Determination of a complex crystal structure in the absence of single crystals: analysis of powder X-ray diffraction data, guided by solid-state NMR and periodic DFT calculations, reveals a new 2'-deoxyguanosine structural motif

Part I: determination of a structure/property transformation mechanism responsible for changes in the point of zero change of anatase titania with decreasing particle size

Below a diameter of approximately 28 nm, the surface crystal structure of anatase titania is known to change. These changes include surface bond lengths and crystal lattice parameter expansion/contractions. Concurrent with these structure changes, the materials point of zero charge (PZC) has been observed to shift toward lower pH values. Therefore, the objective of this work was to determine if a correlation exists between these known structural changes and the shift in the materials PZC values with decreasing particle size. To achieve this a method was developed to identify and minimize the effect of all known variables, save particle size, affecting the materials pHPZC. This led to the discovery of two regions for point of zero charge. Above the average spherical primary particle diameter ≅ 29 nm for anatase titania, denoted as Region I, PZC values remain constant. In Region I the materials surface crystal structure and properties were also found to remain constant. Below the average spherical primary particle diameter ≅29 nm is the second zone, defined as Region II, where pHPZC values decrease almost linearly. An examination of possible surface structure factors and properties responsible for the shift in these PZC values (Region II) identified three underlying causes. These being changes in the materials band gap (i.e. surface bond lengths), lattice parameters and bond ionic content.

An unusual ionic cocrystal of ponatinib hydrochloride: characterization by single-crystal X-ray diffraction and ultra-high field NMR spectroscopy

This study describes the discovery of a unique ionic cocrystal of the active pharmaceutical ingredient (API) ponatinib hydrochloride (pon·HCl), and characterization using single-crystal X-ray diffraction (SCXRD) and solid-state NMR (SSNMR) spectroscopy. Pon·HCl is a multicomponent crystal that features an unusual stoichiometry, with an asymmetric unit containing both monocations and dications of the ponatinib molecule, three water molecules, and three chloride ions. Structural features include (i) a charged imidazopyridazine moiety that forms a hydrogen bond between the ponatinib monocations and dications and (ii) a chloride ion that does not feature hydrogen bonds involving any organic moiety, instead being situated in a "square" arrangement with three water molecules. Multinuclear SSNMR, featuring high and ultra-high fields up to 35.2 T, provides the groundwork for structural interpretation of complex multicomponent crystals in the absence of diffraction data. A 13C CP/MAS spectrum confirms the presence of two crystallographically distinct ponatinib molecules, whereas 1D 1H and 2D 1H-1H DQ-SQ spectra identify and assign the unusually deshielded imidazopyridazine proton. 1D 35Cl spectra obtained at multiple fields confirm the presence of three distinct chloride ions, with density functional theory calculations providing key relationships between the SSNMR spectra and H⋯Cl- hydrogen bonding arrangements. A 2D 35Cl → 1H D-RINEPT spectrum confirms the spatial proximities between the chloride ions, water molecules, and amine moieties. This all suggests future application of multinuclear SSNMR at high and ultra-high fields to the study of complex API solid forms for which SCXRD data are unavailable, with potential application to heterogeneous mixtures or amorphous solid dispersions.

Unveiling the topology of partially disordered micro-crystalline nitro-perylenediimide with X-aggregate stacking: an integrated approach

Profound knowledge of the molecular structure and supramolecular organization of organic molecules is essential to understand their structure-property relationships. Herein we demonstrate the packing arrangement of partially disordered nitro-perylenediimide (NO2-PDI), revealing that the perylenediimide units exhibit an X-shaped packing pattern. The packing of NO2-PDI is derived using a complementary approach that utilises solid-state NMR (ssNMR) and 3D electron diffraction (3D ED) techniques. Perylenediimide (PDI) molecules are captivating due to their high luminescence efficiency and optoelectronic properties, which are related to supramolecular self-assembly. Increasing the alkyl chain length on the imide substituent poses a more significant challenge in crystallizing the resulting molecule. In addition to the alkyl tails, other functional groups, like the nitro group attached as a bay substituent, can also cause disorder. Such heterogeneity could lead to diffuse scattering, which then complicates the interpretation of diffraction experiment data, where perfect periodicity is expected. As a result, there is an unmet need to develop a methodology for solving the structures of difficult-to-crystallize materials. A synergistic approach is utilised in this manuscript to understand the packing arrangement of the disordered material NO2-PDI by making use of 3D ED, ssNMR and density functional theory calculations (DFT). The combination of these experimental and theoretical approaches provides great promise in enabling the structural investigation of novel materials with customized properties across various applications, which are, due to the internal disorder, very difficult to study by diffraction techniques. By effectively addressing these challenges, our methodology opens up new avenues for material characterization, thereby driving exciting advancements in the field.

Combining heteronuclear correlation NMR with spin-diffusion to detect relayed Cl-H-H and N-H-H proximities in molecular solids

Analysis of short-to-intermediate range intermolecular interactions offers a great way of characterizing the solid-state organization of small molecules and materials. This can be achieved by two-dimensional (2D) homo- and heteronuclear correlation NMR spectroscopy, for example, by carrying out experiments at high magnetic fields in conjunction with fast magic-angle spinning (MAS) techniques. But, detecting 2D peaks for heteronuclear dipolar coupled spin pairs separated by greater than 3 Å is not always straightforward, particularly when low-gamma quadrupolar nuclei are involved. Here, we present a 2D correlation NMR experiment that combines the advantages of heteronuclear-multiple quantum coherence (HMQC) and proton-based spin-diffusion (SD) pulse sequences using radio-frequency-driven-recouping (RFDR) to probe inter and intramolecular ¹H-X (X = ¹⁴N, ³⁵Cl) interactions. This experiment can be used to acquire 2D ¹H{X}-HMQC filtered ¹H-¹H correlation as well as 2D ¹H-X HMQC spectra. Powder forms of dopamine·HCl and l-histidine·HCl·H₂O are characterized at high fields (21.1 T and 18.8 T) with fast MAS (60 kHz) using the 2D HMQC-SD-RFDR approach. Solid-state NMR results are complemented with NMR crystallography analyses using the gauge-including projector augmented wave (GIPAW) approach. For histidine·HCl·H₂O, 2D peaks associated with ¹⁴N-¹H-¹H and ³⁵Cl-¹H-¹H distances of up to 4.4 and 3.9 Å have been detected. This is further corroborated by the observation of 2D peaks corresponding to ¹⁴N-¹H-¹H and ³⁵Cl-¹H-¹H distances of up to 4.2 and 3.7 Å in dopamine·HCl, indicating the suitability of the HMQC-SD-RFDR experiments for detecting medium-range proximities in molecular solids.

Ambiguous structure determination from powder data: four different structural models of 4,11-di-fluoro-quinacridone with similar X-ray powder patterns, fit to the PDF, SSNMR and DFT-D

Four different structural models, which all fit the same X-ray powder pattern, were obtained in the structure determination of 4,11-di-fluoro-quinacridone (C20H10N2O2F2) from unindexed X-ray powder data by a global fit. The models differ in their lattice parameters, space groups, Z, Z', molecular packing and hydrogen bond patterns. The molecules form a criss-cross pattern in models A and B, a layer structure built from chains in model C and a criss-cross arrangement of dimers in model D. Nevertheless, all models give a good Rietveld fit to the experimental powder pattern with acceptable R-values. All molecular geometries are reliable, except for model D, which is slightly distorted. All structures are crystallochemically plausible, concerning density, hydrogen bonds, intermolecular distances etc. All models passed the checkCIF test without major problems; only in model A a missed symmetry was detected. All structures could have probably been published, although 3 of the 4 structures were wrong. The investigation, which of the four structures is actually the correct one, was challenging. Six methods were used: (1) Rietveld refinements, (2) fit of the crystal structures to the pair distribution function (PDF) including the refinement of lattice parameters and atomic coordinates, (3) evaluation of the colour, (4) lattice-energy minimizations with force fields, (5) lattice-energy minimizations by two dispersion-corrected density functional theory methods, and (6) multinuclear CPMAS solid-state NMR spectroscopy (1H, 13C, 19F) including the comparison of calculated and experimental chemical shifts. All in all, model B (perhaps with some disorder) can probably be considered to be the correct one. This work shows that a structure determination from limited-quality powder data may result in totally different structural models, which all may be correct or wrong, even if they are chemically sensible and give a good Rietveld refinement. Additionally, the work is an excellent example that the refinement of an organic crystal structure can be successfully performed by a fit to the PDF, and the combination of computed and experimental solid-state NMR chemical shifts can provide further information for the selection of the most reliable structure among several possibilities.

A structure determination protocol based on combined analysis of 3D-ED data, powder XRD data, solid-state NMR data and DFT-D calculations reveals the structure of a new polymorph of l-tyrosine

We report the crystal structure of a new polymorph of l-tyrosine (denoted the β polymorph), prepared by crystallization from the gas phase following vacuum sublimation. Structure determination was carried out by combined analysis of three-dimensional electron diffraction (3D-ED) data and powder X-ray diffraction (XRD) data. Specifically, 3D-ED data were required for reliable unit cell determination and space group assignment, with structure solution carried out independently from both 3D-ED data and powder XRD data, using the direct-space strategy for structure solution implemented using a genetic algorithm. Structure refinement was carried out both from powder XRD data, using the Rietveld profile refinement technique, and from 3D-ED data. The final refined structure was validated both by periodic DFT-D calculations, which confirm that the structure corresponds to an energy minimum on the energy landscape, and by the fact that the values of isotropic 13C NMR chemical shifts calculated for the crystal structure using DFT-D methodology are in good agreement with the experimental high-resolution solid-state 13C NMR spectrum. Based on DFT-D calculations using the PBE0-MBD method, the β polymorph is meta-stable with respect to the previously reported crystal structure of l-tyrosine (now denoted the α polymorph). Crystal structure prediction calculations using the AIRSS approach suggest that there are three other plausible crystalline polymorphs of l-tyrosine, with higher energy than the α and β polymorphs.

Solid-State Photo-NMR Study on Light-Induced Nitrosyl Linkage Isomers Uncovers Their Structural, Electronic, and Diamagnetic Nature

A light-induced linkage NO isomer (MS1) in trans-[Ru(¹⁵NO)(py)₄¹⁹F](ClO₄)₂ is detected and measured for the first time by solid-state MAS NMR. Chemical shift tensors of ¹⁵N and ¹⁹F, along with ⁿJ(¹⁵N-¹⁹F) spin-spin couplings and T₁ relaxation times of MS1, are compared with the ground state (GS) at temperatures T < 250 K. Isotropic chemical shifts (¹⁵N and ¹⁹F) are well resolved for two crystallographically independent cations (A and B) [Ru(¹⁵NO)(py)₄¹⁹F]²⁺, allowing to define separately both populations of MS1 isomers and thermal decay rates for two structural sites. The relaxation times T₁ of ¹⁹F in the case of GS (30/38.6 s for sites A/B) and MS1 (11.6/11.8 s for sites A/B) indicate that both isomers are diamagnetic, which is the first experimental evidence of diamagnetic properties of MS1 in ruthenium nitrosyl. After light irradiation (λ = 420 nm), the NO ligand rotates by nearly 180° from F-Ru-N-O to F-Ru-O-N, whereby the isotropic chemical shifts of δ_iso(¹⁵N) increase and those of δ_iso(¹⁹F) decrease. The ⁿJ(¹⁵N-¹⁹F) couplings increase from ²J(¹⁵N-Ru-¹⁹F)_GS = 71 Hz to ³J(¹⁵N-O-Ru-¹⁹F)_MS1 = 105 Hz. These results are interpreted on the basis of DFT-CASTEP calculations including Bader-, Mulliken-, and Hirshfeld-charge density distributions of both states.

Resolving Atomic-Scale Interactions in Nonfullerene Acceptor Organic Solar Cells with Solid-State NMR Spectroscopy, Crystallographic Modelling, and Molecular Dynamics Simulations

Fused-ring core nonfullerene acceptors (NFAs), designated "Y-series," have enabled high-performance organic solar cells (OSCs) achieving over 18% power conversion efficiency (PCE). Since the introduction of these NFAs, much effort has been expended to understand the reasons for their exceptional performance. While several studies have identified key optoelectronic properties that govern high PCEs, little is known about the molecular level origins of large variations in performance, spanning from 5% to 18% PCE, for example, in the case of PM6:Y6 OSCs. Here, a combined solid-state NMR, crystallography, and molecular modeling approach to elucidate the atomic-scale interactions in Y6 crystals, thin films, and PM6:Y6 bulk heterojunction (BHJ) blends is introduced. It is shown that the Y6 morphologies in BHJ blends are not governed by the morphology in neat films or single crystals. Notably, PM6:Y6 blends processed from different solvents self-assemble into different structures and morphologies, whereby the relative orientations of the sidechains and end groups of the Y6 molecules to their fused-ring cores play a crucial role in determining the resulting morphology and overall performance of the solar cells. The molecular-level understanding of BHJs enabled by this approach will guide the engineering of next-generation NFAs for stable and efficient OSCs.

Enantiotropy of Simvastatin as a Result of Weakened Interactions in the Crystal Lattice: Entropy-Driven Double Transitions and the Transient Modulated Phase as Seen by Solid-State NMR Spectroscopy

In crystalline molecular solids, in the absence of strong intermolecular interactions, entropy-driven processes play a key role in the formation of dynamically modulated transient phases. Specifically, in crystalline simvastatin, the observed fully reversible enantiotropic behavior is associated with multiple order–disorder transitions: upon cooling, the dynamically disordered high-temperature polymorphic Form I is transformed to the completely ordered low-temperature polymorphic Form III via the intermediate (transient) modulated phase II. This behavior is associated with a significant reduction in the kinetic energy of the rotating and flipping ester substituents, as well as a decrease in structural ordering into two distinct positions. In transient phase II, the conventional three-dimensional structure is modulated by periodic distortions caused by cooperative conformation exchange of the ester substituent between the two states, which is enabled by weakened hydrogen bonding. Based on solid-state NMR data analysis, the mechanism of the enantiotropic phase transition and the presence of the transient modulated phase are documented.

Solid-State Structural Properties of Alloxazine Determined from Powder XRD Data in Conjunction with DFT-D Calculations and Solid-State NMR Spectroscopy: Unraveling the Tautomeric Identity and Pathways for Tautomeric Interconversion

We report the solid-state structural properties of alloxazine, a tricyclic ring system found in many biologically important molecules, with structure determination carried out directly from powder X-ray diffraction (XRD) data. As the crystal structures containing the alloxazine and isoalloxazine tautomers both give a high-quality fit to the powder XRD data in Rietveld refinement, other techniques are required to establish the tautomeric form in the solid state. In particular, high-resolution solid-state ¹⁵N NMR data support the presence of the alloxazine tautomer, based on comparison between isotropic chemical shifts in the experimental ¹⁵N NMR spectrum and the corresponding values calculated for the crystal structures containing the alloxazine and isoalloxazine tautomers. Furthermore, periodic DFT-D calculations at the PBE0-MBD level indicate that the crystal structure containing the alloxazine tautomer has significantly lower energy. We also report computational investigations of the interconversion between the tautomeric forms in the crystal structure via proton transfer along two intermolecular N-H···N hydrogen bonds; DFT-D calculations at the PBE0-MBD level indicate that the tautomeric interconversion is associated with a lower energy transition state for a mechanism involving concerted (rather than sequential) proton transfer along the two hydrogen bonds. However, based on the relative energies of the crystal structures containing the alloxazine and isoalloxazine tautomers, it is estimated that under conditions of thermal equilibrium at ambient temperature, more than 99.9% of the molecules in the crystal structure will exist as the alloxazine tautomer.

Orbital Mapping of Semiconducting Perylenes on Cu(111)

Semiconducting O-doped polycyclic aromatic hydrocarbons constitute a class of molecules whose optoelectronic properties can be tailored by acting on the π-extension of the carbon-based frameworks and on the oxygen linkages. Although much is known about their photophysical and electrochemical properties in solution, their self-assembly interfacial behavior on solid substrates has remained unexplored so far. In this paper, we have focused our attention on the on-surface self-assembly of O-doped bi-perylene derivatives. Their ability to assemble in ordered networks on Cu(111) single-crystalline surfaces allowed a combination of structural, morphological, and spectroscopic studies. In particular, the exploitation of the orbital mapping methodology based on angle-resolved photoemission spectroscopy, with the support of scanning tunneling microscopy and low-energy electron diffraction, allowed the identification of both the electronic structure of the adsorbates and their geometric arrangement. Our multi-technique experimental investigation includes the structure determination from powder X-ray diffraction data for a specific compound and demonstrates that the electronic structure of such large molecular self-assembled networks can be studied using the reconstruction methods of molecular orbitals from photoemission data even in the presence of segregated chiral domains.

Pure Isotropic Proton Solid State NMR

Resolution in proton solid state magic angle sample spinning (MAS) NMR is limited by the intrinsically imperfect nature of coherent averaging induced by either MAS or multiple pulse sequence methods. Here, we suggest that instead of optimizing and perfecting a coherent averaging scheme, we could approach the problem by parametrically mapping the error terms due to imperfect averaging in a k-space representation, in such a way that they can be removed in a multidimensional correlation leaving only the desired pure isotropic signal. We illustrate the approach here by determining pure isotropic ¹H spectra from a series of MAS spectra acquired at different spinning rates. For six different organic solids, the approach is shown to produce pure isotropic ¹H spectra that are significantly narrower than the MAS spectrum acquired at the fastest possible rate, with linewidths down to as little as 48 Hz. On average, we observe a 7-fold increase in resolution, and up to a factor of 20, as compared with spectra acquired at 100 kHz MAS. The approach is directly applicable to a range of solids, and we anticipate that the same underlying principle for removing errors introduced here can be applied to other problems in NMR spectroscopy.

Direct-Space Structure Determination of Covalent Organic Frameworks from 3D Electron Diffraction Data

Structure determination of covalent organic frameworks (COFs) with atomic precision is a bottleneck that hinders the development of COF chemistry. Although three-dimensional electron diffraction (3D-ED) data has been used to solve structures of sub-micrometer-sized COFs, successful structure solution is not guaranteed as the data resolution is usually low. We demonstrate that the direct-space strategy for structure solution, implemented using a genetic algorithm (GA), is a successful approach for structure determination of COF-300 from 3D-ED data. Structural models with different geometric constraints were considered in the GA calculations, with successful structure solution achieved from room-temperature 3D-ED data with a resolution as low as ca. 3.78 Å. The generality of this strategy was further verified for different phases of COF-300. This study demonstrates a viable strategy for structure solution of COF materials from 3D-ED data of limited resolution, which may facilitate the discovery of new COF materials in the future.

Theoretical and Spectroscopic Characterization of API-Related Azoles in Solution and in Solid State

Azoles are a family of five-membered azacyclic compounds with relevant biological and pharmacological activity. Different subclasses of azoles are defined depending on the atomic arrangement and the number of nitrogen atoms present in the ring: pyrazoles, indazoles, imidazoles, benzimidazoles, triazoles, benzotriazoles, tetrazoles and pentazoles. The complete characterization of their structure and the knowledge about their crystal packing and physical and chemical properties are of vital importance for the advancement in the design of new azole-containing drugs. In this review, we report the latest recent contributions to azole chemistry, in particular, those in which theoretical studies have been performed.

Effects of incorporating regioisomers and flexible rotors to direct aggregation induced emission to achieve stimuli-responsive luminogens, security inks and chemical warfare agent sensors

Efficient transformation of ACQ materials to AIE luminogens using simple design principles of positional isomerization and C-C bond exclusion is presented here. Consequently, the bond link, position and packing influence the photophysical properties that can be utilized in erasable secret inks, pressure sensors and chemical warfare sensors.

Revealing weak histidine 15N homonuclear scalar couplings using Solid-State Magic-Angle-Spinning NMR spectroscopy

The tautomeric structure and chemistry of the histidine imidazole ring play active roles in many structurally and functionally important proteins and polypeptides. While in NMR spectroscopy histidine chemical shifts (e.g. 15N, 13C, and 1H) have been commonly used to characterize the tautomeric structure, hydrogen bonding, and torsion angles, homonuclear 15N scalar couplings in histidine have rarely been reported. Here, we propose double spin-echo sequences to compare the observed signals with and without a 90° pulse between the two spin-echo periods, such that their signal ratio as a function of the echo time solely depends on homonuclear scalar couplings, allowing for measuring weak homonuclear scalar couplings without influence from transverse dephasing effects, thus capable of revealing hydrogen-bond mediated 15N-15N J-couplings that can provide direct and definitive evidence for the formation of N…H…N hydrogen-bonding associated with the imidazole ring. We used two 13C,15N labeled histidine samples recrystallized from solutions at pH 6.3 and pH 11.0 to demonstrate the feasibility of this method and reveal the existence of a weak two-bond scalar coupling between the Nδ1 and Nε2 sites in the histidine imidazole ring in three tautomeric states and the presence of a hydrogen-bond mediated scalar coupling between the Nδ1 site in the imidazole ring and the backbone Nα site in the histidine neutral τ and π states. Our results demonstrate that weak 15N homonuclear scalar couplings can be measured even when their values are less than their corresponding intrinsic natural linewidths, thus providing direct and definitive evidence for the formation of N…H…N hydrogen bonding that is associated with the histidine imidazole ring.

NMR crystallography of molecular organics

Developments of NMR methodology to characterise the structures of molecular organic structures are reviewed, concentrating on the previous decade of research in which density functional theory-based calculations of NMR parameters in periodic solids have become widespread. With a focus on demonstrating the new structural insights provided, it is shown how "NMR crystallography" has been used in a spectrum of applications from resolving ambiguities in diffraction-derived structures (such as hydrogen atom positioning) to deriving complete structures in the absence of diffraction data. As well as comprehensively reviewing applications, the different aspects of the experimental and computational techniques used in NMR crystallography are surveyed. NMR crystallography is seen to be a rapidly maturing subject area that is increasingly appreciated by the wider crystallographic community.

Imidazole-Imidazole Hydrogen Bonding in the pH-Sensing Histidine Side Chains of Influenza A M2

The arrangement of histidine side chains in influenza A M2 tetramer determines their pKa values, which define pH-controlled proton conduction critical to the virus lifecycle. Both water-associated and hydrogen-bonded imidazole-imidazolium histidine quaternary structures have been proposed, based on crystal structures and NMR chemical shifts, respectively. Here we show, using the conduction domain construct of M2 in lipid bilayers, that the imidazole rings are hydrogen bonded even at a pH of 7.8 in the neutral charge state. An intermolecular 8.9 ± 0.3 Hz 2hJNN hydrogen bond is observed between H37 Nε and Nδ recorded in a fully protonated sample with 100 kHz magic-angle spinning. This interaction could not be detected in the drug-bound sample.

Observation of the Imidazole-Imidazolium Hydrogen Bonds Responsible for Selective Proton Conductance in the Influenza A M2 Channel

The integral membrane M2 protein is a 97-residue membrane protein that assembles as a tetramer to conduct protons at a slow rate (10²-10³/s) when activated by low pH. The proton conductance mechanism has been extensively debated in the literature, but it is accepted that the proton conductance is facilitated by hydrogen bonds involving the His37 residues. However, the hydrogen bonding partnership remains unresolved. Here, we report on the measurement of ¹⁵N-¹⁵N J-couplings of ¹⁵N His37-labeled full length M2 (M2FL) protein from Influenza A virus embedded in synthetic liquid crystalline lipid bilayers using two-dimensional J-resolved NMR spectroscopy. We experimentally observed the hydrogen-bond mediated J-couplings between N_δ1 and N_ε2 of adjacent His37 imidazole rings, providing direct evidence for the existence of various imidazolium-imidazole hydrogen-bonding geometries in the histidine tetrad at low pH, thus validating the proton conduction mechanism in the M2FL protein by which the proton is transferred through the breaking and reforming of the hydrogen bonds between pairs of His37 residues.

Weak Intermolecular CH···N Hydrogen Bonding: Determination of 13CH-15N Hydrogen-Bond Mediated J Couplings by Solid-State NMR Spectroscopy and First-Principles Calculations

Weak hydrogen bonds are increasingly hypothesized to play key roles in a wide range of chemistry from catalysis to gelation to polymer structure. Here, ¹⁵N/¹³C spin-echo magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) experiments are applied to "view" intermolecular CH···N hydrogen bonding in two selectively labeled organic compounds, 4-[¹⁵N] cyano-4'-[¹³C₂] ethynylbiphenyl (1) and [¹⁵N₃,¹³C₆]-2,4,6-triethynyl-1,3,5-triazine (2). The synthesis of 2-¹⁵N₃,¹³C₆ is reported here for the first time via a multistep procedure, where the key element is the reaction of [¹⁵N₃]-2,4,6-trichloro-1,3,5-triazine (5) with [¹³C₂]-[(trimethylsilyl)ethynyl]zinc chloride (8) to afford its immediate precursor [¹⁵N₃,¹³C₆]-2,4,6-tris[(trimethylsilyl)ethynyl]-1,3,5-triazine (9). Experimentally determined hydrogen-bond-mediated ^2hJ_CN couplings (4.7 ± 0.4 Hz (1) and 4.1 ± 0.3 Hz (2)) are compared with density functional theory (DFT) gauge-including projector augmented wave (GIPAW) calculations, whereby species-independent coupling values ^2hK_CN (29.0 × 10¹⁹ kg m^-2 s^-2 A^-2 (1) and 27.9 × 10¹⁹ kg m^-2 s^-2 A^-2 (2)) quantitatively demonstrate the J couplings for these "weak" CH···N hydrogen bonds to be of a similar magnitude to those for conventionally observed NH···O hydrogen-bonding interactions in uracil (^2hK_NO: 28.1 and 36.8 × 10¹⁹ kg m^-2 s^-2 A^-2). Moreover, the GIPAW calculations show a clear correlation between increasing ^2hJ_CN (and ^3hJ_CN) coupling and reducing C(H)···N and H···N hydrogen-bonding distances, with the Fermi contact term accounting for at least 98% of the isotropic ^2hJ_CN coupling.

Manometric real-time studies of the mechanochemical synthesis of zeolitic imidazolate frameworks

We demonstrate a simple method for real-time monitoring of mechanochemical synthesis of metal–organic frameworks, by measuring changes in pressure of gas produced in the reaction. Using this manometric method to monitor the mechanosynthesis of the zeolitic imidazolate framework ZIF-8 from basic zinc carbonate reveals an intriguing feedback mechanism in which the initially formed ZIF-8 reacts with the CO₂ byproduct to produce a complex metal carbonate phase, the structure of which is determined directly from powder X-ray diffraction data. We also show that the formation of the carbonate phase may be prevented by addition of excess ligand. The excess ligand can subsequently be removed by sublimation, and reused. This enables not only the synthesis but also the purification, as well as the activation of the MOF to be performed entirely without solvent.

We demonstrate a simple method for real-time monitoring of mechanochemical synthesis of metal–organic frameworks, by measuring changes in pressure of gas produced in the reaction.

Metallo-responsive self-assembly of lipophilic guanines in hydrocarbon solvents: a systematic SAXS structural characterization

Lipophilic guanines (LipoGs) in aprotic solvents undergo different self-assembly processes based on different H-bonded motifs. Cylindrical nanotubes made by π-π stacked guanine tetramers (G-quadruplexes) and flat, tape-like aggregates (G-ribbons) have been observed depending on the presence of alkali metal ions. To obtain information on the structural properties and stability of these LipoG aggregates, Small-Angle X-ray Scattering (SAXS) experiments have been performed in dodecane, both in the presence and in the absence of potassium ions. As a result, the occurrence of the two different metallo-responsive architectures (nanoribbons or columnar nanotubes) was confirmed and we reported here for the first time a systematic study on the dependence of the aggregate properties on composition, temperature and molecular unit structure. Even if dodecane was selected to favour LipoG solubility, a strong tendency to self-organize into ordered lyotropic phases was indeed detected.

13C-13C spin-coupling constants in crystalline 13C-labeled saccharides: conformational effects interrogated by solid-state 13C NMR spectroscopy

Solid-state ¹³C NMR spectroscopy has been used in conjunction with selectively ¹³C-labeled mono- and disaccharides to measure ¹³C-¹³C spin-couplings (J_CC) in crystalline samples. This experimental approach allows direct correlation of J_CC values with specific molecular conformations since, in crystalline samples, molecular conformation is essentially static and can be determined by X-ray crystallography. J_CC values measured in the solid-state in known molecular conformations can then be compared to corresponding J_CC values calculated in the same conformations using density functional theory (DFT). The latter comparisons provide important validation of DFT-calculated J-couplings, which is not easily obtained by other approaches and is fundamental to obtaining reliable experiment-based conformational models from redundant J-couplings by MA'AT analysis. In this study, representative ¹J_CC, ²J_CCC and ³J_COCC values were studied as either intra-residue couplings in the aldohexopyranosyl rings of monosaccharides or inter-residue (trans-glycoside) couplings in disaccharides. The results demonstrate that (a) accurate J_CC values can be measured in crystalline saccharides that have been suitably labeled with ¹³C, and (b) DFT-calculated J_CC values compare favorably with those determined by solid-state ¹³C NMR when molecular conformation is a constant in both determinations.

A Million Crystal Structures: The Whole Is Greater than the Sum of Its Parts

The founding in 1965 of what is now called the Cambridge Structural Database (CSD) has reaped dividends in numerous and diverse areas of chemical research. Each of the million or so crystal structures in the database was solved for its own particular reason, but collected together, the structures can be reused to address a multitude of new problems. In this Review, which is focused mainly on the last 10 years, we chronicle the contribution of the CSD to research into molecular geometries, molecular interactions, and molecular assemblies and demonstrate its value in the design of biologically active molecules and the solid forms in which they are delivered. Its potential in other commercially relevant areas is described, including gas storage and delivery, thin films, and (opto)electronics. The CSD also aids the solution of new crystal structures. Because no scientific instrument is without shortcomings, the limitations of CSD research are assessed. We emphasize the importance of maintaining database quality: notwithstanding the arrival of big data and machine learning, it remains perilous to ignore the principle of garbage in, garbage out. Finally, we explain why the CSD must evolve with the world around it to ensure it remains fit for purpose in the years ahead.

Applications of Powder X-Ray Diffraction in Small Molecule Pharmaceuticals: Achievements and Aspirations

Since the discovery of X-ray diffraction and its potential to elucidate crystal symmetry, powder X-ray diffraction has found diverse applications in the field of pharmaceutical sciences. This review summarizes significant achievements of the technique during various stages of dosage form development. Improved understanding of the principle involved and development of automated hardware and reliable software have led to increased instrumental sensitivity and improved data analysis. These advances continue to expand the applications of powder X-ray diffraction to emerging research fields such as amorphous systems, mechanistic understanding of phase transformations, and "Quality by Design" in formulation development.

A theoretical NMR study of selected benzazoles: Comparison of GIPAW and GIAO-PCM (DMSO) calculations

This paper compares the absolute shieldings obtained by gauge-including-projected-augmented-wave (GIPAW) to those obtained by gauge-invariant atomic orbital/Becke, 3-parameter, Lee-Yang-Parr (GIAO/B3LYP)/6-311++G(d,p)-polarizable continuum model (PCM, dimethyl sulfoxide) for nine benzazoles (benzimidazoles, indazoles, and benzotriazoles) recorded in the solid-state. Three nuclei were explored, ¹³ C, ¹⁵ N, and ¹⁹ F, and the gauge-including-projected-augmented-wave approach only proved better for ¹⁵ N MAS NMR.

Cocrystals of zileuton with enhanced physical stability

Zileuton can form two promising pharmaceutical cocrystals with nicotinamide and isonicotinamide, which demonstrate superior phase stability against moisture.

Ab initio random structure searching of organic molecular solids: assessment and validation against experimental data

This paper explores the capability of using the DFT-D ab initio random structure searching (AIRSS) method to generate crystal structures of organic molecular materials, focusing on a system (m-aminobenzoic acid; m-ABA) that is known from experimental studies to exhibit abundant polymorphism. Within the structural constraints selected for the AIRSS calculations (specifically, centrosymmetric structures with Z = 4 for zwitterionic m-ABA molecules), the method is shown to successfully generate the two known polymorphs of m-ABA (form III and form IV) that have these structural features. We highlight various issues that are encountered in comparing crystal structures generated by AIRSS to experimental powder X-ray diffraction (XRD) data and solid-state magic-angle spinning (MAS) NMR data, demonstrating successful fitting for some of the lowest energy structures from the AIRSS calculations against experimental low-temperature powder XRD data for known polymorphs of m-ABA, and showing that comparison of computed and experimental solid-state NMR parameters allows different hydrogen-bonding motifs to be discriminated.