Pressure-Dependent Structural and Luminescence Properties of 1-(Pyren-1-yl)but-2-yn-1-one.

The crystal structure of 1-(pyren-1-yl)but-2-yn-1-one ( 1 a , a polynuclear aromatic hydrocarbon displaying enhanced luminescence in the solid state, has been re-determined at several pressures ranging from atmospheric up to 3 GPa using a Diamond Anvil Cell (DAC). These experiments were augmented by periodic DFT calculations at pressures up to 4.4 GPa. UV-Vis fluorescence of 1 a at non-ambient pressures has also been investigated. The crystal structure consists of infinite π -stacks of anti-parallel 1 a molecules with discernible dimers, which may exemplify aggregates formed by pyrene derivatives in solution and thin films, and is predominantly stabilized by dispersion. The average inter-planar distance between individual molecules within π -stacks decreases with pressure in the investigated range. This results in piezochromic properties of 1 a : a red-shift of sample color, as well as a bathochromic shift of fluorescence with pressure (by ca. 100 nm at 3.5 GPa). Two-component fluorescence spectra support the hypothesis that at least two types of excimers are involved in the electronic excitation processes in crystalline 1 a .

Ruthenium Complexes Bearing Thiophene-Based Unsymmetrical N-Heterocyclic Carbene Ligands as Selective Catalysts for Olefin Metathesis in Toluene and Environmentally Friendly 2-Methyltetrahydrofuran.

Three mono-N-heterocyclic carbene (NHC) ruthenium 2-isopropoxybenzylidene (10 a-c) and one bis(NHC) indenylidene complex (8) bearing an unsymmetrical N-heterocyclic carbene ligand were synthesized and structurally characterized by single-crystal X-ray diffraction. The catalytic activity of the newly obtained complexes were evaluated in ring-closing metathesis (RCM) and ene-yne (RCEYM) reactions in toluene and environmentally friendly 2-MeTHF under air. The results confirmed that although all tested reactions can be successfully mediated by catalysts 10 a-c, their general reactivity is lower than the benchmark all-purpose Ru catalysts with symmetrical NHC ligands. However, the latter cannot compete with specialized ruthenium complex 10 a in industrially relevant self-CM of terminal olefins in neat conditions.

Charge density redistribution with pressure in a zeolite framework.

As a result of external compression applied to crystals, ions relax, in addition to shortening the bond lengths, by changing their shape and volume. Modern mineralogy is founded on spherical atoms, i.e., the close packing of spheres, ionic or atomic radii, and Pauling and Goldschmidt rules. More advanced, quantum crystallography has led to detailed quantitative studies of electron density in minerals. Here we innovatively apply it to high-pressure studies up to 4.2 GPa of the mineral hsianghualite. With external pressure, electron density redistributes inside ions and among them. For most ions, their volume decreases; however, for silicon volume increases. With growing pressure, we observed the higher contraction of cations in bonding directions, but a slighter expansion towards nonbonding directions. It is possible to trace the spatial redistribution of the electron density in ions even at the level of hundredths parts of an electron per cubic angstrom. This opens a new perspective to experimentally characterise mineral processes in the Earth's mantle. The use of diamond anvil cells with quantum crystallography offers more than interatomic distances and elastic properties of minerals. Interactions, energetic features, a branch so far reserved only to the first principle DFT calculations at ultra-high-pressures, become available experimentally.

Charge density studies of single and transient (single to double) boron-oxygen bonds in (NH4)2B4O5(OH)4·2H2O.

A H4B4O92- ion which makes up the (NH4)2B4O5(OH)4·2H2O crystal structure has two types of boron-oxygen bonds, i.e. single B-O bonds and an intermediate between single and double BO bonds. Differences between these two bond types are visible not only because they differ by their lengths but also a topology of electron density distribution differs. This also gives a hint as to how to distinguish between these two bond types. Experimental results based on multipole model refinement gave excellent agreement with theoretical calculations and literature data. Calculations at bond critical points for B-O and BO (electron density, the Laplacian of electron density and the localized-orbital locator function) suggest us how boron-oxygen bonds should be categorised with respect to compounds previously reported in the literature. Additionally, a novel synthesis method for the investigated compound has been developed, which involves crystallization from an aqueous solution of BH3NH3 dissolved in a mixture of tetrahydrofuran and water.

Tracing electron density changes in langbeinite under pressure.

Pressure is well known to dramatically alter physical properties and chemical behaviour of materials, much of which is due to the changes in chemical bonding that accompany compression. Though it is relatively easy to comprehend this correlation in the discontinuous compression regime, where phase transformations take place, understanding of the more subtle continuous compression effects is a far greater challenge, requiring insight into the finest details of electron density redistribution. In this study, a detailed examination of quantitative electron density redistribution in the mineral langbeinite was conducted at high pressure. Langbeinite is a potassium magnesium sulfate mineral with the chemical formula [K2Mg2(SO4)3], and crystallizes in the isometric tetartoidal (cubic) system. The mineral is an ore of potassium, occurs in marine evaporite deposits in association with carnallite, halite and sylvite, and gives its name to the langbeinites, a family of substances with the same cubic structure, a tetrahedral anion, and large and small cations. Single-crystal X-ray diffraction data for langbeinite have been collected at ambient pressure and at 1 GPa using a combination of in-house and synchrotron techniques. Experiments were complemented by theoretical calculations within the pressure range up to 40 GPa. On the basis of changes in structural and thermal parameters, all ions in the langbeinite structure can be grouped into 'soft' (potassium cations and oxygens) and 'hard' (sulfur and magnesium). This analysis emphasizes the importance of atomic basins as a convenient tool to analyse the redistribution of electron density under external stimuli such as pressure or temperature. Gradual reduction of completeness of experimental data accompanying compression did not significantly reduce the quality of structural, electronic and thermal parameters obtained in experimental quantitative charge density analysis.

Accurate crystal structure of ice VI from X-ray diffraction with Hirshfeld atom refinement.

Water is an essential chemical compound for living organisms, and twenty of its different crystal solid forms (ices) are known. Still, there are many fundamental problems with these structures such as establishing the correct positions and thermal motions of hydrogen atoms. The list of ice structures is not yet complete as DFT calculations have suggested the existence of additional and - to date - unknown phases. In many ice structures, neither neutron diffraction nor DFT calculations nor X-ray diffraction methods can easily solve the problem of hydrogen atom disorder or accurately determine their anisotropic displacement parameters (ADPs). Here, accurate crystal structures of H2O, D2O and mixed (50%H2O/50%D2O) ice VI obtained by Hirshfeld atom refinement (HAR) of high-pressure single-crystal synchrotron and laboratory X-ray diffraction data are presented. It was possible to obtain O-H/D bond lengths and ADPs for disordered hydrogen atoms which are in good agreement with the corresponding single-crystal neutron diffraction data. These results show that HAR combined with X-ray diffraction can compete with neutron diffraction in detailed studies of polymorphic forms of ice and crystals of other hydrogen-rich compounds. As neutron diffraction is relatively expensive, requires larger crystals which can be difficult to obtain and access to neutron facilities is restricted, cheaper and more accessible X-ray measurements combined with HAR can facilitate the verification of the existing ice polymorphs and the quest for new ones.

Experimental charge density of grossular under pressure - a feasibility study.

X-ray diffraction studies of crystals under pressure and quantitative experimental charge density analysis are among the most demanding types of crystallographic research. A successful feasibility study of the electron density in the mineral grossular under 1 GPa pressure conducted at the CRISTAL beamline at the SOLEIL synchrotron is presented in this work. A single crystal was placed in a diamond anvil cell, but owing to its special design (wide opening angle), short synchrotron wavelength and the high symmetry of the crystal, data with high completeness and high resolution were collected. This allowed refinement of a full multipole model of experimental electron distribution. Results are consistent with the benchmark measurement conducted without a diamond-anvil cell and also with the literature describing investigations of similar structures. Results of theoretical calculations of electron density distribution on the basis of dynamic structure factors mimic experimental findings very well. Such studies allow for laboratory simulations of processes which take place in the Earth's mantle.

Electrostatic matching versus close-packing molecular arrangement in compressed dimethyl sulfoxide (DMSO) polymorphs.

Single crystals of dimethyl sulfoxide (DMSO, (CH3)2SO) were in situ frozen at isochoric conditions in a diamond-anvil cell and their structures determined at 0.37, 0.56, and 2.4 GPa. At ambient pressure, DMSO freezes at 291 K in monoclinic phase alpha, space group P2(1)/c, stable in all the temperature range from its melting point down to 2 K. On increasing the pressure, DMSO freezes at 140 MPa/296 K in phase alpha too, but above 540 MPa it collapses into a more compact triclinic phase beta, space group P1. The molecular aggregation in the crystal structure of DMSO is dominated by electrostatic attraction between negative and positive sites on the molecular surface and CH...O hydrogen bonds linking the molecules into dimers and chains. Most of this electrostatic matching and CH...O bonds present in phase alpha are preserved above 540 MPa, but the more tight packing in phase beta is achieved at the cost of broken dipole-dipole interactions between antiparallel SO groups and repulsing contacts between electropositive H-atoms squeezed to distances commensurate with the sum of van der Waals radii. The isostructural relation between phases alpha and beta is strictly connected with the molecular aggregation governed by electrostatic matching of interatomic contacts.

Differences and similarities among hypoxanthinium nitrate hydrate structures.

Crystals of hypoxanthinium (6-oxo-1H,7H-purin-9-ium) nitrate hydrates were investigated by means of X-ray diffraction at different temperatures. The data for hypoxanthinium nitrate monohydrate (C5H5N4O+·NO3-·H2O, Hx1) were collected at 20, 105 and 285 K. The room-temperature phase was reported previously [Schmalle et al. (1990). Acta Cryst. C46, 340-342] and the low-temperature phase has not been investigated yet. The structure underwent a phase transition, which resulted in a change of space group from Pmnb to P21/n at lower temperature and subsequently in nonmerohedral twinning. The structure of hypoxanthinium dinitrate trihydrate (H3O+·C5H5N4O+·2NO3-·2H2O, Hx2) was determined at 20 and 100 K, and also has not been reported previously. The Hx2 structure consists of two types of layers: the `hypoxanthinium nitrate monohydrate' layers (HX) observed in Hx1 and layers of Zundel complex H3O+·H2O interacting with nitrate anions (OX). The crystal can be considered as a solid solution of two salts, i.e. hypoxanthinium nitrate monohydrate, C5H5N4O+·NO3-·H2O, and oxonium nitrate monohydrate, H3O+(H2O)·NO3-.

An insight into real and average structure from diffuse X-ray scattering - a case study.

Two-dimensional diffuse X-ray scattering from an organic salt [N-(3-(2,6-dimethylanilino)-1-methylbut-2-enylidene)-2,6-dimethylanilinium chloride, C21H27N2(+)Cl(-)] was interpreted with the help of an analytical model of diffuse scattering. An analysis of the relationship between symmetry and diffuse scattering for the studied system has been undertaken. The symmetry of the system explains the extinction pattern, taking the form of curves, on the diffuse scattering planes. We have also tested the relationship between the average structure model and scattering intensities. Two models, differing in their representation of overlapping atoms, were used. In the case of diffuse scattering the difference between resulting intensities is immense, while for the Bragg intensities it is much smaller. This sensitivity of diffuse scattering could potentially be used to improve the description of the average structure.

Pressure-freezing with conformational conversion of 3-aminopropan-1-ol molecules.

3-Aminopropan-1-ol, NH(2)(CH(2))(3)OH, was pressure-frozen and its structure determined at 0.2, 0.9 and 1.31 GPa by single-crystal X-ray diffraction. The freezing pressure of 0.13 GPa at 296 K was measured by ruby fluorescence in the diamond-anvil cell and from compressibility measurement in the piston-and-cylinder reaction press. The molecules assume an extended conformation in the crystalline state, different from the pseudo-ring conformers, with the terminal groups linked by an intramolecular hydrogen bond, present in the gaseous and liquid states. The polar arrangement in the 3-aminopropan-1-ol crystals is explained in terms of the pattern of intermolecular hydrogen bonds.

In-situ high-pressure study of the ordered phase of ethyl propionate.

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 beta) is different from phase alpha 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 beta form no contacts shorter than 2.58 A. A considerable molecular volume difference of 24.2 A3 between phases alpha and beta 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 alpha, but are not sufficient for obtaining a densely packed pattern.

Compressed hydrogen-bond effects in the pressure-frozen chloroacetic acid.

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 P2(1)/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) A. Within the pressure range investigated the hydrogen bonds are squeezed and the shear displacement of the molecules compensate, and the H atoms remain ordered.

Absence of halogen...halogen interactions in chlorotrimethylsilane polymorphs.

The structures of in situ pressure-frozen chlorotrimethylsilane crystals, (CH3)3SiCl, have been determined at 0.23, 0.30 and 0.58 GPa. The molecular arrangements in the low-temperature and high-pressure phases are two-dimensionally isostructural, but different in the third perpendicular direction. Consequently, a striking similarity exists between the unit-cell dimensions of these polymorphs. The absence of short Cl...Cl contacts, both in the low-temperature or pressure-frozen phases of (CH3)3SiCl, has been rationalized in terms of the favoured packing patterns and comparable energies of halogen...halogen interactions and other van der Waals forces.