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Two new methods for calculating lineshapes in solid-state NMR spectra are described. The first method, which we refer to as semi-analytical, allows the rapid calculation of quadrupolar central-transition lineshapes in both static and magic-angle spinning cases. The second method, which is fully numerical, allows the calculation of lineshapes resulting from any combination of interactions, including quadrupolar, dipolar and chemical shift anisotropy, and is not restricted to cases in which the principal axis systems for the different interactions are aligned. Both methods are derived from consideration of the contour lines on a plot of the resonance frequency against the Euler angles, allowing the intensity of the lineshape to be calculated at each frequency. Consequently, highly accurate lineshapes can be calculated more rapidly than previously possible, since only orientations contributing to each specific frequency are considered. For our semi-analytical method, the intensity of each point in the lineshape can be directly calculated in tens of milliseconds on a standard PC. In contrast, established methods can take several hours to calculate the same lineshape. [ABSTRACT FROM AUTHOR]

Crystallization processes play a crucial role in many aspects of biological and physical sciences. Progress in deepening our fundamental understanding of such processes relies, to a large extent, on the development and application of new experimental strategies that allow direct in-situ monitoring of the process. In this paper, we give an overview of an in-situ solid-state NMR strategy that we have developed in recent years for monitoring the time-evolution of different polymorphic forms (or other solid forms) that arise as the function of time during crystallization from solution. The background to the strategy is described and several examples of the application of the technique are highlighted, focusing on both the evolution of different polymorphs during crystallization and the discovery of new polymorphs. [ABSTRACT FROM AUTHOR]

Abstract: Hydration of the ammonium form of the solid acid catalyst ZSM-5 is investigated by applying a technique that has been developed recently for carrying out in situ solid-state NMR studies of adsorption processes. From 1H MAS NMR spectra recorded as a function of time and temperature during the hydration process, insights are established on the nature of the interaction between the adsorbed water molecules and the ammonium cations in the ZSM-5 material. The change in isotropic chemical shift for the ammonium cations is consistent with the formation of N–H⋯O hydrogen bonding with the water molecules. Studies of the adsorption of deuterated water, and dehydration of the hydrated material, are also reported. [Copyright &y& Elsevier]

Abstract: Structure determination of organic molecular solids from powder X-ray diffraction data is nowadays carried out widely, in particular using the direct-space strategy for structure solution. In our implementation of this approach, structure solution involves the use of a genetic algorithm to explore the powder-profile R-factor hypersurface R wp( Γ ) to locate the global minimum, where Γ represents the set of variables that define trial structures. Conventionally, the set of variables comprises, for each molecule in the asymmetric unit, the position {x, y, z} and orientation {θ, φ, ψ} of the molecule, and a set of n variable torsion angles {τ 1, τ 2,...,τn }. An alternative definition of variable-space has been explored recently, based on a multiple-fragment representation of molecular geometry, and has been demonstrated to be successful in solving the structures of conformationally complex molecules. This paper explores details of the methodology for the definition of multiple-fragment representations of molecular geometry within the context of the application of the direct-space genetic algorithm technique for structure solution. [Copyright &y& Elsevier]

Abstract: Fundamental aspects relating to the development of the direct-space genetic algorithm technique for carrying out structure solution from powder X-ray diffraction data are reviewed, and several examples are given to illustrate the application of this technique to solve crystal structures for different types of organic materials. [Copyright &y& Elsevier]

For those solids that can be prepared only as microcrystalline powders, and are not suitable for investigation by single crystal diffraction methods, it is necessary to carry out structure determination using powder diffraction data. Here we highlight recent developments in the opportunities for solving molecular crystal structures from powder diffraction data, focusing on a direct-space strategy in which a hypersurface based on the powder profile R-factor R wp is searched using a Genetic Algorithm. Recent fundamental developments are described, and the application of the method is illustrated by the structure determination of two oligopeptide materials. [ABSTRACT FROM AUTHOR]

Many solids can be prepared only as microcrystalline powders and are not suitable for structural characterization by single crystal diffraction techniques. In such cases, it is necessary to carry out structure determination using powder diffraction data. There has been considerable recent interest in this field, and considerable progress has been achieved both in the development of new techniques and in the application of these techniques to tackle a wide range of structural problems. This article highlights recent developments of techniques in this field, and their application in the field of molecular solid state chemistry. [Copyright &y& Elsevier]

A general strategy is introduced for analysing the evolutionary events that occur during a genetic algorithm calculation, and the application of this approach is illustrated for the specific case of understanding the evolutionary trajectory leading to the correct structure solution in structure determination from powder diffraction data. Methods providing visual representations of the evolution of the population during a genetic algorithm calculation are also introduced. It is demonstrated that our strategy can provide much more detailed insights concerning the evolutionary behaviour than was previously accessible, and can thus be exploited to further develop our genetic algorithm methodology. [Copyright &y& Elsevier]

Recently, new methods based on the use of genetic algorithms have been explored and developed for solving crystal structures directly from powder diffraction data. In implementing genetic algorithms in such applications, several different aspects of the technique and strategy are open to optimization, leading to a versatile and powerful approach. In this paper, the fundamental concepts underlying genetic algorithms are discussed and the implementation of the genetic algorithm for structure solution from powder diffraction data is described. The opportunities, scope and potential for future developments in the foundations and applications of genetic algorithms in this field are highlighted. The genetic algorithm approach adopts the 'direct-space' philosophy for structure solution, with trial structures generated independently of the experimental diffraction data and the quality of each structure assessed by comparing the calculated and experimental powder diffraction patterns: in this work, this comparison is made using the profile R factor R[subwp]. In the genetic algorithm, a population of trial structures is allowed to evolve subject to well defined rules governing mating, mutation and 'natural selection'. The 'fitness' of each structure in the population is a function of its profile R factor. The successful application of the genetic algorithm approach for structure solution of molecular crystals from powder diffraction data is demonstrated with examples of previously known and previously unknown structures. [ABSTRACT FROM AUTHOR]

Determines the structure of crystalline solids from powder diffraction data using the Monte Carlo algorithm. Generation of structural models by random movement of atoms; Basis of the Monte Carlo method used; Use of difference Fourier analysis.

Reports on variable-temperature high-resolution solid-state 13C nuclear magnetic resonance studies of thiourea inclusion compounds. Compounds containing monosubstituted cyclohexane guest molecules; Direct determination of relative amounts of axial and equatorial conformers.

We report the preparation and structural properties of complexes of metal halides and thiourea with composition MX[thiourea]4 (MX = KBr, KI, RbI, CsI) together with the mixed-halide materials KBrnI1-n[thiourea]4 (0 < n < 1) and the mixed-metal materials KnCs1-nI[thiourea]4 (0 < n < 1). These materials are isostructural with a tetragonal structure (space group P4/mnc) characterized by M+[thiourea]4 coordination columns along the 4-fold axis and halide anions located in channels in the region of space between adjacent columns and running parallel to the columns. For the mixed-halide materials, the stoichiometry KBrnI1-n[thiourea]4 depends on the bromide/iodide ratio in the crystallization solution; the crystalline complexes have a higher bromide/iodide ratio than the crystallization solution, indicating preferential incorporation of bromide within the complex. Soaking crystals of KBr0.61I0.39[thiourea]4 in iodide-containing solutions leads to halide exchange with the iodide to bromide ratio increasing relative to that of the parent crystal. Further experiments produced no evidence that these thiourea complexes can accommodate extended polyiodide networks. [ABSTRACT FROM AUTHOR]

Abstract: We report the crystal structure of the 5-residue peptide acetyl-YEQGL-amide, determined directly from powder X-ray diffraction data recorded on a conventional laboratory X-ray powder diffractometer. The YEQGL motif has a known biological role, as a trafficking motif in the C-terminus of mammalian P2X4 receptors. Comparison of the crystal structure of acetyl-YEQGL-amide determined here and that of a complex formed with the μ2 subunit of the clathrin adaptor protein complex AP2 reported previously, reveals differences in conformational properties, although there are nevertheless similarities concerning aspects of the hydrogen-bonding arrangement and the hydrophobic environment of the leucine sidechain. Our results demonstrate the potential for exploiting modern powder X-ray diffraction methodology to achieve complete structure determination of materials of biological interest that do not crystallize as single crystals of suitable size and quality for single-crystal X-ray diffraction. [Copyright &y& Elsevier]

Abstract: We report the formation and structural properties of co-crystals containing gemfibrozil and hydroxy derivatives of t-butylamine H2NC(CH3)3− n (CH2OH) n , with n=0, 1, 2 and 3. In each case, a 1:1 co-crystal is formed, with transfer of a proton from the carboxylic acid group of gemfibrozil to the amino group of the t-butylamine derivative. All of the co-crystal materials prepared are polycrystalline powders, and do not contain single crystals of suitable size and/or quality for single crystal X-ray diffraction studies. Structure determination of these materials has been carried out directly from powder X-ray diffraction data, using the direct-space Genetic Algorithm technique for structure solution followed by Rietveld refinement. The structural chemistry of this series of co-crystal materials reveals well-defined structural trends within the first three members of the family (n=0, 1, 2), but significantly contrasting structural properties for the member with n=3. [Copyright &y& Elsevier]

A method for calculation of the whole-profile powder diffraction R-factor Rwp, adapted specifically for use in direct-space structure solution, is shown to be faster by a factor of approximately 20 than standard methods for calculating Rwp. [Copyright &y& Elsevier]

Solid state 2H NMR has been used to study molecular motion in deuterated ammonia trimethylalane (CH3)3AlND3. From analysis of the 2H NMR lineshape between 123 and 298 K, reorientation of the –ND3 group about the molecular Al–N axis is shown to occur at a rate higher than 108 s–1, and simulation of partially relaxed 2H NMR lineshapes shows that the reorientation can be described as a 3-site 120° jump motion. From the temperature dependence of the 2H spin–lattice relaxation time, the activation energy for this motion is estimated to be 9.3±0.3 kJ mol–1. There is no evidence from either 2H or 27Al NMR data for any site-exchange between the sites occupied by the –ND3 and –CH3 groups. The anisotropy of the dynamics of (CH3)3AlND3 indicates that the orientation of the Al–N bond is highly constrained, presumably by a strong interaction between the electric dipoles of neighboring molecules. [Copyright &y& Elsevier]

Presents information on the effects of ion association on the conductivity of the aqueous electrolyte solution. Background information on electrolyte solution; Detailed information on a study conducted; Results of the study; Information on an exepriment for the undergraduate laboratory.

Abstract We report a high-field in-situ solid-state NMR study of the hydration of CaAl 2 O 4 (the most important hydraulic phase in calcium aluminate cement), based on time-resolved measurements of solid-state 27Al NMR spectra during the early stages of the reaction. A variant of the CLASSIC NMR methodology, involving alternate recording of direct-excitation and MQMAS 27Al NMR spectra, was used to monitor the 27Al species present in both the solid and liquid phases as a function of time. Our results provide quantitative information on the changes in the relative amounts of 27Al sites with tetrahedral coordination (the anhydrous reactant phase) and octahedral coordination (the hydrated product phases) as a function of time, and reveal significantly different kinetic and mechanistic behaviour of the hydration reaction at the different temperatures (20 °C and 60 °C) studied. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

The lower detection limit for 2 distinct crystalline phases by 1 H magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) is investigated for a minority amount of cimetidine (anhydrous polymorph A) in a physical mixture with the anhydrous HCl salt of cimetidine. Specifically, 2-dimensional 1 H double-quantum (DQ) MAS NMR spectra of polymorph A and the anhydrous HCl salt constitute fingerprints for the presence of each of these solid forms. For solid-state NMR data recorded at a 1 H Larmor frequency of 850 MHz and a MAS frequency of 30 kHz on ∼10 mg of sample, it is shown that, by following the pair of cross-peaks at a 1 H DQ frequency of 7.4 + 11.6 = 19.0 ppm that are unique to polymorph A, the level of detection for polymorph A in a physical mixture with the anhydrous HCl salt is a concentration of 1% w/w. [ABSTRACT FROM AUTHOR]

Abstract: Structural properties of 2,4,6-trimethoxybenzaldehyde, 2,4,6-trimethoxybenzyl alcohol and 2,4,6-trimethoxyacetophenone have been determined directly from powder X-ray diffraction data, using the direct-space Genetic Algorithm (GA) technique for structure solution followed by Rietveld refinement. Structural similarities and contrasts within this family of materials are elucidated. The work illustrates the value of utilizing information from other sources, including spectroscopic data and computational techniques, as a means of augmenting the structural knowledge established from the powder X-ray diffraction data. [Copyright &y& Elsevier]

Abstract: In solid triphenylmethanol, the molecules are arranged in hydrogen-bonded tetramers, and it is already well established that the hydrogen bonding in this material undergoes a dynamic switching process between different hydrogen bonding arrangements. In addition to this motion, we show here, from solid-state 2H NMR studies of the deuterated material (C6D5)3COH, that each phenyl ring in this material undergoes a 180°-jump reorientation about the C6D5–C(OH) bond, with an activation energy of ca. 50kJmol−1. The timescale for the phenyl ring dynamics is several orders of magnitude longer than the timescale for the hydrogen bond dynamics in this material, and is uncorrelated with the dynamics of the hydrogen bonding arrangement. [Copyright &y& Elsevier]

The "rigid-core" material 3,5-dimethyl-2,3'-bis(3-methylthiophene)-dithieno[3,2-b:',3'-d]thiophene4 ,4-dioxide (DTTOMe4) has the highest photoluminescence ever reported for thiophene-based molecules in the solid state. We report the structure of this material, determined directly from powder X-ray diffraction data using the Genetic Algorithm method for structure solution, followed by Rietveld refinement, and the structural properties are discussed in relation to the structures of the corresponding subsystems DTTO and DTTOMe. While the crystal structures of the latter compounds contain cofacial dimers, the crystal structure of DTTOMe4 comprises layers of molecules aligned in an antiparallel fashion. Intermediate neglect of differential overlap with single configuration interaction (INDO/SCI) calculations on the intermolecular interactions in the three crystal structures show that the different solid-state photoluminescence efficiencies of DTTOMe4, DTTOMe, and DTTO cannot be correlated with the different types of dipole-dipole alignment in the solid state. Instead, photoluminescence efficiencies correlate well with the rate of formation of nonradiatively decaying charge-transfer pairs upon photoexcitation. Because of larger intermolecular distances in DTTOMe4, the photoluminescence is less effectively quenched by charge-transfer processes than in DTTOMe and DTTO. [ABSTRACT FROM AUTHOR]