Energy frameworks: insights into interaction anisotropy and the mechanical properties of molecular crystals

Evaluating the importance of fractional Z′ polymorphs in a trifluoromethylated N,N′-diphenyloxalamide derivative

A curious case of crystal dimorphism reveals an adjusted fractional number of molecules in their respective crystallographic asymmetric units.

On the kinetics of solvate formation through mechanochemistry

We demonstrate that solvates obtained through mechanochemistry are the thermodynamic products, and that the kinetics of solvate formation are related to the easiness of breaking the reactant crystals.

Piroxicam–clonixin drug–drug cocrystal solvates with enhanced hydration stability

Piroxicam and clonixin can form drug–drug crystalline complexes with the help of suitable solvent molecules.

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’.

Experimental and computational approaches to produce and characterise isostructural solvates

A combination of experiment and theory was applied to rationalise the formation, stability and phase transitions of isostructural dapsone hemisolvates. Critical solvent properties as well as structural and energetic features are discussed.

Conformational aspects of polymorphs and phases of 2-propyl-1H-benzimidazole

This paper reports on the polymorphism of 2-propyl-1H-benzimidazole (2PrBzIm) induced by temperature change. Upon heating, an irreversible reconstructive-type phase transition at T = 384 K from the ordered form I (P212121) to a new polymorph, form II HT (Pcam), was observed. The structural transformation between forms I and II involves significant changes in the crystal packing, as well as a key conformational variation around the propyl chain of the molecule. After the first irreversible phase transition, the II HT form undergoes two further (reversible) phase transitions upon cooling at 361 K (II RT) and 181 K (II LT). All three phases (forms II HT, II RT and II LT) have almost identical crystal packing and, given the reversibility of the conversions as a function of temperature, they are referred to as form II temperature phases. They differ, however, with respect to conformational variations around the propyl chain of 2PrBzIm. Energy calculations of the gas-phase conformational energy landscape of this compound about its flexible bonds allowed us to classify the observed conformational variations of all forms into changes and adjustments of conformers. This reveals that forms I and II are related by conformational change, and that two of the form II phases (HT and RT) are related by conformational adjustment, whilst the other two (RT and LT) are related by conformational change. We introduce the term 'conformational phases' for different crystal phases with almost identical packing but showing changes in conformation.

Crystal structure, interaction energies and experimental electron density of the popular drug ketoprophen

The crystal and molecular structure of the pure (S)-enantiomer of the popular analgesic and anti-inflammatory drug ketoprophen (α-ket) is reported. A detailed aspherical charge-density model based on high-resolution X-ray diffraction data has been refined, yielding a high-precision geometric description and classification of the O-H⋯O interactions as medium strength hydrogen bonds. The crystal structure of the racemic form of ketoprophen (β-ket) was also redetermined at 100 K, at 0.5 Å resolution. A previously unreported disorder (10% occupancy) was discovered. In contrast to the racemic β-ket case, the (S)-enantiomer crystallizes with two independent molecules in the asymmetric unit with two distinct conformations. The major difference between the β-ket and α-ket crystal forms lies in the formation of distinct hydrogen-bonded motifs: a closed ring motif in β-ket versus infinite chains of hydrogen bonds in the chiral α-ket structure. However, the overall crystal packing of both forms is surprisingly similar, with close-packed layers of antiparallel-oriented benzo-phenone moieties bound by C-H⋯π interactions. Notably, the most important stabilizing term in the total lattice energies in both instances proved to be the dispersion related to these interactions. Both forms of the title compound (α- and β-ket) were additionally characterized by differential scanning calorimetry and thermogravimetric analysis.

A novel binuclear hydrazone-based Cd(II) complex is a strong pro-apoptotic inducer with significant activity against 2D and 3D pancreatic cancer stem cells

A novel binuclear Cd complex (1) with hydrazone-based ligand was prepared and characterized by spectroscopy and single crystal X-ray diffraction techniques. Complex 1 reveals a strong pro-apoptotic activity in both human, mammary adenocarcinoma cells (MCF-7) and pancreatic AsPC-1 cancer stem cells (CSCs). While apoptosis undergoes mostly caspase-independent, 1 stimulates the activation of intrinsic pathway with noteworthy down regulation of caspase-8 activity in respect to non-treated controls. Distribution of cells over mitotic division indicates that 1 caused DNA damage in both cell lines, which is confirmed in DNA interaction studies. Compared to 1, cisplatin (CDDP) does not achieve cell death in 2D cultured AsPC-1 cells, while induces different pattern of cell cycle changes and caspase activation in 2D cultured MCF-7 cells, implying that these two compounds do not share similar mechanism of action. Additionally, 1 acts as a powerful inducer of mitochondrial superoxide production with dissipated trans-membrane potential in the majority of the treated cells already after 6 h of incubation. On 3D tumors, 1 displays a superior activity against CSC model, and at 100 μM induces disintegration of spheroids within 2 days of incubation. Fluorescence spectroscopy, along with molecular docking show that compound 1 binds to the minor groove of DNA. Compound 1 binds to the human serum albumin (HSA) showing that the HSA can effectively transport and store 1 in the human body. Thus, our current study strongly supports further investigations on antitumor activity of 1 as a drug candidate for the treatment of highly resistant pancreatic cancer.

Crystal anisotropy explains structure-mechanics impact on tableting performance of flufenamic acid polymorphs

Anisotropic features with other crystallographic properties like d-spacing, and attachment energy (E_att) can predict material performance during the secondary pharmaceutical processing. A newly developed state-of-the-art compression cell lodged in a powder X-ray diffractometer was used to measure anisotropic Young's moduli (YM) of flufenamic acid (FFA) polymorphs in this study. Methodology is based on the generation of a single crystal deformation in this cell, which reflects as a change in the d-spacing in the PXRD pattern. Anisotropic YM was calculated from such information gathered along different FFA planes. Measured FFA crystallographic molecular features were concatenated to understand macroscopic compaction (Heckel and Shapirao's parameters) and tableting performance. Block shaped crystals of FFA form I, and III after initial characterization with SEM, DSC, PXRD, and FTIR were compressed normal to X, Y, and Z-planes, identified from calculated PXRD pattern using the reported single crystal structure. YM of X and Y planes of form I was significantly higher than corresponding planes of form III. Z plane of form III showed significantly higher YM than that for form I. Low YM of form III can be attributed to its large d-spacing regardless of their high E_att than form I, as well as orientation of supramolecular acid dimer (OH⋯O) homosynthon chains in the FFA planes. FFA form I stiffness was further confirmed with lower densification and higher yield pressure of deformation than form III. Clearly, form III exhibited better compressibility, compactibility, and tableting performance than form I due to favorable molecular and macroscopic features. Thus, developed anisotropic measurement approach can be used to distinguish material performance in the early development stage of the pharmaceutical processes.

Relating the tableting behavior of piroxicam polytypes to their crystal structures using energy-vector models

Piroxicam crystallises into two polytypes, α₁ and α_2, with crystal structures that contain identical molecular layers but differ in the way that these layers are stacked. In spite of having close structural similarity, the polytypes have significantly different powder tabletting behaviour: α₂ forms only weak tablets at low pressures accompanied by extensive capping and lamination, which make it impossible to form intact tablets above 100 MPa, while α₁ exhibits superior tabletability over the investigated pressure range (up to 140 MPa). The potential structural origin of the different behaviour is sought using energy-vector models, produced from pairwise intermolecular interaction energies calculated using the PIXEL method. The analysis reveals that the most stabilising intermolecular interactions define columns in both crystal structures. In α₂, a strongly stabilising interaction between inversion-related molecules links these columns into a 2-D network, while no comparable interaction exists in α₁. The higher dimensionality of the energy-vector model in α₂ may be one contributor to its inferior tabletability. A consideration of probable slip planes in the structures identifies regions where the benzothiazine groups of the molecules meet. The energy-vector models in this region are geometrically similar for both structures, but the interactions are more stabilising in α₂ compared to α₁. This feature may also contribute to the inferior tabletability of α₂.

Quantifying intermolecular interactions for isoindole derivatives: substituent effect vs. crystal packing

The aim of the study was to examine noncovalent interactions in considerably different crystal packings of three isoindole compounds. Their structures were compared to three other closely-related derivatives described earlier in the literature. Here we discuss the crystal structures in the context of the hydrogen-bonded motifs and other weak interactions. The hierarchy of investigated intermolecular interactions was examined in a quantitative manner through pairwise interaction energies and energy framework analysis.

Intermolecular interactions in crystals of small unsubstituted cyclic ethers and substituted epoxides

CE-B3LYP model energies are used to investigate intermolecular interactions in crystals of the relatively weakly bound cyclic ethers, as well as a number of substituted epoxides that have been the focus of high-quality experimental electron density studies. This approach readily provides a complete picture of all intermolecular interactions in these molecular crystals, and CE-B3LYP lattice energies for the unsubstituted cyclic ethers are in excellent agreement with available thermodynamic data. When compared with the outcomes of multipole modelling of X-ray diffraction data, these results suggest that experimental interaction energies are typically underestimated and, contrarily, experimental lattice energies are typically overestimated. These observations deserve careful investigation.

An exploration of O-H⋯O and C-H⋯π inter-actions in a long-chain-ester-substituted phenyl-phenol: methyl 10-[4-(4-hydroxyphenyl)phenoxy]decanoate

An understanding of the driving forces resulting in crystallization vs organogel formation is essential to the development of modern soft materials. In the mol-ecular structure of the title compound, methyl 10-[4-(4-hydroxyphenyl)phen-oxy]decanoate (MBO10Me), C23H30O4, the aromatic rings of the biphenyl group are canted by 6.6 (2)° and the long-chain ester group has an extended conformation. In the crystal, mol-ecules are linked by O-H⋯O hydrogen bonds, forming chains along [10[Formula: see text]]. The chains are linked by C-H⋯O hydrogen bonds, forming layers parallel to the ac plane. The layers are linked by C-H⋯π inter-actions, forming a three-dimensional supra-molecular structure. The extended structure exhibits a lamellar sheet arrangement of mol-ecules stacking along the b-axis direction. Each mol-ecule has six nearest neighbors and the seven-mol-ecule bundles stack to form a columnar superstructure. Inter-action energies within the bundles are dominated by dispersion forces, whereas inter-columnar inter-actions have a greater electrostatic component.

Insights on the Similarity of Supramolecular Structures in Organic Crystals Using Quantitative Indexes

The quest for concepts of isostructurality in organic crystals has been long and mostly based on geometric data, even with the development of modern software. This field of study is of great interest to the pharmaceutical industry and for the prediction of crystal structures. Despite this, there is still no methodology that provides broad quantitative and comparable similarity data between two complete crystalline structures. The present study demonstrated that the similarity between two crystalline structures could be estimated from the similarity between the two "supramolecular clusters". Quantitative indexes for similarity comparisons of crystal structures were shown using nine 5-aryl-1-(1,1-dimethylethyl)-1H-pyrazoles as a model. This proposal includes the quantitative data of a geometric parameter (I D), a contact area parameter (I C), and an energetic parameter (I G). The proposed indexes exhibited good perspective regarding the similarity data and distinct regions of similarity. The range of similarity was set at I X ≥ 0.80, 0.80 > I X > 0.60, and I X ≤ 0.60 (X = D, C, or G). Indexes with a value near 1.0 indicate systems with isostructural, isocontact, and isoenergetic behavior. The results indicated that supramolecular structures with high similarity must have high values for all three indexes (I D, I C, and I G).

Supramolecular Organization of Nonstoichiometric Drug Hydrates: Dapsone

The observed moisture- and temperature dependent transformations of the dapsone (4,4'-diaminodiphenyl sulfone, DDS) 0. 33-hydrate were correlated to its structure and the number and strength of the water-DDS intermolecular interactions. A combination of characterization techniques was used, including thermal analysis (hot-stage microscopy, differential scanning calorimetry and thermogravimetric analysis), gravimetric moisture sorption/desorption studies and variable humidity powder X-ray diffraction, along with computational modeling (crystal structure prediction and pair-wise intermolecular energy calculations). Depending on the relative humidity the hydrate contains between 0 and 0.33 molecules of water per molecule DDS. The crystal structure is retained upon dehydration indicating that DDS hydrate shows a non-stoichiometric (de)hydration behavior. Unexpectedly, the water molecules are not located in structural channels but at isolated-sites of the host framework, which is counterintuitively for a hydrate with non-stoichiometric behavior. The water-DDS interactions were estimated to be weaker than water-host interactions that are commonly observed in stoichiometric hydrates and the lattice energies of the isomorphic dehydration product (hydrate structure without water molecules) and (form III) differ only by ~1 kJ mol-1. The computational generation of hypothetical monohydrates confirms that the hydrate with the unusual DDS:water ratio of 3:1 is more stable than a feasible monohydrate structure. Overall, this study highlights that a deeper understanding of the formation of hydrates with non-stoichiometric behavior requires a multidisciplinary approach including suitable experimental and computational methods providing a firm basis for the development and manufacturing of high quality drug products.

Energy frameworks and a topological analysis of the supramolecular features in in situ cryocrystallized liquids: tuning the weak interaction landscape via fluorination

Weak intermolecular interactions observed in crystalline materials are often influenced or forced by stronger interactions such as classical hydrogen bonds. Room temperature liquids offer a scenario where such strong interactions are absent so that the role and nature of the weak interactions can be studied more reliably. In this context, we have analyzed the common organic reagent benzoyl chloride (BC) and a series of its fluorinated derivatives using in situ cryocrystallography. The intermolecular interaction energies have been estimated and their topologies explored using energy framework analysis in a series of ten benzoyl chloride analogues, which reveal that the ππ stacking interactions serve as the primary building blocks in these crystal structures. The crystal packing is also stabilized by a variety of interaction motifs involving weak C-HO/F/Cl hydrogen bonds and FF, FCl, and ClCl interactions. It is found that fluorination alters the electrostatic nature of the benzoyl chlorides, with subsequent changes in the formation of different weak interaction motifs. The effects of fluorination on these weak intermolecular interactions have been systematically analyzed further via detailed inputs from a topological analysis of the electron density and Hirshfeld surface analysis.

Characterization of the short OC⋯OC π-hole tetrel bond in the solid state

An in-depth structure database investigation and experimental charge density analysis of the OC⋯OC π-hole tetrel bonds.

Hand-twistable plastically deformable crystals of a rigid small organic molecule

The crystals of the small rigid molecule 4-bromobenzonitrile exhibit highly flexible plastic bending behaviour that occurs on two perpendicular faces of the crystal, a rare situation, leading to the formation of helical/twisted and curled crystals.

Diversity of methyl group dynamics in felodipine: a DFT supported NMR and QENS study

Computationally-supported NMR and neutron scattering experiments were combined to provide new insights into the structure–dynamics relationship in the most stable polymorph of felodipine.

Observation of bending, cracking and jumping phenomena on cooling and heating of tetrahydrate berberine chloride crystals

Tetrahydrate berberine chloride crystals undergo cracking, bending and jumping on cooling as well as on heating at room temperature with a rapid conversion to a dihydrate phase.

Computational and analytical approaches for investigating hydrates: the neat and hydrated solid-state forms of 3-(3-methylimidazolium-1-yl)propanoate

The interconversion pathways and stability ranges of OOCEMIM solid-state forms have been elucidated.

Tetraiodoallene, I2C=C=CI2 – the missing link between I2C=CI2 and I2C=C=C=CI2 – and the oxidation product, 2,2-diiodoacrylicacid, I2C=CH(CO2H)

The X-ray structure of tetraiodoallene is reported. On standing, atmospheric hydrolysis converts this compound into 2,2-diiodoacrylic acid, for which a structure has also been determined. Energy framework diagrams have been constructed for the two compounds.

Lack of dependence of mechanical properties of baicalein cocrystals on those of the constituent components

Three baicalein (BAI) cocrystals with nicotinamide (NCT), caffeine (CAF), and isoniazid (ISN) exhibited excellent tabletability despite the markedly different tabletability of the coformers.

The Polymorphs of ROY: A Computational Study of Lattice Energies and Conformational Energy Differences

The remarkable structural diversity observed in polymorphs of 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (commonly known as ROY) challenges computational attempts to predict or rationalize their relative stability. This modest study explores the applicability of CE-B3LYP model energy calculation of lattice energies (using experimental crystal structures), supplemented by a systematic approach to account for conformational energy differences. The CE-B3LYP model provides sensible estimates of absolute and relative lattice energies for the polymorphs, provided care is taken to achieve convergence in the summation of pairwise terms. Conformational energy differences based on density functional theory (DFT) energies are shown to be unreliable, but MP2 energies based on DFT-optimized structures show considerable promise.

New Insights into Solid Form Stability and Hydrate Formation: o-Phenanthroline HCl and Neocuproine HCl

The moisture- and temperature dependent stabilities and interrelation pathways of the practically relevant solid forms of o-phenanthroline HCl (1) and neocuproine HCl (2) were investigated using thermal analytical techniques (HSM, DSC and TGA) and gravimetric moisture sorption/desorption studies. The experimental stability data were correlated with the structural changes observed upon dehydration and the pairwise interaction and lattice energies calculated. For 1 the monohydrate was identified as the only stable form under conditions of RH typically found during production and storage, but at RH values >80% deliquescence occurs. The second compound, 2, forms an anhydrate and two different hydrates, mono- (2-Hy1) and trihydrate (2-Hy3). The 2-Hy1 structure was solved from SCXRD data and the anhydrate structure derived from a combination of PXRD and CSP. Depending on the environmental conditions (moisture) either 2-Hy1 or 2-Hy3 is the most sable solid form of 2 at RT. The monohydrates 1-Hy1 and 2-Hy1 show a high enthalpic stabilization (≥20 kJ mol⁻¹) relative to the anhydrates. The anhydrates are unstable at ambient conditions and readily transform to the monohydrates even in the presence of traces of moisture. This study demonstrates how the right combination of experiment and theory can unravel the properties and interconversion pathways of solid forms.

Structural studies of (E)-2-(benzylidene)-1-tetralone derivatives: crystal structures and Hirshfeld surface analysis

Crystal structures are reported from data collected at 100 K of (E)-2-(X-benzylidene)-1-tetralone (2: X=3-O2N; 3: X=4-O2N; 5: X=4-HO; 6: X=4-Me2N; 7: 4-NC), (E)-2-(X-benzylidene)-6-MeO-1-tetralone, 8, and (E)-2-(X-benzylidene)-5-MeO-1-tetralone 9. Also reported herein are the Hirshfeld surface calculations for these compounds as well as those of (E)-2-(X-benzylidene)-1-tetralone (1: X=H; 4: X=4-MeO), whose structures were previously reported. The molecules are not planar as shown by the dihedral angles of 45.66(5)–69.78(5)° between the phenyl groups and by the puckered cyclohexenyl rings. A common feature of the molecular conformations is the C–H···O1(carbonyl) intramolecular hydrogen bond. The carbonyl oxygen atom plays significant roles in the interactions in all compounds baring compound 8. However, there is no consistent set of intermolecular interaction in this group of compounds. Intermolecular interactions present in each compound are some of the O–H···O, C–H···A (A=O, N or π), A–O···π (A=C or N) and π···π interactions. The only compound exhibiting a classical O–H···O hydrogen bond is compound 5. C–H···π interactions are found in all compounds, and while π···π interactions are present in compounds 2, 3, 7 and 9, no consistent type of stacking arrangement is shown. The Hirshfeld surface calculations, while generally concurring with the intermolecular interactions indicated by PLATON analyses, also reveal short interactions, which fall below the PLATON cut-off parameters.

Structural characterization, gelation ability, and energy-framework analysis of two bis(long-chain ester)-substituted 4,4'-biphenyl compounds

There are few examples of single-crystal structure determinations of gelators, as gel formation requires that the dissolved gelator self-assemble into a three-dimensional network structure incorporating solvent via noncovalent interactions rather than self-assembly followed by crystallization. In the solid-state structures of the isostructural compounds 4,4'-bis[5-(methoxycarbonyl)pentyloxy]biphenyl (BBO6-Me), C₂₆H₃₄O₆, and 4,4'-bis[5-(ethoxycarbonyl)pentyloxy]biphenyl (BBO6-Et), C₂₈H₃₈O₆, the molecules sit on a crystallographically imposed center of symmetry, resulting in strictly coplanar phenyl rings. BBO6-Me behaves as an organogelator in various alcohol solvents, whereas BBO6-Et does not. The extended structure reveals bundles of molecules that form a columnar superstructure. Framework-energy calculations reveal much stronger interaction energies within the columns (-52 to -78 kJ mol^-1) than between columns (-2 to -16 kJ mol^-1). The intracolumnar interactions are dominated by a dispersion component, whereas the intercolumnar interactions have a substantial electrostatic component.

Hydrogen-Bonding Interactions in Luminescent Quinoline-Triazoles with Dominant 1D Crystals

Quinoline-triazoles 2-((4-(diethoxymethyl)-1H-1,2,3-triazol-1-yl)methyl)quinoline (1), 2-((4-(m-tolyl)-1H-1,2,3-triazol-1-yl)methyl)quinoline (2) and 2-((4-(p-tolyl)-1H-1,2,3-triazol-1-yl)methyl)quinoline (3) have been prepared with CuAAC click reactions and used as a model series to probe the relationship between lattice H-bonding interaction and crystal direction of growth. Crystals of 1-3 are 1D tape and prism shapes that correlate with their intermolecular and solvent 1D lattice H-bonding interactions. All compounds were thermally stable up to about 200 C and blue-green emissive in solution.

Molecular Level Understanding of the Reversible Phase Transformation between Forms III and II of Dapsone

The reversible solid-state phase transformation between the neat forms II and III of dapsone (DDS) was studied using thermal analytical methods, variable temperature X-ray diffraction and solid-state modeling at the electronic level. The first order III ↔ II phase transformation occurs at 78 ± 4 °C with a heat of transition of 2 kJ mol^-1 and a small hysteresis. The two isosymmetric polymorphs (both P2₁2₁2₁) differ only in movement of layers of molecules and show a small variation in conformation. The combination of variable-temperature single-crystal structure determinations and pair-wise intermolecular energy calculations allowed us to unravel the single-to-single crystal transformation at a molecular level, to estimate the molecular contributions to the heat of transformation and to rationalize why the room and low temperature form III is the less dense polymorphic form, which is a rare phenomenon in enantiotropically related pairs of polymorphs in molecular crystals.

CrystalExplorer model energies and energy frameworks: extension to metal coordination compounds, organic salts, solvates and open-shell systems

The application domain of accurate and efficient CE-B3LYP and CE-HF model energies for intermolecular interactions in molecular crystals is extended by calibration against density functional results for 1794 molecule/ion pairs extracted from 171 crystal structures. The mean absolute deviation of CE-B3LYP model energies from DFT values is a modest 2.4 kJ mol-1 for pairwise energies that span a range of 3.75 MJ mol-1. The new sets of scale factors determined by fitting to counterpoise-corrected DFT calculations result in minimal changes from previous energy values. Coupled with the use of separate polarizabilities for interactions involving monatomic ions, these model energies can now be applied with confidence to a vast number of molecular crystals. Energy frameworks have been enhanced to represent the destabilizing interactions that are important for molecules with large dipole moments and organic salts. Applications to a variety of molecular crystals are presented in detail to highlight the utility and promise of these tools.

Expedited development of a high dose orally disintegrating metformin tablet enabled by sweet salt formation with acesulfame

Salt formation has been extensively used to improve drug properties, including solubility, stability and mechanical properties. A sweet salt of metformin with acesulfame, prepared though an anion exchange reaction, showed superior properties over the commercial hydrochloride salt. These included both remarkable improvement of taste and significant enhancement in tabletability, which is explained by the different crystal structures and lower hardness as measured by nanoindentation. The relationship among crystal structure, mechanical properties and tabletability was rationalized through an energy framework analysis. This approach led to the successful development of an orally disintegrating tablet product containing 60% of metformin-acesulfame salt by direct compaction.

Exploring the rare S—H...S hydrogen bond using charge density analysis in isomers of mercaptobenzoic acid

Experimental and theoretical charge density analyses on isomers of mercaptobenzoic acid have been carried out to quantify the hydrogen bonding of the hitherto less explored thiols, to assess the strength of the interactions using the topological features of the electron density. The electron density study offers interesting insights into the nature of the S—H...S interaction. The interaction energy is comparable with that of a weak hydrogen bond. The strength and directionality of the S—H...S hydrogen bond is demonstrated to be mainly due to the conformation locking potential of the intramolecular S...O chalcogen bond in 2-mercaptobenzoic acid and is stronger than in 3-mercaptobenzoic acid, which lacks the intramolecular S...O bond. Thepara-substituted mercaptobenzoic acid depicts a type I S...S interaction.

Is there a future for topological analysis in experimental charge-density research?

Topological analysis using Bader and co-worker'sAtoms in Moleculestheory has seen many applications in theoretical chemistry and experimental charge-density research. A brief overview of successful early developments, establishing topological analysis as a research tool for characterizing intramolecular chemical bonding, is provided. A lack of vision in many `descriptive but not predictive' subsequent studies is discussed. Limitations of topology for providing accurate energetic estimates of intermolecular interaction energies are put into perspective. It is recommended that topological analyses of well understood bonding situations are phased out and are only reported for unusual bonding. Descriptive studies of intermolecular interactions should have a clear research focus.

Atom interaction propensities of oxygenated chemical functions in crystal packings

The crystal contacts of several families of hydrocarbon compounds substituted with one or several types of oxygenated chemical groups were analyzed statistically using the Hirshfeld surface methodology. The propensity of contacts to occur between two chemical types is described with the contact enrichment descriptor. The systematic large enrichment ratios of some interactions like the O-H⋯O hydrogen bonds suggests that these contacts are a driving force in the crystal packing formation. The same statement holds for the weaker C-H⋯O hydrogen bonds in ethers, esters and ketones, in the absence of polar H atoms. The over-represented contacts in crystals of oxygenated hydrocarbons are generally of two types: electrostatic attractions (hydrogen bonds) and hydrophobic interactions. While Cl⋯O interactions are generally avoided, in a minority of chloro-oxygenated hydrocarbons, significant halogen bonding does occur. General tendencies can often be derived for many contact types, but outlier compounds are instructive as they display peculiar or rare features. The methodology also allows the detection of outliers which can be structures with errors. For instance, a significant number of hydroxylated molecules displaying over-represented non-favorable oxygen-oxygen contacts turned out to have wrongly oriented hydroxyl groups. Beyond crystal packings with a single molecule in the asymmetric unit, the behavior of water in monohydrate compounds and of crystals with Z' = 2 (dimers) are also investigated. It was found in several cases that, in the presence of several oxygenated chemical groups, cross-interactions between different chemical groups (e.g. water/alcohols; alcohols/phenols) are often favored in the crystal packings. While some trends in accordance with common chemical principles are retrieved, some unexpected results can however appear. For example, in crystals of alcohol-phenol compounds, the strong O-H⋯O hydrogen bonds between two phenol groups turn out to be extremely rare, while cross contacts between phenols and alcohols have enriched occurrences.

Intermolecular interactions in molecular crystals: what’s in a name?

Structure–property relationships are the key to modern crystal engineering, and for molecular crystals this requires both a thorough understanding of intermolecular interactions, and the subsequent use of this to create solids with desired properties. There has been a rapid increase in publications aimed at furthering this understanding, especially the importance of non-canonical interactions such as halogen, chalcogen, pnicogen, and tetrel bonds. Here we show how all of these interactions – and hydrogen bonds – can be readily understood through their common origin in the redistribution of electron density that results from chemical bonding. This redistribution is directly linked to the molecular electrostatic potential, to qualitative concepts such as electrostatic complementarity, and to the calculation of quantitative intermolecular interaction energies. Visualization of these energies, along with their electrostatic and dispersion components, sheds light on the architecture of molecular crystals, in turn providing a link to actual crystal properties.

Accurate hydrogen parameters for the amino acid L-leucine

The structure of the primary amino acid L-leucine has been determined for the first time by neutron diffraction. This was made possible by the use of modern neutron Laue diffraction to overcome the previously prohibitive effects of crystal size and quality. The packing of the structure into hydrophobic and hydrophilic layers is explained by the intermolecular interaction energies calculated using the PIXEL method. Variable-temperature data collections confirmed the absence of phase transitions between 120 and 300 K in the single-crystal form.

Structural Collapse of the Hydroquinone-Formic Acid Clathrate: A Pressure-Medium-Dependent Phase Transition

The energy landscape governing a new pressure-induced phase transition in the hydroquinone-formic acid clathrate is reported in which the host structure collapses, opening up the cavity channels within which the guest molecules migrate and order. The reversible isosymmetric phase transition causes significant changes in the morphology and the birefringence of the crystal. The subtle intermolecular interaction energies in the clathrate are quantified at varying pressures using novel model energies and energy frameworks. These calculations show that the high-pressure phase forms a more stable host network at the expense of less-stable host-guest interactions. The phase transition can be kinetically hindered using a nonhydrostatic pressure-transmitting medium, enabling the comparison of intermolecular energies in two polymorphic structures in the same pressure range. Overall this study illustrates a need for accurate intermolecular energies when analyzing self-assembly structures and supramolecular aggregates.

‘Quasi-isostructural polymorphism’ in molecular crystals: inputs from interaction hierarchy and energy frameworks

The polymorphs of (Z)-2-fluoro-N′-phenyl benzamidamide with multiple Z′ form “equi-energetic” crystal structures and exhibit “quasi-isostructurality”.