Kenneth Shankland

DASH : a program for crystal structure determination from powder diffraction data

DASH is a user-friendly graphical-user-interface-driven computer program for solving crystal structures from X-ray powder diffraction data, optimized for molecular structures. Algorithms for multiple peak fitting, unit-cell indexing and space-group determination are included as part of the program. Molecular models can be read in a number of formats and automatically converted to Z-matrices in which flexible torsion angles are automatically identified. Simulated annealing is used to search for the global minimum in the space that describes the agreement between observed and calculated structure factors. The simulated annealing process is very fast, which in part is due to the use of correlated integrated intensities rather than the full powder pattern. Automatic minimization of the structures obtained by simulated annealing and automatic overlay of solutions assist in assessing the reproducibility of the best solution, and therefore in determining the likelihood that the global minimum has been obtained.

A probabilistic approach to space-group determination from powder diffraction data

An algorithm for the determination of the space-group symmetry of a crystal from powder diffraction data, based upon probability theory, is described. Specifically, the relative probabilities of different extinction symbols are assessed within a particular crystal system. In general, only a small number of extinction symbols are relatively highly probable and a single extinction symbol is often significantly more probable than any other. Several examples are presented to illustrate this approach.

Routine determination of molecular crystal structures from powder diffraction data

The state of the art in determining molecular crystal structures from powder diffraction data using a global optimisation method is illustrated with a fast, automated simulated annealing approach to solving the previously unknown crystal structures of capsaicin, thiothixene and promazine hydrochloride.

Temperature- and Pressure-induced Proton Transfer in the 1:1 Adduct Formed between Squaric Acid and 4,4 '-Bipyridine

We have applied a combination of spectroscopic and diffraction methods to study the adduct formed between squaric acid and bypridine, which has been postulated to exhibit proton transfer associated with a single-crystal to single-crystal phase transition at ca. 450 K. A combination of X-ray single-crystal and very-high flux powder neutron diffraction data confirmed that a proton does transfer from the acid to the base in the high-temperature form. Powder X-ray diffraction measurements demonstrated that the transition was reversible but that a significant kinetic energy barrier must be overcome to revert to the original structure. Computational modeling is consistent with these results. Modeling also revealed that, while the proton transfer event would be strongly discouraged in the gas phase, it occurs in the solid state due to the increase in charge state of the molecular ions and their arrangement inside the lattice. The color change is attributed to a narrowing of the squaric acid to bipyridine charge-transfer energy gap. Finally, evidence for the possible existence of two further phases at high pressure is also presented.

Solving molecular crystal structures from laboratory X-ray powder diffraction data with DASH: the state of the art and challenges

The crystal structures of 35 molecular compounds have been redetermined from laboratory monochromatic capillary transmission X-ray powder diffraction data using the simulated-annealing approach embodied within the DASH structure solution package. The compounds represent industrially relevant areas (pharmaceuticals; metal coordination compounds; nonlinear optical materials; dyes) in which the research groups in this multi-centre study are active. The molecules were specifically selected to form a series within which the degree of structural complexity (i.e. degrees of freedom in the global optimization) increased systematically, the degrees of freedom increasing with increasing number of optimizable torsion angles in the structural model and with the inclusion of positional disorder or multiple fragments (counterions; crystallization solvent; Z′ > 1). At the lower end of the complexity scale, the structure was solved with excellent reproducibility and high accuracy. At the opposite end of the scale, the more complex search space offered a significant challenge to the global optimization procedure and it was demonstrated that the inclusion of modal torsional constraints, derived from the Cambridge Structural Database, offered significant benefits in terms of increasing the frequency of successful structure solution by restricting the magnitude of the search space in the global optimization.

Structure solution of Ibuprofen from powder diffraction data by the application of a genetic algorithm combined with prior conformational analysis

The crystal structure of Ibuprofen has been solved from synchrotron X-ray powder diffraction data using a genetic algorithm based model building method. The performance of the algorithm is enhanced if additional prior chemical information is incorporated in the form of hard limits on the values that can be assumed by flexible torsion angles within the molecule.

Routine ab initio structure determination of chlorothiazideby X-ray powder diffraction using optimised data collection and analysisstrategies

The likelihood of solving crystal structures from powder diffraction data is greatly enhanced if data collection and analysis strategies can be designed to effectively remove Bragg peak overlap. In this way, accurate normalised structure factors of essentially single-crystal quality are obtained. Such strategies are illustrated here with theab initio solution, from powder diffraction data using traditional direct methods, of the clinically used diuretic compound chlorothiazide. The structure solution is outstanding in that, despite the non-centrosymmetric, triclinic symmetry, all 17 non-hydrogen atom positions are clearly visible in the E-map generated from the top direct methods solution.

Mechanistic Insights into a Gas–Solid Reaction in Molecular Crystals: The Role of Hydrogen Bonding

Hydrogen bonding leads the way: Reaction of nonporous crystalline coordination compound 1 with HCl gas results in conversion into the crystalline salt 2 (see picture) following chemisorption and insertion of HCl into CuN bonds. Powder diffraction studies show that [CuBr2Cl2]2− ions formed in 2 are reoriented to maximize the strength of hydrogen bonds and halogen bonds by preferentially involving Cl rather than Br ligands as acceptor sites.

Organocatalytic enantioselective construction of nitrocyclohexanes containing multiple chiral centres via a cascade reaction

The stereoselective construction of complex molecules with multiple stereogenicity in a single step represents an extremely useful, but challenging approach to complexity in chemical synthesis. The development of organocatalytic cascade processes has proven useful in these studies, but reports where four or more stereocentres are created in a single step from just two achiral reagents are rare. Herein we report the development of a novel asymmetric domino Michael-Michael reaction between nitrohex-4-enoates and nitro-olefins to generate cyclohexanes of high complexity, including one with a quaternary centre, and three with five contiguous stereocentres. This methodology provides access to a range of useful nitrocyclohexane derivatives, including a novel class of α-lycorane-like structures.

Control and prediction of packing motifs: a rare occurrence of carbamazepine in a catemeric configuration

A predicted orthorhombic crystal structure of carbamazepine is isostructural with an experimentally determined dihydrocarbamazepine crystal structure, providing a rationale for synthesising a novel 1 : 1 solid solution that has carbamazepine in a rare catemeric configuration.