High-pressure studies of the hydrogen bond networks in molecular crystals

High pressures studies on bis(L-alaninate)copper(II) by Raman spectroscopy and synchrotron X-ray diffraction

The crystal of bis(L-alaninate)copper(II) [Cu(C3H6NO2)2] was studied by Raman spectroscopy and synchrotron X-ray diffraction as a function of hydrostatic pressure, and its vibrational and structural behavior were investigated to analyze its stability at high pressures. The Raman spectra of bis(L-alaninate)copper(II) show changes in vibrational modes that are associated with deformations and stretching of units involving the copper atom. These results indicate that molecular fragments involving the copper atom undergo rotations and discontinuities in bond lengths. The lattice parameters of bis(L-alaninate)copper(II) obtained from Le Bail fits also exhibit changes in the same pressure ranges as the Raman spectra. The discontinuities in the angular parameter beta are compatible with the rotations of the molecular fragments. Bis(L-alaninate)copper(II) undergoes changes, but maintains monoclinic symmetry in the range of 0-20.1 GPa.

Synchrotron radiation X-ray diffraction and Raman spectroscopy study of l-asparagine monohydrate doped with Fe(III) at high pressure

Crystals of l-asparagine monohydrate doped with Fe(III) were studied by Raman spectroscopy in a diamond anvil cell (DAC) in the spectral range from 100 to 3200 cm^-1 and pressures up to 9.2 GPa. The behavior of external modes suggests conformational changes between 3.0 and 4.0 GPa mainly affecting the CH₂ group. X-ray diffraction measurements with synchrotron radiation were performed in the angular range from 3 to 12 degrees (2θ) up to 9.3 GPa. The lattice parameters contract up to 9.3 GPa, with the exception of parameter b, which exhibits expansion from 7.2 GPa. The lattice parameters exhibit discontinuities between 3.0 and 4.0 GPa, this effect is compatible with conformational changes. Such modifications occur without a change in symmetry, at least up to 9.3 GPa. Under decompression, down to atmospheric pressure, the original Raman spectrum is recovered, showing that the conformational change and the other changes are all reversible.

High-Pressure Polymorphism in Hydrogen-Bonded Crystals: A Concise Review

High-pressure polymorphism is a developing interdisciplinary field. Pressure up to 20 GPa is a powerful thermodynamic parameter for the study and fabrication of hydrogen-bonded polymorphic systems. This review describes how pressure can be used to explore polymorphism and surveys the reports on examples of compounds that our group has studied at high pressures. Such studies have provided insight into the nature of structure–property relationships, which will enable crystal engineering to design crystals with desired architectures through hydrogen-bonded networks. Experimental methods are also briefly surveyed, along with two methods that have proven to be very helpful in the analysis of high-pressure polymorphs, namely, the ab initio pseudopotential plane–wave density functional method and using Hirshfeld surfaces to construct a graphical overview of intermolecular interactions.

Phase stability analysis of shocked ammonium dihydrogen phosphate by X-ray and Raman scattering studies

Impact of shock waves on non-linear optical materials bring about a lot of unknown behaviors of materials and such kinds of shock wave recovery experiments are highly required for the better understanding of material-property relationship. In the present context, we have performed experiments on the impact of structural properties of ammonium dihydrogen phosphate (ADP) samples under shock wave loaded conditions and the results of the test samples have been evaluated by X-ray diffraction (XRD), Raman spectroscopy, diffused reflectance spectroscopy (DRS) and field emission scanning electron microscopic (FESEM) technique. Interestingly, prismatic face of ADP shows loss of degree of crystallinity whereas pyramidal face shows enhancement of crystalline nature with respect to number of shock pulses due to shock wave induced dynamic re-crystallization. Hence, the present problem is worthy enough to unearth and understand the anisotropic nature of the ADP crystal and their structural modifications at shock wave loaded conditions.

Semi-empirical and ab initio calculations for crystals under pressure at fixed temperatures: the case of guanidinium perchlorate

A simple semi-empirical approach is proposed to calculate structure and properties of crystals under pressure at fixed temperatures. The computed semi-empirical pressure dependencies for guanidinium perchlorate are in good agreement with available experimental data. Ab initio results within quasi-harmonic approximation for guanidinium perchlorate are also presented.

Shock wave induced defect engineering on structural and optical properties of pure and dye doped potassium dihydrogen phosphate crystals

Based on the importance of the shock recovery experiments, the authors report the structural and optical properties of pure and 0.001 M dye-doped potassium dihydrogen phosphate (KDP) crystals for virgin and shock wave loaded samples. Rhodamine B and Methylene blue dyes are selected as dopants to be doped with KDP crystal for the present investigation. The test crystals of pure and doped KDP crystals are grown by slow evaporation technique and cut and polished crystals of (200) face are used for the present investigation. Table-top pressure driven shock tube is utilized for the shock wave generation and the used functional Mach number is 1.7. Virgin and shock wave loaded test crystals’ surface morphology, structural properties and optical transmissions are observed using optical microscope, powder X-ray diffractometer and UV-Visible spectrometer, respectively. Crystalline nature and optical transmission of pure and doped KDP crystals are found to have reduced by the impact of shock waves. It occurs due to the enhancement of defect concentration on the surface of the test crystals. From the observed results, we assert that the pure KDP crystal is relatively more stable to shock wave induced damage compared to doped KDP crystals as reflected by structural and optical studies.

Pressure-induced phase transitions in DL-glutamic acid monohydrate crystal

DL-glutamic acid monohydrate crystal was synthesized from an aqueous solution by slow evaporation technique. The crystal was submitted to high-pressure (1 atm-14.3 GPa) to investigate its vibrational behavior and the occurrence of phase transitions. We performed Raman spectroscopy as probe and through the analysis of the spectra we discovered three structural phase transitions. The first one occurs around 0.9 GPa. In this phase transition, glutamic acid molecules suffer modifications in their conformations while water molecules are less affected. The second phase transition at 4.8 GPa involves conformational changes related to CO₂^-, NH₃⁺ units and the water molecules, while the third one, between 10.9 and 12.4 GPa, involves motions of several parts of the glutamic acid as well as the water molecules. Considering the dynamic of high pressure, the second phase of DL-glutamic acid monohydrate crystal presented a better stability compared with the second phase of its polymorphs α and β L-glutamic acid. In addition, water molecules seem to play important role on this structural stability. All changes are reversible.

Pressure-Induced Amorphization of Diisopropylammonium Perchlorate Studied by Raman Spectroscopy and X-ray Diffraction

Diisopropylammonium salts have drawn attention in recent years due to their room-temperature ferroelectric properties. Triclinic diisopropylammonium perchlorate (DIPAP) exhibits ferroelectricity at room temperature. We have carried out density functional theory calculations to assign the phonon modes in DIPAP. High-pressure Raman spectra of DIPAP are recorded up to ∼3 GPa. Discontinuity in the NH₂ bending and stretching mode frequencies and the appearance of new bands at 0.7 GPa suggest a phase transition by a rearrangement in the hydrogen network. Broadening of lattice modes at 1.3-1.7 GPa indicates a loss of crystalline nature above 1.7 GPa. High-pressure synchrotron X-ray diffraction of DIPAP shows an isostructural phase transition at 0.6 GPa and confirms amorphization at 1.5 GPa that may lead to a loss of ferroelectricity above this pressure. The ambient phase becomes reversible after releasing the pressure. The bulk modulus of DIPAP is determined to be 16.5 GPa.

Effect of shock waves on structural and dielectric properties of ammonium dihydrogen phosphate crystal

In this research article, the authors pay attention to investigate the effect of structural and dielectric properties of ammonium dihydrogen phosphate (ADP) crystal under pre and post shock loaded conditions. A shock wave of Mach number 1.9 was utilized for the present investigation which was generated by a table-top pressure driven shock tube. The crystalline nature and grain size variations were estimated by powder X-ray diffraction technique. The grain size of post shock wave loaded ADP crystal is found to be larger than that of the pre shock wave loaded ADP crystal. The dielectric properties of the pre and post shock loaded crystals were analyzed by impedance analyzer as a function of frequency (1 kHz–1 MHz) at ambient temperature. The dielectric constant is observed to be varying from 346 to 362 at the frequency of 400 kHz for pre and post shock wave loaded ADP crystals, respectively. The obtained results suggest that shock waves can be an alternate tool to tailor the physical properties of materials without creating any change in the original crystal system and surface morphology.

DORI Reveals the Influence of Noncovalent Interactions on Covalent Bonding Patterns in Molecular Crystals Under Pressure

The study of organic molecular crystals under high pressure provides fundamental insight into crystal packing distortions and reveals mechanisms of phase transitions and the crystallization of polymorphs. These solid-state transformations can be monitored directly by analyzing electron charge densities that are experimentally obtained at high pressure. However, restricting the analysis to the featureless electron density does not reveal the chemical bonding nature and the existence of intermolecular interactions. This shortcoming can be resolved by the use of the DORI (density overlap region indicator) descriptor, which is capable of simultaneously detecting both covalent patterns and noncovalent interactions from electron density and its derivatives. Using the biscarbonyl[14]annulene crystal under pressure as an example, we demonstrate how DORI can be exploited on experimental electron densities to reveal and monitor changes in electronic structure patterns resulting from molecular compression. A novel approach based on a flood-fill-type algorithm is proposed for analyzing the topology of the DORI isosurface. This approach avoids the arbitrary selection of DORI isovalues and provides an intuitive way to assess how compression packing affects covalent bonding in organic solids.

The nature of the chemical bond in oxyanionic crystals based on QTAIM topological analysis of electron densities

The QTAIM topological analysis of the calculated electron densities in oxyanionic crystals revealed the covalency criteria for metal–oxygen and hydrogen bonds.

Pressure-induced phase transition of 4-aminobenzonitrile: the formation and enhancement of N–H⋯N weak hydrogen bonds

A reversible pressure-induced structural phase transition of 4-aminobenzonitrile was found at about 0.3 GPa by conducting in situ high-pressure synchrotron angle-dispersive X-ray diffraction (ADXRD) experiments. The discontinuous changes of Raman modes at 0.2 GPa confirmed the occurrence of phase transition. In situ high-pressure Raman spectra indicated that the molecular arrangement and intermolecular interactions changed abruptly. The process of this phase transition continued up to about 1.0 GPa. When the pressure reached 1.1 GPa, the initial N–H⋯N interaction transformed into a new weak hydrogen bond, which was enhanced by further compression. The ab initio calculations and Hirshfeld surfaces were used to illustrate the above views. This study gives an example that demonstrates that the pressure can induce the formation of hydrogen bonds, which contributes to the development of supramolecular chemistry.

The initial N–H⋯N interactions in 4-aminobenzonitrile crystals are enhanced and changed into weak hydrogen bonds by high pressure.

Pressure-induced phase transition in N–H⋯O hydrogen-bonded crystalline malonamide

In this study, malonamide (C₃H₆N₂O₂) was compressed under up to 10.4 GPa of pressure in a diamond anvil cell at room temperature.

Pressure-induced phase transition in hydrogen-bonded molecular crystal acetamide: combined Raman scattering and X-ray diffraction study

Two structural phase transitions are observed at ∼0.9 and ∼3.2 GPa in acetamide using in situ synchrotron X-ray diffraction (XRD) and Raman scattering techniques.

Weak hydrogen bonds formed by thiol groups in N-acetyl-(L)-cysteine and their response to the crystal structure distortion on increasing pressure

The effect of hydrostatic pressure on single crystals of N-acetyl-l-cysteine was followed at multiple pressure points from 10(-4) to 6.2 GPa with a pressure step of 0.2-0.3 GPa by Raman spectroscopy and X-ray diffraction. Since in the crystals of N-acetyl-l-cysteine the thiol group is involved in intermolecular hydrogen bonds not as a donor only (bonds S-H···O) but also as an acceptor (bonds N-H···S), increasing the pressure does not result in phase transitions. This makes a contrast with the polymorphs of l- and dl-cysteine, in which multiple phase transitions are observed already at relatively low hydrostatic pressures and are related to the changes in the conformation of the thiol side chains only weakly bound to the neighboring molecules in the structure and thus easily switching over the weak S-H···O and S-H···S hydrogen bonds. No phase transitions occur in N-acetyl-l-cysteine with increasing pressure, and changes in cell parameters and volume vs pressure do not reveal any peculiar features. Nevertheless, a more detailed analysis of the changes in intermolecular distances, in particular, of the geometric parameters of the hydrogen bonds based on X-ray single crystal diffraction analysis, complemented by an equally detailed study of the positions of all the significant bands in Raman spectra, allowed us to study the fine details of subtle changes in the hydrogen bond network. Thus, as pressure increases, a continuous shift of the hydrogen atom of the thiol group from one acceptor (a carboxyl group) to another acceptor (a carbonyl group) is observed. Precise single-crystal X-ray diffraction and polarized Raman spectroscopy structural data reveal the formation of a bifurcated S-H···O hydrogen bond with increasing pressure starting with ∼1.5 GPa. The analysis of the vibrational bands in Raman spectra has shown that different donor and acceptor groups start "feeling" the formation of the bifurcated S-H···O hydrogen bond in different pressure ranges. The results are discussed in relation to some of the previously published data on the effect of high pressure on the polymorphs of l-cysteine, dl-cysteine, and glutathione, that show similarity with the effects reported here for N-acetyl-l-cysteine. The results obtained in this work allow one to suggest new models for the pressure-induced structural rearrangements in the whole family of cysteine-containing crystals.

A high-pressure single-crystal to single-crystal phase transition in DL-alaninium semi-oxalate monohydrate with switching-over hydrogen bonds

A single-crystal to single-crystal transition in DL-alaninium semi-oxalate monohydrate at a pressure between 1.5 and 2.4 GPa was studied by single-crystal X-ray diffraction and Raman spectroscopy. This is the first example of a single-crystal diffraction study of a high-pressure phase transition in a crystalline amino acid salt hydrate. Selected hydrogen bonds switch over and become bifurcated, whereas the others are compressed continuously. The transition is accompanied by pronounced discontinuities in the cell parameters and volume versus pressure, although no radical changes in the molecular packing are induced. Although, in contrast to DL-alanine, in the crystal structure of the salt there are short O-H···O hydrogen bonds, the structure of the salt is more compressible. At the same time, the structure of DL-alanine does not undergo pressure-induced phase transitions, whereas the structure of DL-alaninium semi-oxalate monohydrate does, and at a relatively low pressure. The anisotropy of lattice strain for the low-pressure phase differs from that on cooling at ambient pressure; interestingly, the anisotropy of the pressure-induced compression of the high-pressure phase is quite similar to the lattice strain of the low-pressure phase on cooling.

Pressure-induced phase transition in N-H···O hydrogen-bonded molecular crystal oxamide

The effect of high pressure on the structural stability of oxamide has been investigated in a diamond anvil cell by Raman spectroscopy up to ∼14.6 GPa and by angle-dispersive X-ray diffraction (ADXRD) up to ∼17.5 GPa. The discontinuity in Raman shifts around 9.6 GPa indicates a pressure-induced structural phase transition. This phase transition is confirmed by the change of ADXRD spectra with the symmetry transformation from P1 to P1. On total release of pressure, the diffraction pattern returns to its initial state, implying this transition is reversible. We discuss the pressure-induced variations in N-H stretching vibrations and the amide modes in Raman spectra and propose that this phase transition is attributed to the distortions of the hydrogen-bonded networks.

Effect of pressure on crystalline L- and DL-serine: revisited by a combined single-crystal X-ray diffraction at a laboratory source and polarized Raman spectroscopy study

Information on the effect of pressure on hydrogen bonds, which could be derived from single-crystal X-ray diffraction at a laboratory source and polarized Raman spectroscopy, has been compared. L-Serine and DL-serine were selected for this case study. The role of hydrogen bonds in pressure-induced phase transitions in the first system and in the structural stability of the second one are discussed. Non-monotonic distortion of selected hydrogen bonds in the pressure range below ∼ 1–2 GPa, a change in the compression mechanism at ∼ 2–3 GPa, and the evidence of formation of bifurcated N—H...O hydrogen bonds in DL-serine at ∼ 3–4 GPa are considered.

Structural properties and halogen bonds of cyanuric chloride under high pressure

The effects of high pressure on cyanuric chloride (C(3)N(3)Cl(3)), a remarkable crystal structure dominated by halogen bonds, have been studied by synchrotron X-ray diffraction and Raman spectroscopy in a diamond anvil cell. The results of high pressure experiments revealed that there was no obvious phase transition up to 30 GPa, indicating that halogen bonding is an effective noncovalent interaction to stabilize the crystal structure. Moreover, cyanuric chloride exhibited a high compressibility and a strong anisotropic compression, which can be explained by the layered crystal packing. Ab initio calculations were also performed to account for the high pressure Raman spectra and the high pressure behavior of halogen bonding.

Effect of hydrostatic pressure on the γ-polymorph of glycine. 1. A polymorphic transition into a new δ-form

The results of a high-resolution powder diffraction study of the effect of high hydrostatic pressure up to 8 GPa on the pure γ-polymorph of glycine (P31) are discussed. A phase transition with a jumpwise change of cell volume and cell parameters was observed. The transition starts at about 2.73 GPa and is still not complete even at 7.85 GPa. The crystal structure of the previously unknown high-pressure polymorph of glycine (δ-polymorph) could be solved and refined in the space group Pn. In this structure, glycine zwitter-ions are linked via NH…O hydrogen bonds into layers, which form double-layered bands via additional NH…O hydrogen bonds. The structure of the individual layers in the high-pressure polymorph is similar to that in the previously known α- (P21/n) and β- (P21) forms, but the packing of the layers is essentially different. The pressure-induced polymorphic transformation in the γ-glycine can be compared with a change in the secondary structure of a peptide, when a helix is transformed into a sheet.

A comparative study of the anisotropy of lattice strain induced in the crystals of L-serine by cooling down to 100 K or by increasing pressure up to 4.4 GPa

The anisotropy of lattice strain in the crystals of L-serine (P212121, at ambient conditions a = 5.615(1) Å, b = 8.589(2) Å, c = 9.346(2) Å) on cooling down to 100 K and with increasing hydrostatic pressure up to 4.4 GPa was compared with each other and also with the results previously obtained for the polymorphs of glycine. On cooling, the structure expanded slightly along the crystallographic a-direction, compression along the crystallographic b- and c-directions (normal to the chains of the serine zwitter-ions) was very similar. With increasing pressure, the same structure compressed in all the crystallographic directions, linear strain along c-axis was the largest, linear strain along a-axis — the smallest, linear compression along the b-axis with increasing pressure was slightly larger than that along the a-axis. The different anisotropy of lattice strain of the same structure on cooling and under pressure could be correlated with different response of intermolecular hydrogen bonds to these two scalar actions.

Variable temperature (100—295 K) single-crystal X-ray diffraction study of the α-polymorph of glycylglycine and a glycylglycine hydrate

The crystal structures of theα-polymorph of glycylglycine (glygly) and of its hydrate (glygly × 1.5 H2O) were refined by single-crystal X-ray diffraction at 100, 150, 220, and 295 K (glygly) and at 100, 200, 295 K (glygly × 1.5 H2O). The values of the volume thermal expansions of glygly and its hydrate were shown to be larger than for the three polymorphs of glycine. The anisotropy of strain on cooling was analyzed. Despite a smaller bulk thermal expansion measured for glygly, linear strain (both compression and expansion) along the axes of the strain ellipsoids was larger for the structure of glygly, than for the structure of glygly × 1.5 H2O. The contributions of the distortion of the intermolecular hydrogen bonds and of the conformational changes of the zwitter-ions to the lattice strain are considered.

A comparative study of the anisotropy of lattice strain induced in the crystals of DL-serine by cooling down to 100 K, or by increasing pressure up to 8.6 GPa. A comparison with L-serine

The anisotropy of lattice strain in the crystals of DL-serine (P21/n) on cooling down to 100 K and with increasing hydrostatic pressure up to 8.6 GPa was studied by single-crystal X-ray diffraction. In contrast to L-serine undergoing pressure-induced phase transitions at about 5 and 8 GPa, no phase transitions were observed in DL-serine at least up to 8.6 GPa (the highest pressure reached in the experiment). The anisotropy of strain in DL-serine on cooling was shown to be radically different from that with increasing pressure. The response of the crystal structure of DL-serine to cooling and to increasing pressure was considerably different from that of L-serine.

Stability of hydrogen-bonded supramolecular architecture under high pressure conditions: pressure-induced amorphization in melamine-boric acid adduct

The effects of high pressure on the structural stability of the melamine-boric acid adduct (C3N6H(6).2H3BO3, M.2B), a three-dimensional hydrogen-bonded supramolecular architecture, were studied by in situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy. M.2B exhibited a high compressibility and a strong anisotropic compression, which can be explained by the layerlike crystal packing. Furthermore, evolution of XRD patterns and Raman spectra indicated that the M.2B crystal undergoes a reversible pressure-induced amorphization (PIA) at 18 GPa. The mechanism for the PIA was attributed to the competition between close packing and long-range order. Ab initio calculations were also performed to account for the behavior of hydrogen bonding under high pressure.

High-pressure diffraction studies of molecular organic solids. A personal view

This paper discusses the trends in the experimental studies of molecular organic solids at high pressures by diffraction techniques. Crystallization of liquids, crystallization from solutions and solid-state transformations are considered. Special attention is paid to the high-pressure studies of pharmaceuticals and of biomimetics.