Maryjane Tremayne

Contemporary Advances in the Use of Powder X-Ray Diffraction for Structure Determination

Many crystalline solids cannot be prepared in the form of single crystals of sufficient size and/or quality for investigation using single-crystal X-ray diffraction techniques, and the opportunity to carry out structure determination using powder diffraction data is therefore essential to understand the structural properties of such materials. Although the refinement stage of the structure determination process can be carried out fairly routinely from powder diffraction data using the Rietveld profile refinement technique, solving crystal structures directly from powder data is associated with several intrinsic difficulties. Nevertheless, substantial progress has been made in recent years in the scope and potential of techniques in this field. This article aims to highlight the types of structural problems for which structure determination may now be tackled directly from powder diffraction data, and contemporary applications across several chemical disciplines are presented. A brief survey of the underlying methodologies is given, with some emphasis on recently developed techniques for carrying out the structure-solution stage of the structure-determination process.

The impact of powder diffraction on the structural characterization of organic crystalline materials

The bulk properties of organic crystalline materials depend on their molecular and crystal structures but, as many of these materials cannot be prepared in a suitable form for conventional single–crystal diffraction studies, structural characterization and rationalization of these properties must be obtained from powder diffraction data. The recent development of direct–space structure solution methods has enabled the study of a wide range of organic materials using powder diffraction data, many of structural complexity only made tractable by these advances in methodology. These direct–space methods are based on a number of global optimization techniques including Monte Carlo, simulated annealing, genetic algorithm and differential evolution approaches. In this article, the implementation and relative efficiency and reliability of these methods are discussed, and their impact on the structural study of organic materials is illustrated by examples of polymorphic systems, pharmaceutical, pigment and polypeptide structures and compounds used in the study of intermolecular networks.

Strategies for the synthesis of porous metal phosphonate materials

The use of bifunctional phosphonate anions and metal cations that can adopt different co-ordination environments is proposed as a strategy for synthesising porous metal phosphonates. The hydrothermal methods described use phosphonic acids and dialkylphosphonates, which are hydrolysed in situ by the acidic metal solutions, to derive the desired phosphonate anions. A series of isostructural hydroxymethylphosphonates, M(O3PCH2OH) [M = Zn, Co, Cu and Mn], which contain a hexagonal array of cylindrical channels (approximate diameter 5.7 A), were prepared by reaction of the relevant metal acetate with diethyl hydroxymethylphosphonate. Zn(O3PCH2C(O)NH2)·H2O was prepared by reaction of zinc acetate and diethyl cyanomethylphosphonate. The structure was determined by single crystal X-ray diffraction and contains two orthogonal channel systems with water molecules situated at the intersections of the channels. Zn3(O3PCH2CO2)2·nH2O [n = 3, 4] was prepared by reaction of phosphonoacetic acid and zinc acetate. Single crystal analysis reveals the presence of channels in the [100] direction with a cross-section of 10.02 × 6.78 A. All three of the Zn-based materials can be isomorphously doped with Co.

Neue Fortschritte bei Strukturbestimmungen durch Röntgen-Pulverdiffraktometrie

Aus vielen kristallinen Festkorpern lassen sich keine hinreichend grosen und/oder fur die Untersuchung mit Einkristall-Rontgenbeugungsverfahren qualitativ geeigneten Einkristalle erhalten. Darum ist fur das Verstandnis der strukturellen Eigenschaften solcher Stoffe die Moglichkeit zur Strukturbestimmung aus Pulverbeugungsdaten absolut notwendig. Obwohl man den Schritt der Verfeinerung bei der Strukturbestimmung aus Pulverbeugungsdaten mit der Rietveld-Methode zur Verfeinerung des Profils routinemasig durchfuhren kann, sind mit der Losung der Kristallstruktur direkt aus den Daten von Pulveraufnahmen einige Schwierigkeiten verbunden. Gleichwohl hat man in den letzten Jahren auf diesem Gebiet hinsichtlich der Anwendungsmoglichkeiten und der Aussagekraft wesentliche Fortschritte erzielt. In diesem Beitrag werden die strukturellen Probleme solcher Strukturbestimmungen hervorgehoben, die man inzwischen direkt ausgehend von Pulverbeugungsdaten angehen kann, ebenso werden neue Anwendungen aus anderen Bereichen der Chemie vorgestellt. Die zugrunde liegenden Methoden werden kurz behandelt, wobei wir schwerpunktmasig auf kurzlich entwickelte Methoden fur den Schritt der Strukturlosung im Strukturbestimmungsverfahren eingehen.

Topochemical Rationalization of the Solid-State Polymerization Reaction of Sodium Chloroacetate: Structure Determination from Powder Diffraction Data by the Monte Carlo Method

It has been known for a long time that, at sufficiently high temperature, solid sodium chloroacetate undergoes a polymerization reaction to produce polyglycolide and sodium chloride. However, an understanding of this reaction and its mechanism was not previously possible, as sodium chloroacetate is microcrystalline and structural information could not be determined from conventional single-crystal X-ray diffraction methods. In this paper, we report the structure of sodium chloroacetate; this advance has been possible through the application of new methodology, based on Monte Carlo sampling techniques, that we have developed recently for crystal structure determination from powder diffraction data. The reported structure of sodium chloroacetate provides a direct rationalization, based on topochemical principles, of the polymerization reaction to produce polyglycolide.

Solution of an organic crystal structure from X-ray powder diffraction data by a generalized rigid-body Monte Carlo method: crystal structure determination of 1-methylfluorene

Many important crystalline materials do not form single crystals of sufficient size and quality for single crystal X-ray diffraction studies, and in such cases it is essential that the crystal structure can be solved from powder diffraction data; however, there are many difficulties associated with solving crystal structures directly from powder diffraction data. In this paper, we report the successful application of a Monte Carlo technique to solve the previously unknown crystal structure of 1-methylfluorene from X-ray powder diffraction data collected on a conventional laboratory diffractometer. The ‘structural fragment’ used in the Monte Carlo calculation comprised the non-hydrogen atoms of the fluorenyl (C13) group, and this rigid structural fragment was subjected to simultaneous translation and rotation within the unit cell. The correct position of the structural fragment was discriminated readily (on the basis of the agreement between experimental and calculated X-ray powder diffractograms) from wrong positions sampled during the Monte Carlo calculation, and the correct position was then used as the initial structural model in Rietveld refinement and difference Fourier calculations. The work reported in this paper represents the first demonstration of the application of the Monte Carlo approach to solve a previously unknown crystal structure from powder diffraction data in the general case requiring simultaneous translation and rotation of a rigid structural fragment, and paves the way for the future application of the Monte Carlo approach to a much wider array of structural problems.

The crystal structures and melting point properties of isonicotinamide cocrystals with alkanediacids HO2C(CH2)n−2CO2Hn = 7–9

The tunability of physiochemical properties of crystalline materials through controlled cocrystal formulation is an aim for both the pharmaceutical industry and crystal engineers. In this paper we report that the melting point alternation behaviour of odd and even alkanedicarboxylic acids is mimicked in a set of cocrystals in which isonicotinamide is combined in a 1 : 1 stoichiometry with pimelic acid, suberic acid and azelaic acid. All three structures contain hydrogen bonded chains of alternating acid and amide molecules between which acid–pyridine and acid–amide synthons are formed. Both isonicotinamide : pimelic acid and isonicotinamide : azelaic acid form structures in which the acid moiety adopts a twisted alkyl backbone conformation similar to that observed in the pure odd alkanediacid materials. The isonicotinamide : suberic acid cocrystal differs from the others reported here retaining both the elevated melting point and the planar acid conformation displayed by even alkanediacid materials in this series.

Structures of three substituted arenesulfonamides from X-ray powder diffraction data using the differential evolution technique

The structures of three substituted arenesulfonamides have been solved from laboratory X-ray powder diffraction data, using a new direct-space structure solution method based on a differential evolution algorithm, and refined by the Rietveld method. In 2-toluenesulfonamide, C(7)H(9)NO(2)S (I) (tetragonal I4(1)/a, Z = 16), the molecules are linked by N-H...O=S hydrogen bonds into a three-dimensional framework. In 3-nitrobenzenesulfonamide, C(6)H(6)N(2)O(4)S (II) (monoclinic P2(1), Z = 2), N-H...O=S hydrogen bonds produce molecular ladders, which are linked into sheets by C-H...O=S hydrogen bonds: the nitro group does not participate in the hydrogen bonding. Molecules of 4-nitrobenzenesulfonamide, C(6)H(6)N(2)O(4)S (III) (monoclinic P2(1)/n, Z = 4), are linked into sheets by four types of hydrogen bond, N-H...O=S, N-H...O(nitro), C-H...O=S and C-H...O(nitro), and the sheets are weakly linked by aromatic pi...pi stacking interactions.

Differential evolution: crystal structure determination of a triclinic polymorph of adipamide from powder diffraction data

The crystal structure of a previously unknown triclinic polymorph of adipamide has been solved from laboratory X-ray powder diffraction data using a new direct space global optimisation method based on differential evolution.

Molecular versus crystal symmetry in tri-substituted triazine, benzene and isocyanurate derivatives

The crystal structures of triethyl-1,3,5-triazine-2,4,6-tricarboxylate (I), triethyl-1,3,5-benzenetricarboxylate (II) and tris-2-hydroxyethyl isocyanurate (III) have been determined from conventional laboratory X-ray powder diffraction data using the differential evolution structure solution technique. The determination of these structures presented an unexpectedly wide variation in levels of difficulty, with only the determination of (III) being without complication. In the case of (I) structure solution resulted in a Rietveld refinement profile that was not ideal, but was subsequently rationalized by single-crystal diffraction as resulting from disorder. Refinement of structure (II) showed significant variation in side-chain conformation from the initial powder structure solution. Further investigation showed that the structure solution optimization had indeed been successful, and that preferred orientation had a dramatic effect on the structure-solution R-factor search surface. Despite the presence of identical side chains in (I) and (II), only the triazine-based system retains threefold molecular symmetry in the crystal structure. The lack of use of the heterocyclic N atom as a hydrogen-bond acceptor in this structure results in the formation of a similar non-centrosymmetric network to the benzene-based structure, but with overall three-dimensional centrosymmetry. The hydrogen-bonded layer structure of (III) is similar to that of other isocyanurate-based structures of this type.