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 H₄B₄O₉^2- 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.

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.

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.

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.

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^-.

Crystal morphology fixed by interplay of π-stacking and hydrogen bonds – the case of 1-hydroxypyrene

Crystal structure of 1-hydroxypyrene has been determined and its luminescence in the solid state described. An interplay of π-stacking and H-bonds results in a conserved morphology and great flexibility of the crystals. This crystal structure can be described as a set of ‘molecular springs’.

Are heart rate monitors valuable tools for diagnosing arrhythmias in endurance athletes?

Millions of physically active individuals worldwide use heart rate monitors (HRMs) to control their exercise intensity. In many cases, the HRM indicates an unusually high heart rate (HR) or even arrhythmias during training. Unfortunately, studies assessing the reliability of these devices to help control HR disturbances during exercise do not exist. We examined 142 regularly training endurance runners and cyclists, aged 18-51 years, with unexplained HR abnormalities indicated by various HRMs to assess the utility of HRMs in diagnosing exertion-induced arrhythmias. Each athlete simultaneously wore a Holter electrocardiogram (ECG) recorder and an HRM during typical endurance training in which they had previously detected "arrhythmias" to verify the diagnosis. Average HRs during exercise were precisely recorded by all types of HRMs. No signs of arrhythmia were detected during exercise in approximately 39% of athletes, and concordant HRs were recorded by the HRMs and Holter ECG. HRMs indicated surprisingly high short-term HRs in 45% of athletes that were not detected by the Holter ECG and were artifacts. In 15% of athletes, single ventricular/supraventricular beats were detected by the Holter ECG but not by the HRM. We detected a serious tachyarrhythmia in the HRM and Holter ECG data with concomitant clinical symptoms in only one athlete, who was forced to cease exercising. We conclude that the HRM is not a suitable tool for monitoring heart arrhythmias in athletes and propose an algorithm to exclude the suspicion of exercise-induced arrhythmia detected by HRMs in asymptomatic, physically active individuals.

Hoveyda–Grubbs catalyst analogues bearing the derivatives of N-phenylpyrrol in the carbene ligand – structure, stability, activity and unique ruthenium–phenyl interactions

N-Phenylpyrrole-2,6-diisopropylphenyl ruthenium complex and its perbrominated derivative are active in ring-closing metathesis at 80 °C, but inactive at room temperature.

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.

Synthesis and catalytic activity of ruthenium indenylidene complexes bearing unsymmetrical NHC containing a heteroaromatic moiety

New ruthenium(ii) indenylidene catalysts were synthesized and used in olefin metathesis reactions in toluene under air, leading to high conversions and good selectivities.

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.

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 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.

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 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.

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.