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A phenomenological model of anisotropic lattice vibrations is proposed, using a temperature-dependent spectral cutoff and varying Debye temperatures for the vibrational normal components. The internal lattice energy, entropy and Debye–Waller B factors of non-cubic elemental crystals are derived. The formalism developed is non-perturbative, based on temperature-dependent linear dispersion relations for the normal modes. The Debye temperatures of the vibrational normal components differ in anisotropic crystals; their temperature dependence and the varying spectral cutoff can be inferred from the experimental lattice heat capacity and B factors by least-squares regression. The zero-point internal energy of the phonons is related to the low-temperature limits of the mean-squared vibrational amplitudes of the lattice measured by X-ray and γ-ray diffraction. A specific example is discussed, the thermodynamic variables of the hexagonal close-packed zinc structure, including the temperature evolution of the B factors of zinc. In this case, the lattice vibrations are partitioned into axial and basal normal components, which admit largely differing B factors and Debye temperatures. The second-order B factors defining the non-Gaussian contribution to the Debye–Waller damping factors of zinc are obtained as well. Anharmonicity of the oscillator potential and deviations from the uniform phonon frequency distribution of the Debye theory are modeled effectively by the temperature dependence of the spectral cutoff and Debye temperatures.

Being a close analogue of amflutizole, methyl 4-amino-3-phenylisothiazole-5-carboxylate (C11H10N2O2S) was assumed to be capable of forming polymorphic structures. Noncentrosymmetric and centrosymmetric polymorphs have been obtained by crystallization from a series of more volatile solvents and from denser tetrachloromethane, respectively. Identical conformations of the molecule are found in both structures. The two polymorphs differ mainly in the intermolecular interactions formed by the amino group and in the type of stacking interactions between the π-systems. The most effective method for revealing packing motifs in structures with intermolecular interactions of different types (hydrogen bonding, stacking, dispersion, etc.) is to study the pairwise interaction energies using quantum chemical calculations. Molecules form a column as the primary basic structural motif due to stacking interactions in both polymorphic structures under study. The character of a column (straight or zigzag) is determined by the orientations of the stacked molecules (in a `head-to-head' or `head-to-tail' manner). Columns bound by intermolecular N—H...O and N—H...N hydrogen bonds form a double column as the main structural motif in the noncentrosymmetric structure. Double columns in the noncentrosymmetric structure and columns in the centrosymmetric structure interact strongly within the ab crystallographic plane, forming a layer as a secondary basic structural motif. The noncentrosymmetric structure has a lower density and a lower (by 0.59 kJ mol−1) lattice energy, calculated using periodic calculations, compared to the centrosymmetric structure.

Thalassemia is a genetic blood disorder requiring life-long blood transfusions. This process often results in iron overload and can be treated by an iron-chelating agent, like deferiprone (3-hydroxy-1,2-dimethylpyridin-4-one), C7H9NO2, in an oral formulation. The first crystal structure of deferiprone, (Ia), was reported in 1988 [Nelson et al. (1988). Can. J. Chem. 66, 123–131]. In the present study, two novel polymorphic forms, (Ib) and (Ic), of deferiprone were identified concomitantly with polymorph (Ia) during the crystallization experiments. Polymorph (Ia) was redetermined at low temperature for comparison of the structural features and lattice energy values with polymorphs (Ib) and (Ic). Polymorph (Ia) crystallized in the orthorhombic space group Pbca, whereas both polymorphs (Ib) and (Ic) crystallized in the monoclinic space group P21/c. The asymmetric units of (Ia) and (Ib) contain one deferiprone molecule, while polymorph (Ic) has three crystallographically independent molecules (A, B and C). All three polymorphs have similar hydrogen-bonding features, such as an R 2 2(10) dimer formed by O—H...O hydrogen bonds, an R 4 3(20) tetramer formed by C—H...O hydrogen bonds and π–π interactions, but the polymorphs differ in their molecular arrangements in the solid state and are classified as packing polymorphs. O—H...O and C—H...O hydrogen bonds lead to the formation of two-dimensional hydrogen-bonded parallel sheets which are interlinked by π–π stacking interactions. In the three-dimensional crystal packing, the deferiprone molecules were aggregated as corrugated sheets in polymorphs (Ia) and (Ic), whereas in polymorph (Ib), they were aggregated as a square-grid network. The characteristic crystalline peaks of polymorphs (Ia), (Ib) and (Ic) were established through powder X-ray diffraction analysis. The Rietveld analysis was also performed to estimate the contribution of the polymorphs to the bulk material.

Experimental and theoretical electron density distributions and the nature of intermolecular interactions of bicalutamide in its monoclinic polymorph and in androgen receptor complexes are reported., High-resolution single-crystal X-ray measurements of the monoclinic polymorph of bicalutamide and the aspherical atom databank approach have served as a basis for a reconstruction of the charge density distribution of the drug and its androgen receptor (AR) and albumin complexes. The contributions of various types of intermolecular interactions to the total crystal energy or ligand:AR energy were estimated. The cyan and amide groups secured the ligand placement in the albumin (Lys-137) and the AR binding pocket (Leu-704, Asn-705, Arg-752), and also determined the packing of the small-molecule crystals. The total electrostatic interaction energy on average was −230 kJ mol−1, comparable with the electrostatic lattice energy of the monoclinic bicalutamide polymorph. This is the result of similar distributions of electropositive and electronegative regions on the experimental and theoretical molecular electrostatic potential maps despite differences in molecular conformations. In general, bicalutamide interacted with the studied proteins with similar electrostatic interaction energies and adjusted its conformation and electrostatic potential to fit the binding pocket in such a way as to enhance the interactions, e.g. hydrogen bonds and π⋯π stacking.

The consistency and variability as well as self-assembly evolution mechanism of cresol–piperazine cocrystals are investigated by experiments and molecular simulation., The disposition of functional groups can induce variations in the nature and type of interactions and hence affect the molecular recognition and self-assembly mechanism in cocrystals. To better understand the formation of cocrystals on a molecular level, the effects of disposition of functional groups on the formation of cocrystals were systematically and comprehensively investigated using cresol isomers (o-, m-, p-cresol) as model compounds. Consistency and variability in these cocrystals containing positional isomers were found and analyzed. The structures, molecular recognition and self-assembly mechanism of supramolecular synthons in solution and in their corresponding cocrystals were verified by a combined experimental and theoretical calculation approach. It was found that the heterosynthons (heterotrimer or heterodimer) combined with O—H⋯N hydrogen bonding played a significant role. Hirshfeld surface analysis and computed interaction energy values were used to determine the hierarchical ordering of the weak interactions. The quantitative analyses of charge transfers and molecular electrostatic potential were also applied to reveal and verify the reasons for consistency and variability. Finally, the molecular recognition, self-assembly and evolution process of the supramolecular synthons in solution were investigated. The results confirm that the supramolecular synthon structures formed initially in solution would be carried over to the final cocrystals, and the supramolecular synthon structures are the precursors of cocrystals and the information memory of the cocrystallization process, which is evidence for classical nucleation theory.

The synthesis and detailed description of the crystal structure and energetic features of the 1:1 cocrystal of carbamazepine (5H-dibenzo[b,f]azepine-5-carboxamide, CBZ) with 3,5-dinitrobenzoic acid (35DNBA), i.e. C15H12N2O·C7H4N2O6, are reported. The CBZ and 35DNBA molecules are packed in alternately arranged layers. Two characteristic R 2 2(8) and R 2 2(16) hydrogen-bond ring motifs have been found. The supramolecular architecture, besides the network of hydrogen bonds, is also stabilized by numerous C—H...π, C=O...π, N—O...π, N—O...C and C=O...N weak intermolecular contacts involving neighbouring molecules in the crystal network. Identified interactions have been discussed in detail on the basis of a structural and energetic analysis. The latter approach, performed using the Pixel and CrystalExplorer programs, yielded additional information about the lattice energy and energetic landscape of the respective interactions in the crystal of CBZ·3DNBA with the evaluation of electrostatic, polarization, repulsion and dispersion terms.

The application of crystal structure prediction (CSP) to industrially relevant molecules requires the handling of increasingly large and flexible compounds. A revised model for the effect of molecular flexibility on the lattice energy that removes the discontinuities and non-differentiabilities present in earlier models (Sugden et al., 2016), with a view to improving the performance of CSP is presented. The approach is based on the concept of computing a weighted average of local models, and has been implemented within the CrystalPredictor code. Through the comparative investigation of several compounds studied in earlier literature, it is shown that this new model results in large reductions in computational effort (of up to 65%) and in significant increases in reliability. The approach is further applied to investigate, for the first time, the computational polymorphic landscape of flufenamic acid for Z′ = 1 structures, resulting in the successful identification of all three experimentally resolved polymorphs within reasonable computational time.

The title compound, C9H13N3O, crystallizes in the monoclinic space group C2/c and all non-hydrogen atoms are within 0.1 Å of the molecular mean plane. In the crystal, the hydrogen-bonding pattern results in [001] chains built up from fused R 2 2(6) and R 2 2(10) rings; the former consists of N—H...N bonds and the latter N—H...O bonds. Electrostatic and dispersion forces are major contributors to the lattice energy, which was estimated by DFT calculations to be −215.7 kJ mol−1.

The crystal and molecular structures of the popular anti-inflammatory drug ketoprophen were examined. The centrosymmetric form of ketoprophen (β-ket) that has been reported so far, was structurally, geometrically and energetically compared with the non-centrosymmetric form (S-enantiomer) of ketoprophen (α-ket). The molecules in both crystal structures are in similar arrangements, with antiparallel-oriented benzo­phenone moieties., 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 benzo­phenone 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.

The role of different intermolecular interactions in the crystal structures of halogen-substituted indoles which are fused with six-membered or seven-membered cyclic rings is investigated here. Several crystal structures show isostructural characteristics due to the presence of similar supramolecular motifs. In the absence of any strong hydrogen bonds, the molecular packing of reported structures is primarily stabilized by the presence of non-classical N—H...π and C—H...π interactions in addition to C—H...X (X = F/Cl/Br) interactions. The nature and energetics of primary and secondary dimeric motifs are partitioned into the electrostatics, polarization, dispersion and exchange–repulsion components using the PIXEL method. Short and directional N—H...π interactions are further explored by a topological analysis of the electron density based on quantum theory of atoms in molecules.

The syntheses of newmyo-inositol derivatives have received much attention due to their important biological activities. 1,2-O-Cyclohexylidene-myo-inositol is an important intermediate formed during the syntheses of certainmyo-inositol derivatives. We report herein the crystal structure of 1,2-O-cyclohexylidene-myo-inositol dihydrate, C12H20O6·2H2O, which is an intermediate formed during the syntheses ofmyo-inositol phosphate derivatives, to demonstrate the participation of water molecules and hydroxy groups in the formation of several intermolecular O—H...O interactions, and to determine a low-energy conformation. The titlemyo-inositol derivative crystallizes with two water molecules in the asymmetric unit in the space groupC2/c, withZ= 8. The water molecules facilitate the formation of an extensive O—H...O hydrogen-bonding network that assists in the formation of a dense crystal packing. Furthermore, geometrical optimization and frequency analysis was carried out using density functional theory (DFT) calculations with B3LYP hybrid functionals and 6-31G(d), 6-31G(d,p) and 6-311G(d,p) basis sets. The theoretical and experimental structures were found to be very similar, with only slight deviations. The intermolecular interactions were quantitatively analysed using Hirshfeld surface analysis and 2D (two-dimensional) fingerplot plots, and the total lattice energy was calculated.

The crystal energy landscapes of the salts of two rigid pharmaceutically active molecules reveal that the experimental structure of amantadine hydrochloride is the most stable structure with the majority of low-energy structures adopting a chain hydrogen-bond motif and packings that do not have solvent accessible voids. By contrast, memantine hydrochloride which differs in the substitution of two methyl groups on the adamantane ring has a crystal energy landscape where all structures within 10 kJ mol−1of the global minimum have solvent-accessible voids ranging from 3 to 14% of the unit-cell volume including the lattice energy minimum that was calculated after removing water from the hydrated memantine hydrochloride salt structure. The success in using crystal structure prediction (CSP) to rationalize the different hydration propensities of these substituted adamantane hydrochloride salts allowed us to extend the model to predict under blind test conditions the experimental crystal structures of the previously uncharacterized 1-(methylamino)adamantane base and its corresponding hydrochloride salt. Although the crystal structure of 1-(methylamino)adamantane was correctly predicted as the second ranked structure on the static lattice energy landscape, the crystallization of aZ′ = 3 structure of 1-(methylamino)adamantane hydrochloride reveals the limits of applying CSP when the contents of the crystallographic asymmetric unit are unknown.

A combination of single-crystal X-ray and neutron diffraction experiments are used to determine the electron density distribution in orthorhombic rubrene. The topology of electron density, NCI analysis and energetics of intermolecular interactions clearly demonstrate the presence of π⋯π stacking interactions in the crystalline state., Rubrene is one of the most studied organic semiconductors to date due to its high charge carrier mobility which makes it a potentially applicable compound in modern electronic devices. Previous electronic device characterizations and first principles theoretical calculations assigned the semiconducting properties of rubrene to the presence of a large overlap of the extended π-conjugated core between molecules. We present here the electron density distribution in rubrene at 20 K and at 100 K obtained using a combination of high-resolution X-ray and neutron diffraction data. The topology of the electron density and energies of intermolecular interactions are studied quantitatively. Specifically, the presence of Cπ⋯Cπ interactions between neighbouring tetracene backbones of the rubrene molecules is experimentally confirmed from a topological analysis of the electron density, Non-Covalent Interaction (NCI) analysis and the calculated interaction energy of molecular dimers. A significant contribution to the lattice energy of the crystal is provided by H—H interactions. The electron density features of H—H bonding, and the interaction energy of molecular dimers connected by H—H interaction clearly demonstrate an importance of these weak interactions in the stabilization of the crystal structure. The quantitative nature of the intermolecular interactions is virtually unchanged between 20 K and 100 K suggesting that any changes in carrier transport at these low temperatures would have a different origin. The obtained experimental results are further supported by theoretical calculations.

A crystallographic quantum-mechanical/molecular-mechanical model (c-QM/MM model) with full space-group symmetry has been developed for molecular crystals. The lattice energy was calculated by quantum-mechanical methods for short-range interactions and force-field methods for long-range interactions. The quantum-mechanical calculations covered the interactions within the molecule and the interactions of a reference molecule with each of the surrounding 12–15 molecules. The interactions with all other molecules were treated by force-field methods. In each optimization step the energies in the QM and MM shells were calculated separately as single-point energies; after adding both energy contributions, the crystal structure (including the lattice parameters) was optimized accordingly. The space-group symmetry was maintained throughout. Crystal structures with more than one molecule per asymmetric unit,e.g.structures withZ′ = 2, hydrates and solvates, have been optimized as well. Test calculations with different quantum-mechanical methods on nine small organic molecules revealed that the density functional theory methods with dispersion correction using the B97-D functional with 6-31G* basis set in combination with the DREIDING force field reproduced the experimental crystal structures with good accuracy. Subsequently the c-QM/MM method was applied to nine compounds from the CCDC blind tests resulting in good energy rankings and excellent geometric accuracies.

An experimental charge-density analysis of pyrazinamide (a first line antitubercular drug) was performed using high-resolution X-ray diffraction data [(sin θ/λ)max= 1.1 Å−1] measured at 100 (2) K. The structure was solved by direct methods usingSHELXS97 and refined bySHELXL97. The total electron density of the pyrazinamide molecule was modeled using the Hansen–Coppens multipole formalism implemented in theXDsoftware. The topological properties of electron density determined from the experiment were compared with the theoretical results obtained fromCRYSTAL09at the B3LYP/6-31G** level of theory. The crystal structure was stabilized by N—H...N and N—H...O hydrogen bonds, in which the N3—H3B...N1 and N3—H3A...O1 interactions form two types of dimers in the crystal. Hirshfeld surface analysis was carried out to analyze the intermolecular interactions. The fingerprint plot reveals that the N...H and O...H hydrogen-bonding interactions contribute 26.1 and 18.4%, respectively, of the total Hirshfeld surface. The lattice energy of the molecule was calculated using density functional theory (B3LYP) methods with the 6-31G** basis set. The molecular electrostatic potential of the pyrazinamide molecule exhibits extended electronegative regions around O1, N1 and N2. The existence of a negative electrostatic potential (ESP) region just above the upper and lower surfaces of the pyrazine ring confirm the π-electron cloud.

The crystal structures of the disordered hemi-DMSO solvate of (E)-2-oxo-N′-(3,4,5-tri­meth­oxy­benzyl­idene)-2H-chromene-3-carbohydrazide, C20H18N2O6·0.5C2H6OS, and (E)-N′-benzyl­idene-2-oxo-2H-chromene-3-carbohydrazide, C17H12N2O3 (4: R = C6H5), are discussed. The non-hydrogen atoms in compound [4: R = (3,4,5-MeO)3C6H2)] exhibit a distinct curvature, while those in compound, (4: R = C6H5), are essential coplanar. In (4: R = C6H5), C—H⋯O and π–π intra­molecular inter­actions combine to form a three-dimensional array. A three-dimensional array is also found for the hemi-DMSO solvate of [4: R = (3,4,5-MeO)3C6H2], in which the mol­ecules of coumarin are linked by C—H⋯O and C—H⋯π inter­actions, and form tubes into which the DMSO mol­ecules are cocooned. Hirshfeld surface analyses of both compounds are reported, as are the lattice energy and inter­molecular inter­action energy calculations of compound (4: R = C6H5).

The crystal structures of three new structurally related drug-like bicyclo derivatives are correlated with measured thermodynamic quantities for their sublimation and melting processes. The sublimation thermodynamics are determined using the temperature dependencies of the vapour pressure, and the melting processes are examined using differential scanning calorimetry. The three compounds contain a commonN-(3-thia-1-azabicyclo[3.3.1]non-2-ylidene)aniline core, with either a CH3, F or CF3substituent at the 4-position of the aniline ring. Lattice energy calculations are made using both the PIXEL and Coulomb–London–Pauli (CLP) models, and the conformational flexibility of the molecules is examined using gas-phase density functional theory (DFT) calculations. The experimentally measured crystal lattice energies (ΔH0sub) decrease in the order: CH3> F > CF3. The calculated lattice energies using the PIXEL approach are in good agreement with the experimental values, and the partitioned intermolecular interaction energies suggest that dispersion contributions dominate the crystal structures of all three compounds. The sublimation energies and melting points are inversely correlated for the three molecules, with the melting points increasing in the order CF3< F < CH3.

The X-ray single-crystal structure of (2S,5R,6R)-6-amino-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid, commonly known as (+)-6-aminopenicillanic acid (C8H12N2O3S) and a precursor of a variety of semi-synthetic penicillins, has been determined from synchrotron data at 150 K. The structure represents an ordered zwitterion and the crystals are nonmerohedrally twinned. The crystal structure is composed of a three-dimensional network built by three charge-assisted hydrogen bonds between the ammonium and carboxylate groups. The complementary analysis of the crystal packing by thePIXELmethod brings to light the nature and ranking of the energetically most stabilizing intermolecular interaction energies. In accordance with the zwitterionic nature of the structure,PIXELlattice energy calculations confirm the predominance of the Coulombic term (−379.1 kJ mol−1) ahead of the polarization (−141.4 kJ mol−1), dispersion (−133.7 kJ mol−1) and repulsion (266.3 kJ mol−1) contributions.

The structures of the three title monosubstituted ferrocenes, namely 1-chloro­ferrocene, [Fe(C5H5)(C5H4Cl)], (I), 1-bromo­ferrocene, [Fe(C5H5)(C5H4Br)], (II), and 1-iodo­ferrocene, [Fe(C5H5)(C5H4I)], (III), were determined at 100 K. The chloro- and bromo­ferrocenes are isomorphous crystals. The new triclinic polymorph [space group P , Z = 4, T = 100 K, V = 943.8 (4) Å3] of iodo­ferrocene, (III), and the previously reported monoclinic polymorph of (III) [Laus, Wurst & Schottenberger (2005 ▶). Z. Kristallogr. New Cryst. Struct. 220, 229–230; space group Pc, Z = 4, T = 100 K, V = 924.9 Å3] were obtained by crystallization from ethanolic solutions at 253 and 303 K, respectively. All four phases contain two independent mol­ecules in the unit cell. The relative orientations of the cyclo­penta­dienyl (Cp) rings are eclipsed and staggered in the independent mol­ecules of (I) and (II), while (III) demonstrates only an eclipsed conformation. The triclinic and monoclinic polymorphs of (III) contain nonbonded inter­molecular I⋯I contacts, causing different packing modes. In the triclinic form of (III), the mol­ecules are arranged in zigzag tetra­mers, while in the monoclinic form the mol­ecules are arranged in zigzag chains along the a axis. Crystallographic data for (III), along with the computed lattice energies of the two polymorphs, suggest that the monoclinic form is more stable.

A new approach to the crystal structure prediction of flexible molecules is presented. It is applied to piracetam, whose conformational polymorphs exhibit a variety of hydrogen-bond motifs but lack the intramolecular hydrogen bond found in the gas-phase ab initio optimized conformer. Stable crystal packing can result when favourable intermolecular interactions are made possible when the molecule distorts from the gas-phase conformation. If the resulting intermolecular lattice energy is sufficiently favourable to compensate for the intramolecular energy penalty associated with the suboptimal gas-phase conformation, then the crystal structure may be experimentally feasible. The new approach involves searching for low-energy crystal structures using a large number of rigid conformers, firstly to systematically explore which regions of conformational space could give rise to low-energy hydrogen-bonded crystal structures, and then to refine the search using crystallographic insight to optimize particular intermolecular interactions. The timely discovery of a new polymorph (form IV) by an independent experimental team allowed this approach to be validated by way of a `blind test' of crystal structure prediction. Form IV was successfully identified as the most favourable computed crystal structure with a conformation very distinct from that in the previously known polymorphs.

The crystal structures of six nitrotoluene derivatives, synthesized by Wilhelm Koerner about a century ago and retrieved from a depository at the University of Milano, were determined. The correct assignment of molecular structures is verified. The geometry of the nitro groups and factors affecting the orientation of nitro groups with respect to the benzene ring are discussed, also using an auxiliary set of crystal structures retrieved from the Cambridge Structural Database. The crystal packings have been analyzed, and lattice energies have been calculated by atom–atom potential methods and by the newly proposed Pixel method. This method allows a more complete description of intermolecular potentials in terms of the interaction between molecular electron densities and separate Coulombic, polarization, dispersion and overlap repulsion energies. Lattice vibrations and external entropies were calculated by lattice-dynamical procedures. The results of the Pixel energy calculations allow a reliable, quantitative assessment of the relative importance of stacking interactions and hydrogen bonding in the rationalization of the recognition modes of nitrobenzene derivatives, which is impossible to attain using only qualitative atom– atom geometry concepts.

The results of the sixth blind test of organic crystal structure prediction methods are presented and discussed, highlighting progress for salts, hydrates and bulky flexible molecules, as well as on-going challenges., The sixth blind test of organic crystal structure prediction (CSP) methods has been held, with five target systems: a small nearly rigid molecule, a polymorphic former drug candidate, a chloride salt hydrate, a co-crystal and a bulky flexible molecule. This blind test has seen substantial growth in the number of participants, with the broad range of prediction methods giving a unique insight into the state of the art in the field. Significant progress has been seen in treating flexible molecules, usage of hierarchical approaches to ranking structures, the application of density-functional approximations, and the establishment of new workflows and ‘best practices’ for performing CSP calculations. All of the targets, apart from a single potentially disordered Z′ = 2 polymorph of the drug candidate, were predicted by at least one submission. Despite many remaining challenges, it is clear that CSP methods are becoming more applicable to a wider range of real systems, including salts, hydrates and larger flexible molecules. The results also highlight the potential for CSP calculations to complement and augment experimental studies of organic solid forms.

By using simply the numbers of occurrences of different atom types as descriptors, a conceptually transparent and remarkably accurate model for the prediction of the enthalpies of sublimation of organic compounds has been generated. The atom types are defined on the basis of atomic number, hybridization state and bonded environment. Models of this kind were applied firstly to aliphatic hydrocarbons, secondly to both aliphatic and aromatic hydrocarbons, thirdly to a wide range of non-hydrogen-bonding molecules, and finally to a set of 226 organic compounds including 70 containing hydrogen-bond donors and acceptors. The final model gives squared correlation coefficients of 0.925 for the 226 compounds in the training set and 0.937 for an independent test set of 35 compounds. The success of such a simple model implies that the enthalpy of sublimation can be predicted accurately without knowledge of the crystal packing. This hypothesis is in turn consistent with the idea that, rather than being determined by the particular features of the lowest-energy packing, the lattice energy is similar for a number of hypothetical alternative crystal structures of a molecule.

The α phase of SiO2is known as a technologically important material. According to the Hartman–Perdok theory, so-calledFforms are present: the hexagonal prismm{10\bar{1}0}, the major rhombohedronr{10\bar{1}1} and the minor rhombohedronz{01\bar{1}1}.Ffaces grow according to a two-dimensional growth mechanism and are the only forms to be expected on the growth form. Computation of attachment energies, which are considered to be directly related to the growth rates of the correspondingFfaces, has been performed using an electrostatic point-charge model, taking into account Born repulsion and van der Waals contributions computed by means of the Gilbert equation or the van Beest and van Santen potential. All the theoretical growth forms are prismatic with the two aforementioned rhombohedra, which are almost equally important and independent of the corrections for the van der Waals and Born interactions. The equilibrium form is prismatic and shorter as a result of the hexagonal prism form being less important. On the atomic scale, the differences in surface topologies between theFforms depend on the variation in depth below the surface boundary of those central Si atoms that are partially unshielded because of the incomplete tetrahedral coordination.

The crystal structures of the inclusion compounds formed between the host 2,2′-dihydroxy-1,1′-binaphthyl and the three picoline isomers have been elucidated and their lattice energies calculated. The selectivities of enclathration by the host have been measured by competition experiments. The thermal stabilities and activation energies of desolvation of the compounds have been determined. The kinetics of desolvation are correlated to the structures.

A collaborative workshop was held in May 1999 at the Cambridge Crystallographic Data Centre to test how well currently available methods of crystal structure prediction perform when given only the atomic connectivity for an organic compound. A blind test was conducted on a selection of four compounds and a wide range of methodologies representing the principal computer programs currently available were used. There were 11 participants who were allowed to propose at most three structures for each compound. No program gave consistently reliable results. However, seven proposed structures were close to an experimental one and were classified as `correct'. One compound occurred in two polymorphs, but only one form was predicted correctly among the calculated structures. The basic problem with lattice energy based methods of crystal structure prediction is that many structures are found within a few kJ mol−1 of the global minimum. The fine detail of the force-field methodology and parametrization influences the energy ranking within each method. Nevertheless, present methods may be useful in providing a set of structures as possible polymorphs for a given molecular structure.

The compounds tetrakis(trimethylsilyl)methane C[Si(CH3)3]4 (TC) and tetrakis(trimethylsilyl)silane Si[Si(CH3)3]4 (TSi) have crystal structures with the molecules in a cubic closed-packed (c.c.p.) stacking. At room temperature both structures have space group Fm{\bar 3}m (Z = 4) with a = 13.5218 (1) Å, V = 2472.3 (1) Å3 for TSi, and a = 12.8902 (2) Å, V = 2141.8 (1) Å3 for TC. X-ray scattering data can be described by a molecule with approximately sixfold orientational disorder, ruling out a structure with free rotating molecules. Upon cooling, TSi exhibits a first-order phase transition at T c = 225 K, as is characterized by a jump of the lattice parameter of Δa = 0.182 Å and by an exothermal maximum in differential scanning calorimetry (DSC) with ΔH = 11.7 kJ mol−1 and ΔS = 50.0 J mol−1 K−1. The structure of the low-temperature phase is refined against X-ray powder data measured at 200 K. It has space group P2 13 (Z = 4), a = 13.17158 (6) Å and V = 2285.15 (2) Å3. The molecules are found to be ordered as a result of steric interactions between neighboring molecules, as is shown by analyzing distances between atoms and by calculations of the lattice energy in dependence on the orientations of the molecules. TC has a phase transition at T c1 = 268 K, with Δa 1 = 0.065 Å, ΔH 1 = 3.63 kJ mol−1 and ΔS 1 = 13.0 J mol−1 K−1. A second first-order phase transition occurs at T c2 = 225 K, characterized by Δa 2 = 0.073 Å, ΔH 2 = 6.9 kJ mol−1 and ΔS 2 = 30.0 J mol−1 K−1. The phase transition at higher temperature has not been reported previously. New NMR experiments show a small anomaly in the temperature dependence of the peak positions in NMR to occur at T c2. Rietveld refinements were performed for the low-temperature phase measured at T = 150 K [space group P2 13, lattice parameter a = 12.609 (3) Å], and for the intermediate phase measured at T = 260 K [space group Pa{\bar 3}, lattice parameter a = 12.7876 (1) Å]. The low-temperature phase of TC is formed isostructural to the low-temperature phase of TSi. In the intermediate phase the molecules exhibit a twofold orientational disorder.

We fully agree with Spackman’s tribute to the considerable contribution of R. F. Stewart to the field of charge-density analysis and will restrict our comments to the second half of his letter which discusses a number of issues relevant to the results presented in our paper (Volkov, King et al., 2006). Mark Spackman (MS) states that ‘it is worth noting that the map presented by VKCF is based on a direct-space summation of contributions from only the eight nearest neighbours in the crystal’, implying that the figure presented will change significantly if more neighbouring molecules are included in the calculation. In Fig. 1 below, we present analogous maps obtained by taking into account the contributions of pseudoatoms within several unit cells (this option was available in the latest version of XD2006 at the time our original paper was published). Only small changes are observed on the periphery of the map, which do not in any way change any of the conclusions presented in our paper. For the calculation of the electrostatic potential (ESP), electric field (EF) and electric field gradient (EFG), we use the same EPMM approach (Volkov et al., 2004) developed for calculation of the electrostatic interaction energy. Exact formulae are used for atoms located within a certain distance (typically ~6 A for the ESP, EF and EFG in organic systems) from the point at which the properties are calculated. Beyond that distance, approximate formulae, based on the expansion of the ESP, EF and EFG in atomic multipole moments, are used. Table 1 includes a comparison between exact and EPMM methods, demonstrating the satisfactory accuracy of the much faster (Table 2) EPMM method. Table 3 shows the convergence of the x and y components of the EF vector in the plane of the map versus the number of unit cells included in the calculation. An excellent convergence is achieved when including atoms with fractional coordinates of 1 < x, y, z < 2, while the inclusion of eight neighbouring cells is also quite satisfactory, i.e. the r.m.s. deviation is only 0.02–0.03 e A , which is less than 1% of the r.m.s. values of the Ex (~5.4 e A ) and Ey (~6.5 e A ) components of the EF vector in the plane of the map. Note that a similar direct-space approach is used for the calculation of the electrostatic binding energy in the crystal (Volkov et al., 2007), which has also been implemented in XD2006 (Volkov, Macchi et al., 2006). The efficient combination of the exact formulae and the multipole moment approximation eliminates the convergence problems inferred by MS. MS quotes Cummins et al. (1976) that ‘computation of the EF in the crystal due to point dipoles is well known to require lattice summation techniques to achieve convergence’ and adds that ‘these combined approaches are entirely analogous to the Ewald approach used to achieve rapid convergence of lattice sums.’ We are of course well aware of the existence of the Ewald summation techniques which we used previously in the calculation of the lattice energy of ionic (Su & Coppens, 1995) and molecular (Abramov, Volkov, Wu & Coppens, 2000a,b; Abramov, Volkov & Coppens, 2000) crystals. However, the direct-space summation was found to be advantageous in terms of both speed and ease of implementation when combined with the new EPMM method (Volkov et al., 2007). MS states that we misunderstand the purpose of the methods described by Brown & Spackman (1994) as several algorithms published in this paper were used only for debugging of the VALRAY code and should not be considered of practical importance. Table 1 Root-mean-squared differences in electric field components Ex and Ey (e A ) in the plane of the map between exact and EPMM calculations .

In the past fifty years, one of the major challenges for crystallographers and crystal engineers has been the rationalization and exploitation of the molecular pre-aggregation for reactions that occur in the solid state. Indeed, one of the first appearances of the term ‘crystal engineering’ coincides with Schmidt’s investigation of the 2+2 photochemical cycloaddition in the polymorphs of cis-cinammic acid (Schmidt, 1971 ▶). That experiment showed that ‘freezing’ molecules in the appropriate fashion is the key for some reactions to occur, and that the crystalline state can serve as a template for those reactions that depend on the reciprocal molecular orientation (topochemical control). A great deal of research on molecular crystals involves reactions within solids, triggered by photo or thermal events (Toda, 2005 ▶), or at the interface of the solid (James et al., 2012 ▶) with other solids and molten phases. These reactions can be difficult to identify by standard diffraction experiment and require ad hoc crystallographic analysis (Lusi & Barbour, 2013 ▶; Poulain et al., 2014 ▶; Casaretto et al., 2015 ▶). Solid–gas reactions have received particular attention owing to their potential exploitation in chemical sensing, storage and separation (Perry et al., 2009 ▶). Although most of them consist of physisorption of inert guests onto the internal surface of a porous material, some remarkable examples of host–guest chemistry describe molecular diffusion of volatile species within close-packed crystals (Atwood et al., 2002 ▶). In molecular crystals, intricate equilibria are known to exist between the pure compounds and their hydrogen-bonded water and alcohol solvates (Braga et al., 2008 ▶; Braun et al., 2012 ▶). Similarly, ammonia and hydrohalic acids can reversibly coordinate to metals in discrete (Minguez Espallargas et al., 2007 ▶) and polymeric complexes (Adams et al., 2007 ▶), while clathration of aromatic species can be driven by the increased lattice energy deriving from weak van der Waals interactions (Lusi & Barbour, 2012 ▶). Even the simple selective exposure of a given crystallographic face to water vapour can trigger molecular reorientation (Jiang et al., 2013 ▶). These phenomena require dramatic structural reorganization (Caira & Nassimbeni, 1996 ▶), which contrasts with the rigidity typically associated with the crystalline state. Now Brammer and co-workers (Wright et al., 2015 ▶) explore this idea further by investigating a family of silver–phenazine coordination polymers: {[Ag4(O2C(CF2)2­CF3)2(phenazine)2(arene)k]·J(arene)}n (arene = toluene, xylene). In these compounds the aromatic solvent has a double function: as a ligand it is coordinated to open metal sites and as a guest it is included within the cavities present in the structures. It follows that the structure and composition of the product depend on the solvent used in the synthesis. Besides a novel arene–silver coordination geometry, the paper reports on the solid-state reactivity of the family of compounds. The authors observe that volatile solvent can diffuse out of the non-porous crystal causing major structural transformation, which results in a mixture of products. In total, three crystalline phases were identified: two have been characterized as polymorphs – perhaps polytypes (Feller & Cheetham, 2008 ▶) or supramolecular isomers (Moulton & Zaworotko, 2001 ▶) – of the coordination polymer {[Ag4(O2C(CF2)2CF3)2(phenazine)2]}n, while the third remains unknown. Solvent removal can be achieved by heating or exposing the sample to air, but also by exposing the solvated microcrystalline powder to the vapour of alcohols and arenes (other than those already included in the structure). Under the attempted experimental conditions, guest substitution was observed only between toluene and p-xylene, and even in that case it was only partial. Most importantly, the relative amount of the phases produced depends on both the structural features of the reacted material and the method used to eliminate the solvent. A sound structural analysis relates those observations with the topochemical character of the reactions. Remarkably, one of the desolvated forms could not be isolated by wet methods. A single crystal was instead isolated from the thermal elimination experiment. Since the reactions afford crystalline particles that are large enough for single-crystal diffraction it can be imagined that the molecular reorganization is concerted or that there is a certain degree of flexibility in the polymeric chains. An archaic interpretation of the solid physical state portrays crystals as rigid assemblies of atoms and molecules that are buried into fixed positions from which they cannot move (Dunitz et al., 1988 ▶). This view is countered by many examples of solid-state reactions that occur with considerable molecular reorganization. The work of Brammer and co-workers is another such example. It shows that novel coordination polymers can be produced from solvated precursors, the product depending on the molecular arrangement in the reacting structure as well as the conditions used to eliminate the guests. Given the wide potential applications of coordination polymers (as porous or conducting or magnetic materials), all the synthetic strategies shall be considered, which might afford the desired architecture. In this sense, solvent elimination represents a powerful tool to access crystalline products under topochemical control for the rational design of novel materials.

The enthalpy of sublimation of diuron, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, C 9 H 10 ON 2 Cl 2 , was measured at 393 K using a Tian-Calvet calorimeter. The corrected experimental value Δ sub H m o (298 K) = 133.87(74) kJ mol -1 was then compared to the half-crystal energy E k . This energy was calculated from structural data using the 'consistent valence force field', which consists of a 6-12 Lennard-Jones potential and a Coulomb term. The disagreement between Δ sub H m o (298 K) and E k = 152.6 kJ mol -1 is about 14%. It can be partially attributed to the inaccuracy of the atomic coordinates of the crystal structure and to the charges of the force field. For simple molecules such as normal octadecane (n-C 18 H 38 ), hexanedioic acid (C 6 H 10 O 4 ), diphenylmethanone (C 13 H 10 O) and anthracene (C 14 H 10 ), the agreement is very good, about 3%.

The 23 Glazer tilt systems describing octahedral tilting in perovskites have been investigated. The various tilt systems have been compared in terms of their A-cation coordination and it is shown that those tilt systems in which all the A-cation sites remain crystallographically equivalent are strongly favored, when all the A sites are occupied by the same ion. Calculations based on both ionic and covalent models have been performed to compare the seven equivalent A-site tilt systems. Both methods predict that when the tilt angles become large, the orthorhombic a + b − b − tilt system will result in the lowest energy structure. This tilt system gives the lowest energy structure because it maximizes the number of short A—O interactions. The rhombohedral a − a − a − tilt system gives a structure with a slightly lower Madelung energy, but increased ion–ion repulsions destabilize this structure as the tilt angles increase. Consequently, it is stabilized by highly charged A cations and small to moderate tilt angles. The ideal cubic a 0 a 0 a 0 tilt system is only observed when stabilized by oversized A cations and/or M—O π-bonding. Tilt systems with nonequivalent A-site environments are observed when at least two A cations with different sizes and/or bonding preferences are present. In these compounds the ratio of large-to-small cations dictates the most stable tilt system.

Ultrasonic and thermal expansivity measurements have been used to calculate the elastic properties of nearly perfect single crystals of potassium tetrachlorozincate. Determination of the nine components of the elastic tensor indicates that this material has lower elastic constants than comparable tetrahedral compounds containing sulfate anions [von Hausshul (1965). Acta Cryst. 18, 841-845]. The largest elastic constant is along the crystallographic a axis, along which are aligned the pseudo-equivalent tetrachlorozincate anions associated with the structural modulation. Conversely, the b axis exhibits the smallest of the three principal elastic constants and the highest thermal-expansion coefficient. Measurements of the temperature dependence of the elastic constants as the incommensurate phase transition is approached show similar behaviour for most of the components with the exception of the off-diagonal component C 13 , where a significantly different temperature sensitivity is observed. Comparison of the calculated elastic constant values with those for other A 2 BX 4 systems indicates that the introduction of the ZnCl 4 grouping has a significant effect in lowering the C 44 constant. A reduction in these constants is a reflection of the effects that this grouping has on the overall lattice energy, leading eventually to the formation of incommensurate phase behaviour. The unusual temperature dependence of the C 13 component is indicative of soft-mode generation, which may play a role in the onset of the ferroelectric-to-incommensurate phase transformation.

The structure of the title compound, C 17 H 20 O 3 , a substituted cyclohexene system, has been determined. The high degree of disorder in various substituent moieties results in two different orientations of the molecule in the crystal lattice which have unequal occupancy factors. Lattice-energy calculations based on van der Waals attractive and repulsive potentials clearly show minima at the observed disordered positions. The difference in occupancy factors between the two molecular orientations can be explained from the relative values of lattice energies.

The structure was solved with the molecular packing program MOLPAK, starting with a molecular mechanics-geometry optimized model of an isolated molecule. The best predicted crystal structures from the MOLPAK procedure were subjected to lattice energy refinement with the WMIN program. The MOLPAK/WMIN-predicted crystal structure, whose cell parameters were closest to the experimental values, gave an initial R of 0.48 for the 173 data to 0 = 25°. Four cycles of least-squares refinement of x, y, z and U gave an R of 0.27 for the 277 data to 0 = 30°. Final R = 0.038, wR = 0.044. There are no unusual intermolecular distances and crystal packing is normal.

Geometries of 3-buten-2-ol (CH 3 -CHOH-CH=CH 2 ) were optimized using standard ab initio SCF-LCAO-MO methods, as well as the electrostatic crystal field (ECF-MO) approach. Comparisons are made between isolated butenol molecules and molecules packed in a tri-ortho-thymotide (TOT) clathrate, 1,7,13-trimethyl-4,10,16-tris(1-methylethyl)-6H,12H,18H-tribenzo[b,f,j][1,5,9]trioxacyclododecin-6,12,18-trione. Excellent agreement is obtained with available microwave and diffraction data. Despite the neglect of orbital overlap between neighbouring molecules, all the significant geometric differences between the gas phase and the solid state are well reproduced. Specifically we note the differences in C=C-C-O torsion angles. The presence of both R and S configurations in the TOT-clathrate with a low degree of chiral discrimination has been experimentally verified. From the difference in calculated molecular energy between the two butenol enantiomers that were optimized within the TOT-clathrate, the observed enantiomeric excess of 4-6% in favour of (R)-butenol is reproduced.

The crystal structure of the low-temperature phase of 4,4'-dichlorobenzophenone, C 13 H 8 Cl 2 O, has been determined at 140K. Crystal data: T = 140K, M r = 251.1, monoclinic, I2/c, a = 24.62 (1), b = 6.091(1), c = 7.476(2)A, β = 100.14 (3)°, V = 1104(2)A 3 , Z = 4, D x = 1.510 Mg m -3 , λ(Mo Kα) = 0.7107 A, μ = 0.55 mm -1 , F(000) = 512, R = 0.041, wR = 0.050. A mechanism for the sequence of phase transitions is proposed and a semi-empirical atom-atom potential model has been adopted in order to study the energetic stability of the high- and low-temperature crystal phases. A lattice-dynamical model allowing for molecular internal degrees of freedom has been developed in order to interpret the experimental Raman spectra of both phases.

Composite structures, containing two or more sublattices in one crystal, are of importance in the search for new materials. They are classified in terms of the interlattice matrix relating the sublattices, and by the use of chemical criteria The use of superspace to interpret the diffraction pattern and perform calculations on electronic structure and lattice energy is referred to. The composite low-dimensional conductors (BEDT-TTF)Hg 0.776 (SCN) 2 and (BEDT-TTF) 4 Hg 2.88 Br 8 (BEDT-TTF = bisethylene dithiotetrathiafulvalene) are discussed as examples. Of particular interest are the interaction between the sublattices and the change of structure and properties as a function of temperature.

Ewald's method of accelerated convergence [Ewald (1921). Ann. Phys. (Leipzig), 64, 253–287] is generalized to calculate the electrostatic potential of a crystal in which the atoms have overlapping spherical densities. The algorithm is applied to the cubic NaF crystal. The potentials at the Na and F nuclei are calculated for the free-ion model and for the results from a κ refinement of the experimental data of Howard & Jones [Acta Cryst. (1977), A33, 776–783]. The κ refinement indicates an incomplete charge transfer but gives an electrostatic energy close to that of the point-charge model with full charge transfer and a lattice energy that is in good agreement with the experimental value.

The hydrate crystal lithium hydroxide monohydrate LiOH.H2O has been studied by ab initio periodic Hartree-Fock calculations. The influence of the crystalline environment on the local molecular properties (molecular geometry, atomic charges, electron density, molecular vibrations and deuterium quadrupole coupling constants) of the water molecule, the lithium and hydroxide ions has been calculated. A number of crystalline bulk properties are also presented, optimized crystalline structure, lattice energy and electronic band structure. The optimized cell parameters from calculations with a large basis set of triple-zeta quality differ by only 1-3% from the experimental neutron-determined cell, whereas the STO-3g basis set performs poorly (differences of 5-10%). With the triple-zeta basis also the atomic positions and intermolecular distances agree very well with the experiment. The lattice energy differs by approximately 8% from the experimental value, and by at most 3% when a density-functional electron correlation correction is applied. Large electron-density rearrangements occur in the water molecule and in the hydrogen bond and are in qualitative and quantitative agreement with experimental X-ray diffraction results. The quadrupole-coupling constants of the water and hydroxide deuterium atoms are found to be very sensitive to the O-H bond length and are in good agreement with experimental values when the calculation is based on the experimental structure. The anharmonic O-H stretching vibrations in the crystal are presented and found to be very close to results from calculations on molecular clusters. The electronic band and density-of-states spectra are discussed. Model calculations on a hydrogen fluoride chain were used to rationalize the results.

The structure of the title compound, decahydro-2,2,5,5-tetranitro-1,6:3,4-dimethanocyclobuta[1,2: 3, 4]dicyclopentene, C12H12N4O8, was solved with the molecular packing program MOLPAK, starting with an AM1-geometry-optimized model of an isolated molecule. The 20 best predicted crystal structures from the MOLPAK procedure were subjected to lattice energy refinement with the WMIN program. A structure-factor calculation with the top MOLPAK/WMIN derived structure and the 197 Fo data to sin theta/lambda = 0.324 A-1 (theta = 30 degrees) gave an R value of 0.29. Final R = 0.057, wR = 0.067 for 730 reflections with I > 3 sigma (I). There are no unusual intermolecular distances and crystal packing is normal.

A novel quantum-chemical approach to the properties of molecular crystals has recently been developed. This Hartree–Fock-based self-consistent-crystal-field method is outlined and applied to the intramolecular torsion angle in solid hydrogen peroxide, the cold-phase structure of solid dinitrogen and the hot-phase structure of solid acetylene. The potential-energy surfaces are calculated with the assumption of perfectly ordered crystals and coherent molecular potential-energy surfaces are produced. Agreement with experiment is obtained for hydrogen peroxide. It is suggested that discrepancies between calculated and observed structures of N2(s) and C2H2(s) result from static and dynamic disorder, respectively. This interpretation is consistent with the experimentally observed intramolecular-bond shortenings in these two solids. Lattice energies in good agreement with experiment are obtained in all three cases.

Un ensemble consequent de parametres de repulsion a courte distance (A et B) a ete etabli en utilisant le programme WMIN de minimisation de l'energie potentielle. Les valeurs de B ont ete determinees a partir de l'extrapolation quadratique des especes isoelectroniques prealablement derivee des donnees de vibration. Les rayons de repulsions A ont ete affines a partir des structures cristallines connues en minimisant leur energie reticulaire, tandis que B est conserve invariant

The crystal structures of 458 azahydrocarbons have been retrieved from the Cambridge Structural Database. Statistical analyses of molecular and crystal parameters were performed, and the results compared with hydrocarbons and oxygecontaining compounds.

La contribution de chaque operateur de symetrie a l'energie reticulaire est calculee pour un echantillon de 1068 structures cristallines contenant des liaisons autres que des liaisons hydrogene dans les groupes d'espace les plus frequents des molecules organiques

(I) C 29 H 20 O cristallise dans C2/c avec a=26,16, b=8,38, c=21,57 A, β=119,5°, Z=8; affinement jusqu'a R=0,063. (IIa) C 35 H 20 Cl 4 O 3 •0,5C 7 H 8 cristallise dans P2 1 /a avec a=18,26, b=10,50, c=17,74 A, β=109,6°, Z=4; affinement jusqu'a R=0,079. Les calculs d'energie reticulaire suggerent que les structures sont stabilisees par des forces de reseau et que les groupes phenyl ne sont pas coplanaires avec le cycle central dans la molecule libre

Size-effect distortions have been computed in a pair of orientationally disordered isomers of dibromodiethyldimethylbenzene using Monte Carlo simulation. Net forces/torques were computed for each molecule and the direction of movement was determined using variational methods. The sampling was carded out both in sequential and in a random manner. Effects of lattice size, interaction radius and boundary conditions have been investigated. In the lowest-energy configuration, typical centre-of-mass displacements were in the range 0.25-0-30 A. Average molecular librations, represented in terms of Eulerian angles, were in the range 0-4°. The ϕ and ψ components however had large r.m.s. values (~ 10°). Basic results and trends were unaffected by the lattice size and the interaction radius (6-15 A). However, the r.m.s. values of the angles came down considerably for fixed boundaries. The influence of temperature on size effect has also been investigated.

C 9 H 13 NO 3 S cristallise dans P21/c avec a=8,855, b=10,426, c=11,082, β=91,13° a 193K, a=8,913, b=10,487, c=11,166A, β=90,95° a 255K, Z=4; affinement jusqu'a R=0,044. L'analyse des mouvements et les calculs d'energie potentielle d'empilement demontrent que la reaction de transfert du methyl, induite par la temperature, est assistee par des mouvements cooperatifs des molecules des couches le long de l'axe b