane (dabco) monosalts and disalts to form solvates at high-pressure. The high-pressure reactivity of 44′biPyHBr is analogous to that of 1,4-diazabicyclo[2.2.2]octane hydrobromide (dabcoHBr); however, dabcoHBr is asymmetrically N-methylated at one amine site only. This asymmetric mono N-methylation of dabcoHBr has been associated with strong electrostatic interactions between the transannular N-atoms. [ABSTRACT FROM AUTHOR]

5,6-Dimethylbenzimidazole (C9H9N2) crystallizes exclusively in the unsolvated form from an aqueous solution up to 0.25 GPa and from methanol, ethanol or their mixed solutions up to 0.6 GPa. At still higher pressures, solvated crystals with water, methanol and ethanol are formed depending on the solvent used for recrystallization. The NH…N bonds of the neat crystals are replaced by NH…OH…N bonds in the solvates. This preference for high-pressure solvation has been rationalized in terms of changed hydrogen bonds as well as volume balance between neat and solvated compounds. The methanol solvate C9H9N2·MeOH undergoes a pressure-induced phase transition at 1.40 GPa, lowering its symmetry from monoclinic space group P21/c (phase α) to triclinic space group P1̅ (high-pressure phase β) and ordering the H-atoms in hydrogen bonds NH…OH…N, disordered in phase α. [ABSTRACT FROM AUTHOR]

Intermolecular contacts between organic molecules in crystals have been categorized according to the shortest distances between the van der Waals spheres. The distributions of the first, second and next shortest contacts reveal systematic changes in the role of different types of interactions in the aggregation of molecules. These distributions of the shortest contacts can be related to the specific types of cohesion forces as well as to the effect of close packing of molecules. The sequence of close contacts reflects the role of cohesion forces in a given crystal, and their contribution to the stability of crystals in general can be evaluated. Such an analysis has been applied to measure the changes in the types of cohesion forces in polymorphs and in structures undergoing phase transitions. This information is essential to understand the molecular aggregation as well as to validate the crystallographic data. It has been established for the chemical compounds investigated that their phase transitions and polymorphic modifications do not change the closest intermolecular contacts of the first order, and only higher orders of contacts are modified. [ABSTRACT FROM AUTHOR]

The 1:1 complex of N -methylmorpholinium-acetate, MMB, with hydrobromic acid ( 1 ) has been investigated by single-crystal X-ray analysis, infrared spectroscopy and theoretical calculations. The proton-transfer complex has been observed in the crystalline state, with the COO H⋯Br hydrogen bond with the O⋯Br distance of 3.107(2) Å. In the structure optimized at the B3LYP/6-311++G(d,p) levels of theory the proton is closer to the bromide anion, with the Br H⋯OOC distance of 3.069 Å. The potential energy distributions (PED) have been used for the assignments of IR bands. The experimental and theoretical infrared spectra have been discussed. [ABSTRACT FROM AUTHOR]

Anhydrous 5,6-dimethylbenzimidazole (dMBzIm) crystals are obtained from an aqueous solution at ambient pressure, but above 0.25 GPa the hemihydrate dMBzIm·½H2O is formed. It can be stored for over a year in open vials. Counter-intuitively, the volume of dMBzIm·½H2O is smaller than that of the anhydrate. Ambient and high-pressure structures of dMBzIm and dMBzIm·½H2O crystals grown in situ in a diamond-anvil cell have been determined by X-ray diffraction methods. The isostructural phase transition observed in dMBzIm at 0.25 GPa coincides with the changed preference to hemihydrate crystallization. The significantly different ambient and high-pressure hydration preferences of imidazole and benzimidazole derivatives have been explained by the interplay of interactions and the efficiency of crystal packing measured by the size and volume of voids. [ABSTRACT FROM AUTHOR]

Hydroxypyridazinone and pyrroledione rings condensation in the reactions of hydrazine hydrate with citraconic, 2,3-dimethylmaleic, succinic and cis-cyclohexanedicarboxylic anhydrides have been conducted in the HCl aqueous solution. The pyridazine-ring condensation yields products unexpected for these conditions. They have been identified by 1 H/ 13 C NMR and X-ray diffraction. The course of the reaction toward the five- and six-membered ring condensation strongly depends on methyl and other substituents in the anhydrides and in hydrazine. The obtained products indicate that the ring condensation is controlled by the molecular strains and steric hindrances between the substituents in anhydrides and pyridazinone products. The condensation of cyclic anhydrides with hydrazines has been reduced to one-step reaction and its yield significantly increased. [ABSTRACT FROM AUTHOR]

The structure of bis(4-trimethylammoniumbenzoate) hydroiodide hydrate 1 has been studied by X-ray diffraction, B3LYP/6–311G(d,p) calculations, FTIR, Raman and NMR spectroscopic techniques. The crystal is polar in monoclinic space group Cc. Two 4-trimethylammoniumbenzoate moieties are joined by a short and asymmetric hydrogen bond of 2.45(2) Å. Water molecules are gradually released from the structure, causing shifts in the position of iodine anions, which induces their disorder. The water molecule interacts with 4-trimethylammoniumbenzoate moiety and iodide anion via two O(3)–H(1)⋯O(1) and O(3)–H(2)⋯I(1) hydrogen bonds of lengths 2.70(3) and 3.51(1) Å. Hydrogen bonds in theoretically predicted structures of 2 and 3 (in vacuum), and 4 , 5 (in DMSO) optimized by the B3LYP/6-311G(d,p) approach are slightly longer than in crystal 1 . The FTIR spectrum of 1 shows a broad and intense absorption in the 1500–400 cm −1 region, typical of short hydrogen bonds assigned to the ν as (OHO) + γ (OHO) vibrations. The correlations between the experimental 13 C and 1 H chemical shifts ( δ exp ) of the investigated compound in DMSO and the GIAO/B3LYP/6-311G(d,p) magnetic isotropic shielding constants ( σ calc ) calculated by using the screening solvation model (COSMO) are linear, δ exp = a + b σ calc , and they well reproduce the experimental chemical shifts. [ABSTRACT FROM AUTHOR]

ABSTRACT Recalcitrant crystallization and syrup formation are frequent features of natural sugars. This is the case of d-ribose, yielding low-quality crystals of mixed α- and β-pyranose anomers. However, large crystals of dl-ribose can be grown easily. The crystal structures of stable d-ribose forms I and II as well as dl-form II have been analyzed in terms of their compatibility with the molecular aggregation. The comparison of the potential energy of all conformers and their OH···O hydrogen-bonding patterns is consistent with the preferential racemate crystallization in terms of departures from the optimized isolated ribose molecule and its directional interactions. This analysis is aimed at rationalizing the interplay between the molecular structure and spontaneous crystallization of chiral compounds. Chirality 26: 806-810, 2014. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Monotoniccompression of 2-methylbenzimidazole mimics the pressure-induceddiscontinuous transition between benzimidazole phases β andγ. Both are built of similarly arranged chains of NH···Nbonded molecules. Although the methyl substituents prevent the collapsesof voids, 2-methylbenzimidazole monotonically follows the abrupt latticestrain of benzimidazole. This generates the effects of giant linearcompression and negative-area compression in 2-methylbenzimidazole. The observation of converted discontinuous and monotonic compressionprovides a new method for engineering materials of required elasticproperties. [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]

Thereare clearly two maxima in the distribution of the shortestintermolecular contacts, referred to the atomic van der Waals radii,in crystals of organic compounds. Accordingly, the crystals of organiccompounds can be classified into those governed by strong cohesionforces (such as OH···O, NH···O, andNH···N hydrogen bonds) and weak ones (CH···O,CH···π, halogen bonds, and dispersion forces).In about 1/3 of all known structures of organic compounds, there arestrong cohesion interactions, while in 2/3 of structures the shortestcontacts are associated with weak interactions. Characteristic propertiesof organic compounds depend on these either strong or weak cohesionforces. The distributions of the shortest intermolecular contactsof specific types, such as OH···O, NH···O,CH···N, and Br···Br can be approximatedby the Gaussian functions. These Gaussian functions, with mean distanceand standard deviation characteristic of specific interactions, canbe used for predicting molecular arrangements and for validating crystalstructures. [ABSTRACT FROM AUTHOR]

Despitesignificantly different crystal symmetry and packing, thecrystal structures of NH+···N hydrogen-bondedsalts of 4,4′-bipyridine (44′biPy) with mineral acidsHA= HCl, HBr, HI, HClO4, HBF4and H2SiF6exhibit close analogies in thehydration, aggregation of the cations and their twisted conformation,as well as proton disordering. All monosalts have been synthesizedand, at normal conditions, form crystals of general formula [44′biPyH]+A–·xH2O (x= 0.5, 1, or 2), and[44′biPyH]+2SiF62–·5H2O. In the structures, the 44′biPyH+cations are NH+···N bonded intolinear chains, and in most [44′biPyH]+A–·xH2O crystals the protons are disordered, similarly as in anisotropicrelaxors 1,4-diazabicyclo[2.2.2]octane hydroiodide and hydrobromide(dabcoHI and dabcoHBr, respectively). The proton disorder implies generation of point defects of neutral 44′biPy molecules and[44′biPyH2]2+dications in all thesestructures. In all [44′biPyH]+A–structures investigated, theacid anions are hydrogen bonded to water molecules and interact withpyridine hydrogen atoms. Two polymorphs of [44′biPyH]+I–·H2O differ in color: the orthorhombicpolymorph α is yellow, and the triclinic polymorph β isorange. [ABSTRACT FROM AUTHOR]

The 1:2 complex of 1,4-dimethylpiperazine mono-betaine (MBPZ) with squaric acid (H2SQ) has been characterised by single-crystal X-ray analysis, FTIR and NMR spectroscopies, and by DFT calculations. The crystals are monoclinic, space groupP21/c. Two MBPZ cations and four hydrogen squarate anions (HSQ− ) are linked by strong O(1)–H…O(13) (2.525(4) Å), O(14)–H…O(21) (2.511(4) Å) and N(4)–H…O(23) (2.607(3) Å) hydrogen bonds into a cyclamer. In turn, the cyclamers are linked into a helixthrough two O(24)–H…O(11) hydrogen bonds of 2.516(4) Å. The piperazinium ring has a chair conformation with N(4)–CH3and N(1)–CH2COOH substituents in the equatorial positions, and N(1)–CH3in the axial position. The FTIR spectrum is consistent with the crystal data. Two models of the 1:2 complex of MBPZ with H2SQ have been optimised at the B3LYP/6-311++G(d,p) level of theory and have been used to calculate harmonic IR frequencies. One of the models (2) is dominated by electrostatic attraction between NH(4)+and HSQ− , whereas in the other (3) squaric acid interacts with a zwitterionic MBPZ through the O–H…O and O–H…N hydrogen bonds. [ABSTRACT FROM PUBLISHER]

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: The location of H-atoms sites in carboxylic-acid dimers has been related to the crystal environment. It has been shown that the H-sites are coupled to the orientation of carboxylic-acid groups. Consequently, the H-bonds can be strained and the H-atoms destabilized and disordered by the symmetric crystal environment of symmetric molecules. The coupling results in the O⋯O distance contraction by about 0.015Å in carboxylic acids with disordered H-atoms, this magnitude calculated from H-bonds strain being consistent with that observed in crystal structures. This strain analysis provides an easy method of locating the H-atoms in hydrogen bonds between carboxyl groups, verifying experimental data determined by X-ray diffraction, calculating the magnitudes of O⋯O distance contraction and molecular reorientations induced by the H-atoms disordering structural effects, and predicting properties related to the H-atom dynamics. The survey of carboxylic-acid structures corroborates the effects of the molecular environment on the H-sites and hydrogen-bond dimensions. [Copyright &y& Elsevier]

A subtle interplay of supramolecular aggregation, crystal symmetry, and bistable proton sites in NH+···N hydrogen bonds in 1,4-diazabicyclo[2.2.2]octane hydroiodide (dabcoHI, [C6H13N2]+I−) leads to the giant dielectric response in this compound. This unique feature disappears on increasing the temperature above 410 K or on increasing the pressure above 40 MPa at 296 K. Ten polymorphs of dabcoHI have been obtained in varied thermodynamical conditions. Phase V (space group P6̅m2) is stable at normal conditions. For five new polymorphs, the destructive phase transitions have been circumvented by growing the single crystals of specific phases in situ in their stability regions, and their space-group symmetries are Pmm2 (phase IV), Pmc21 (phase VI), Pbcm (phase VII), Cmm2 (phase VIII), and P2/c (phase IX). Of all these 10 dabcoHI phases, the giant dielectric response and anisotropic relaxor properties have been evidenced only for phases V and X. In all the determined structures of dabcoHI, linear or nearly linear NH+···N bonded chains are present, which shows that the relaxor properties of dabcoHI are not inherent to the NH+···N aggregation alone. The polymorphic structures differ mainly in the arrangement of the chains and iodide anions, in the dabco conformation, and in the location of protons, in phases IV, V, VIII, and IX disordered and in phases VI and VII ordered. Polymorph VI is highly metastable at normal conditions and only after days or weeks gradually transforms to phase V. The molecular-scale origin of the changes in dielectric and relaxor properties of dabcoHI results from the conjunction of short-range polarization induced by ordered protons in NH+···N bonds in nanoregions of phase V and the specific crystal environment of hexagonal symmetry, leading to frustration of the local crystal field. A series of hydrates and solvated dabcoHI crystals was obtained above 0.50 GPa. Ten polymorphs have been shown in the phase diagram of 1,4-diazabicyclo[2.2.2]octane hydroiodide, and the structures of six of them have been determined. [ABSTRACT FROM AUTHOR]

Molecular arrangements have been determined at the lowest limits of pressure ranges of benzene phase I, at 0.15 GPa, and phase II at 0.91 and 0.97 GPa, all at 295 K. All intermolecular contacts both in phase I and phase II to about 1.0 GPa exceed the sums of van der Waals radii; however, the transition between phases I and II does not affect the pattern of CH···π(arene) hydrogen bonds. In phase I the molecules are CH···π bonded approximately perpendicular into sheets, and there are substantial voids between the molecules within the sheets. The mechanism of transition to phase II involves a collapse of the voids, simultaneous with a shift of the CH···π bonded sheets. The thickness of sheets increases, which partly compensates the volume reduction due to the voids collapse; hence, the transition exhibits a large hysteresis of two GPa and a sluggish character at 295 K. No other phases of benzene have been observed between 0.15 and 5.0 GPa. High pressure transition between benzene phases I and II, involving a collapse of intermolecular voids, exhibits a huge pressure hysteresis of 2 GPa at 295 K and an intriguingly tiny volume change. It has been explained by determining the crystal structures of benzene I and II at their lowest stability limits. [ABSTRACT FROM AUTHOR]

The synthesis of 6-chloro-3-chloromethyl-1,2,4-triazolo[4,3-b]pyridazine and its vicarious nucleophilic substitution products are described and characterized by spectroscopic methods and X-ray diffraction. The lipophilicities of the title compound, its acetate, and the derived 3-methyl tetrazolopyridazine have been measured and correlated with the chlorine substitution. The title compound co-crystallizes with acetic acid in the structure of the same symmetry and very similar layered arrangement as the crystals of its acetoxymethyl analog, despite the considerably different substituents and intermolecular contacts. The details of the intermolecular interactions are discussed. [ABSTRACT FROM AUTHOR]

A series of 1,2-dihalotetrafluoroethanes X(CF2)2X (X = Br, I) has been in-situpressure crystallized in a diamond-anvil cell (DAC), their structures determined by single crystal X-ray diffraction and rationalized by weak intermolecular interactions. For isoelectronic and isostructural 1,2-dibromotetrafluoroethane (BrCF2CF2Br), 1,2-diiodotetrafluoroethane (ICF2CF2I) and 1-bromo-2-iodotetrafluoroethane (BrCF2CF2I), the crystals are monoclinic, space group P21/n, with the midpoint of the C–C bond located at the centre of inversion. The freezing pressures of these compounds have been determined to be 0.80 GPa, 0.30(5) GPa and 0.10(5) GPa for BrCF2CF2Br, BrCF2CF2I and CF2ICF2I, respectively. In the structure of ICF2CF2I, the –CF2–CF2– moiety is orientationally disordered at 0.16 GPa, but it becomes ordered at 0.86 GPa; in the BrCF2CF2I crystal the Br and I atoms are substitutionally disordered; and the BrCF2CF2Br structure is completely ordered. The formation of isostructural crystals by these compounds and different types of molecular disorder can be rationalized by the intermolecular interactions at varied thermodynamical conditions. The phenomenon of “self-healing” has been observed, in which crystal defects, formed during initial crystallization in the DAC, repaired themselves during the annealing process, which greatly facilitates the formation of single crystals. Pressure considerably affects the crystal morphology of ICF2CF2I, where the elimination of –CF2–CF2– disorder coincides with the appearance of dendritic forms of the crystal phases. [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]

Trimethyl(trifluoromethyl)silane, (CH3)3SiCF3, has been in situ pressure frozen in a diamond anvil cell and its structure determined at 0.90(5) GPa/296 K by single-crystal X-ray diffraction. The crystal is monoclinic, space group P21/m, with the molecules lying on crystallographic mirror planes. The CH3 and CF3 groups assume the fully staggered conformation. The 14-fold coordination scheme of the molecules is similar to those in (CH3)3SiCl polymorphs, but different from that in crystalline tetramethylsilane, (CH3)4Si (TMS). The longest Si—C bond length of 1.943(12) Å observed in the crystal structure has been documented. The shortest intermolecular contacts in the structure of pressure-frozen CF3Si(CH3)3 are observed between the hydrogen atoms, and those involving fluorine atoms are longer than sums of van der Waals’ radii. These structural features explain the facile cleavage of Si—CF3 bond for CF3 group transfer in organic reactions. [Copyright &y& Elsevier]

Abstract: The structure of 4-hydroxy-1-methylpiperidinium perchlorate has been characterized by X-ray diffraction, FTIR, Raman and NMR spectroscopies, and by DFT calculations. The crystals belong to orthorhombic Pbca group. The piperidine ring has a chair conformation with the hydroxyl and methyl groups in the equatorial positions. The hydroxyl group interacts by bifurcated O–H···O hydrogen bonds with the ClO4 − anion of 2.905(5) and 3.045(5)Å. Two optimized structures of the title compound have been calculated by the B3LYP/6–31G(d,p) approach. In one of them the anion is involved in the N+–H···O hydrogen bonds of 2.629Å, while in the other the anion interacts electrostatically with the N+ atom of the 4-hydroxy-1-methylpiperidinium cation. In aqueous solution two 4-hydroxy-1-methylpiperidinium perchlorates are in the conformational equilibrium. Their conformations are discussed on the basis of the NMR spectra. The solid state FTIR spectrum is consistent with the X-ray data. [Copyright &y& Elsevier]

Abstract: Pentafluoropyridine has been pressure frozen in situ in a diamond-anvil cell (DAC) and its structure determined at 0.30(5) and 1.10(5)GPa and at room temperature by single-crystal X-ray diffraction. The freezing pressure of pentafluoropyridine has been determined to be 0.10(5)GPa. The crystals are monoclinic, space group P21/c. The crystal packing is governed by F⋯F and C/N⋯F van der Waals contacts, but no ring stacking is observed. The intermolecular interactions are non-directional, and the crystal compresses nearly isotropically between 0.3 and 1.1GPa. [Copyright &y& Elsevier]

The competing effects of squeezed OH...O bonds, destabilizing the H-atom position, and of displaced hydrogen donor and acceptor groups, favouring the ordered H-atom sites, have been tuned by pressure in the pressure-frozen dichloroacetic acid. Its structure has been determined at 0.1, 0.7, 0.9 and 1.4 GPa: in this pressure range the crystals are stable in the monoclinic space group P21/ n. The molecules are O—H...O hydrogen bonded into dimers, which in turn interact via a unique pattern of halogen...halogen contacts. Between 0.1 and 1.4 GPa the OH...O bond is squeezed from 2.674 (13) to 2.632 (9) Å. Within the pressure range investigated the hydrogen bonds are squeezed and the shear displacement of the molecules compensate, and the H atoms remain ordered. [ABSTRACT FROM AUTHOR]

Bromochloromethane, CH2BrCl, has been temperature-frozen and in situ pressure-frozen and the structure determined by X-ray diffraction at low temperatures of 170 and 100 K at ambient pressure (0.10 MPa), and at high pressures of 1.04 and 1.72 GPa at room temperature (295 K). CH2BrCl exhibits a remarkable polymorphism: at low temperature it crystallizes in the monoclinic space group C2/ c (phase I), isostructural to the crystals of CH2Br2. The pressure-frozen crystal of CH2BrCl is orthorhombic, space group Pbcn, and is isostructural to the crystal of CH2Cl2. In both phases I and II the Br and Cl atoms are substitutionally disordered. The freezing temperatures and pressures of simple dihalomethanes have been correlated to their molecular weight and halogen... ˙˙halogen distances. Calculated electrostatic potential surfaces have been related to the different crystal packing of dihalomethanes investigated. [ABSTRACT FROM AUTHOR]

Abstract: The structures of a series of co-crystals formed by enantiomers of BINOL (2,2′-dihydroxy-1,1′-binaphthalene) with (R,R)-1,2-diaminocyclohexane (DACH) and (R,R)-1,2-diamino-1,2-diphenylethane have been determined by X-ray diffraction to investigate the factors responsible for diastereoselective co-crystallization, the process responsible for the highly effective resolution of rac-BINOL by DACH. The absolute configurations of the molecules and directional preferences of the hydrogen bonds in the supramolecular aggregates have been analyzed. The supramolecular aggregation of hydrogen-bonded BINOL and diamines shows considerable distortions from the helical shape, which indicates that cohesion forces other than hydrogen bonds play a significant role in the diastereoselective resolution. [Copyright &y& Elsevier]

Abstract: The formation of a disproportionation dimer of 4,6-pyrimidinedione 1, 2-(4,6-dioxo-5-pyrimidinyl)-4,6-dioxo-1,2,3,5,5-pentahydropyrimidine 2, results from the reactivity of the inner-salt tautomers or ions of 1, contrasting to the general properties of nucleobases which are all stable in the lactam or dilactam molecular forms. The observation of dimer 2, indefinitely stable in the crystalline state, reveals an unusual chemical reactivity strongly dependent on the H-tautomeric forms of the substrate and product. [Copyright &y& Elsevier]

Ethyl propionate, C5H10O2 (m.p. 199 K), has been in-situ pressure-frozen and its structure determined at 1.34, 1.98 and 2.45 GPa. The crystal structure of the new high-pressure phase (denoted β) is different from phase α obtained by lowering the temperature. The freezing pressure of ethyl propionate at 296 K is 1.03 GPa. The molecule assumes an extended chain s-trans–trans–trans conformation, only slightly distorted from planarity. The closest intermolecular contacts in both phases are formed between carbonyl O and methyl H atoms; however, the ethyl-group H atoms in phase β form no contacts shorter than 2.58 Å. A considerable molecular volume difference of 24.2 Å3 between phases α and β can be rationalized in terms of degrees of freedom of molecules arranged into closely packed structures: the three degrees of freedom allowed for rearrangements of molecules confined to planar sheets in phase α, but are not sufficient for obtaining a densely packed pattern. [ABSTRACT FROM AUTHOR]

Centrosymmetric pyrazinium NH+⋯N bonded complexes allow their dielectric properties to be analyzed separately from the bulk ferroelectric polarization observed in the noncentrosymmetric DABCO monosalts. The method of dielectric permittivity measurements has been employed for monitoring polarization fluctuations generated by proton transfers in the NH+⋯N bonded linear polycations. The revealed dielectric response of [C4H5N2]+BF4- and [C4H5N2]+ClO4- cannot be reconciled with the centrosymmetric symmetry of their structures, but suggests formation of polar defects or nanoregions. Unique transformations of the pyrazine [C4H5N2]+BF4- complex between linear NH+⋯N hydrogen-bonded polycationic aggregates antiparallel in phase γ, perpendicular chains in phase β, and with NH+⋯N bonds broken in phase α have been observed. The [C4H5N2]+ClO4- and [C4H5N2]+BF4- complexes as grown at 290 K are isostructural in orthorhombic phase 7, space group Pbcm, with linear polycationic chains arranged antiparallel. On heating, the tetrafluoroborate and perchlorate salts each undergoes two first-order phase transitions at similar temperatures about 340–360 K, while on cooling only one phase transition has been observed. An extremely unique sequence of two phase transitions subsequently lowering the symmetry of the [C4H5N2]+BF4- crystal when temperature is increased has been evidenced: the orthorhombic phase γ heated above 343 K transforms into the monoclinic C2/c-symmetric phase β, in which the NH+⋯N bonded linear chains assume perpendicular arrangement; and at about 353–357 K the anions and cations adopt a typical ionic-crystal packing without homonuclear NH+⋯N hydrogen bonds in a still lower-symmetry monoclinic P21/n-symmetric structure. The exceptional perpendicular arrangement of the linear NH+⋯N bonded chains in phase β constitutes a unique system where polarization of small regions can assume various orientations within a plane, depending on the H+ sites. No phase transitions or anomalous dielectric response were observed in the NH+⋯O bonded [C4H5N2]+NO3- complex. The unprecedented structure-property relations of the pyrazinium complexes fully confirm the role of the NH+⋯N bond transformations for the dielectric response of analogous DABCO ferroelectrics. [ABSTRACT FROM AUTHOR]

The structure of 1,2,3-trichloropropane, ClCH2CHClCH2Cl, in-situ crystallized in a diamond–anvil cell, has been determined by single-crystal X-ray diffraction at 0.28 and 0.35 GPa. A melting point at 295 K and 0.22 GPa has been determined. The molecular conformation of aliphatic chain and terminal Cl atoms is approximately C2 symmetric. Of the intermolecular contacts, the tightest are the Cl⋯Cl and then the Cl⋯H contacts, whereas the H⋯H distances are considerably longer than the sum of the van der Waals radii and leave narrow voids in the structure. The elevated pressure reduces the H⋯H separations but hardly affects the Cl⋯Cl distances. The crystal growth rates, compression and types of intermolecular interactions in the structures can be correlated with the directions of the Cl⋯Cl contacts, which can be considered the main motif responsible for the molecular arrangement in the structure. [ABSTRACT FROM AUTHOR]

The shadowing of the primary beam and of reflections from a powder crystal sample enclosed in a diamond-anvil high-pressure cell (DAC) is described for unrestricted data-collection procedures. The corrections account for the shadowing of the data measured with point- or area-detector diffractometers and can be applied for any containers of environment devices enclosing a powdered sample. The general procedure for correcting the powder diffraction data can be applied for analysing and optimizing diffractometric procedures of data collection and provides facilities for collecting high-pressure powder diffraction data with the DAC positioned optimally and rotated to counteract the preferred-orientation and particle-size effects, and to increase the resolution of data. The quadrature effects of numerical integration for the accuracy of computed corrections have been analysed. [ABSTRACT FROM AUTHOR]

The structure of the model layered compound of [C(NH2)3]+ · NO3- has been refined using high-resolution powder neutron diffraction data collected at 4 and 395 K. While in the low-temperature the hydrogen bond geometry is typical of the medium-strength heteronuclear hydrogen bonds, at high-temperature the H-atoms shift to the centres between the N and O atoms. The H-centring in NH--O hydrogen bonds has been rationalized in terms of thermal activation of the hydrogens in the diabatically modified potential-energy wells, as well as of specific features of diffraction data scattered of a strongly vibrating structure. [Copyright &y& Elsevier]

The method of exhaustive methylation has been applied for investigating tautomeric stability and its temperature variation of 6-hydroxy-5-methyl-3-pyridazinone (1) and its N-phenyl derivative (2)—two model compounds chosen for studying tautomeric transformations of pyridazine medicinal drugs, either in the process of their production, or then their activity in the living tissue. The resulting N-methylated and O-methylated products were separated, and characterized by 1H NMR and X-ray diffraction. The methylation of 1 and 2 in the dimethyl sulfate as well as in its aqueous solution leads predominantly to the O-methylated forms, but the rising temperature favours the N-methylation. [Copyright &y& Elsevier]

A brief account of transformations of internal structure of hydrogen bonds, and transformations involving the breaking and formation of hydrogen bonds is highlighted for a selected group of crystals. The main stress is laid on the short bistable -OH…O= hydrogen bonds, illustrating several general properties, often blurred for weaker hydrogen bonds. A set of parameters relating the electronic structure of the oxygen atoms and geometry of hydrogen bonds is described. This allows one to combine the hydrogen bond dimensions with changes in the electronic structure of the oxygen atoms, accompanying any transformations of hydrogen bonds. Several resultant effects are described: coupling of displacements of molecules or ions with the H-atom transfer, the isotope effect of deuteration on T c in ferroelectric crystals, a skew trajectory of a disordered H-atom with respect to the O···O direction, or shortening of the O···O distance when the H-atom becomes disordered. It is shown that hydrogen-bond patterns, molecular arrangements and thermodynamic properties of crystals are interdependent. Several examples and possible applications of the presented model are given. [ABSTRACT FROM AUTHOR]

The reaction of chlorination of maleic-hydrazide (1) with POCl3 is specific to the substrate polymorph. Chlorination of either the triclinic polymorph of maleic-hydrazide (1), denoted MH1, or the monoclinic polymorph MH2 leads to a single product of 3,6-dichloropyridazine (2), while chlorination of the monoclinic polymorph MH3 under the same conditions results in two products: 3,6-dichloropyridazine (2) and 6-chloro-3-pyridazinone (3). The structural differences between polymorphs and their topochemical chlorination in phosphorus oxychloride are discussed. The crystal structure of 6-chloro-3-pyridazinone (3) monohydrate, determined in this study by X-ray diffraction, is built of ribbons of hydrogen-bonded 3 and water molecules. It has been established that the chlorination of ground crystals of polymorph MH3 yields exclusively the dichloropyridazine, which suggests that the crystal habit of MH3, the size of crystallites and the lower solubility of MH3 than MH1 and MH2 are the main reasons for the different course of the reaction. [Copyright &y& Elsevier]

The crystals of a 1:1 complex of 1,2-bis(diethylaminomethyl)-4,5-dichlorobenzene perchlorate with HClO4 were studied using X-ray diffraction and FT-IR spectroscopy. There are two symmetry-independent ionic pairs in the crystal, monoclinic, space group P21/n. In both symmetry-independent cations, the diethylaminomethyl substituents are in a similar Z conformation relative to the phenyl moieties, however significant conformational differences in the diethylaminomethyls have been noted. The diethylaminomethyls are linked by intracationic +N–H⋯N hydrogen bonds of 2.758(9) and 2.721(7) A˚ in two symmetry-independent cations. The protons in these H-bonds are disordered in two off-center sites, which is most likely to be connected with the considerable disorder of the perchlorate anions. The IR spectrum of the crystals shows a broad intense band and a relatively intense continuous absorption indicating relatively large proton polarizability in the intramolecular hydrogen bond of a medium length. [Copyright &y& Elsevier]

The mixed crystals of 1,2-bis(diethylaminomethyl)-3,6-di- and 3,4,6-trichloro-benzene perchlorate at 0.5:0.5 ratio have been studied using X-ray diffraction and FT-IR spectroscopy. The molecules of di- and tri-chloro derivatives are distributed randomly in the crystal lattice; however, this disorder manifested mainly by a partial occupation of the chlorine atom at C(4) and by relatively big atomic temperature parameters even at 99 K, particularly for the oxygens of the perchlorate anion. The proton in the [NH⋯N]+ bond refined close to the equidistant position between the nitrogens. The conformations of the diethylaminomethyl substituents are pseudo-symmetrical relative to the plane and twofold axis passing through the midpoint of the hydrogen bond. The IR spectrum of the crystals shows a broad intense band and an intense continuous absorption indicating relatively high proton polarizability in the intramolecular hydrogen bond. In acetonitrile and in chloroform, the proton in the intramolecular hydrogen bond also shows a very high proton polarizability demonstrated by the continuous absorption in the IR spectrum. [Copyright &y& Elsevier]