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Kenneth D M Harris

Publications and source records attributed to Kenneth D M Harris.

At least 19 recordsLinked to original sources
PubMed2006

Structure-reactivity correlations for solid-state enantioselective photochemical reactions established directly from powder X-ray diffraction.

A prerequisite for the development of structure-reactivity correlations for photoreactive crystalline materials is to have detailed knowledge of the structural properties of the reactant crystalline phase. In some cases, however, the materials of interest can be prepared only as microcrystalline powders and are not amenable to structural characterization by single-crystal X-ray diffraction. This paper demonstrates the utility of modern powder X-ray diffraction techniques for obtaining structural understanding in such cases, leading to the development of structure-reactivity correlations. In particular, a series of three photoreactive organic salts are considered, which undergo the same photochemical asymmetric reaction but with high enantiomeric excess in two cases and low enantiomeric excess in the other case. The structural properties of the three salts determined from powder X-ray diffraction data are shown to provide a direct rationalization of these observations.

Journal Article↗
PubMed2006

Abundant polymorphism in a system with multiple hydrogen-bonding opportunities: oxalyl dihydrazide.

To date, only one crystal structure has been reported in the literature for oxalyl dihydrazide [H(2)N.NH.CO.CO.NH.NH(2)]. In the present paper, we report the discovery of four new polymorphs of oxalyl dihydrazide, obtained by crystallization from solution under different conditions, including the use of different crystallization solvents. All polymorphs have the trans-trans-trans conformation of the N-N-C-C-N-N backbone, but the positions of the hydrogen atoms of the NH(2) groups relative to this backbone differ between the different polymorphs through variation of the torsion angle around each NH-NH(2) bond. The different polymorphs display a range of different hydrogen-bonding arrangements, constructed from different types of hydrogen-bonded array. The existence of several different potential hydrogen-bond donor and hydrogen-bond acceptor groups in the oxalyl dihydrazide molecule, together with the fact that the N-H bonds of the NH(2) groups adopt different orientations with respect to the molecular plane, leads to several possible geometric permutations for hydrogen-bonding arrangements in the solid state. It would not be surprising if even more polymorphs of oxalyl dihydrazide are discovered in the future.

Journal Article↗
PubMed2006

Understanding the structural properties of a dendrimeric material directly from powder X-ray diffraction data.

Complete structure determination of an early-generation dendrimeric material 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. This work represents the first application of modern direct-space techniques for structure determination from powder X-ray diffraction data in the case of a dendrimeric material and paves the way for the future application of this approach to enable complete structure determination of other dendrimeric materials that cannot be prepared as single crystal samples suitable for single crystal X-ray diffraction studies.

Letter↗
PubMed2006

Significant conformational changes associated with molecular transport in a crystalline solid.

Confocal Raman microspectrometry has been applied as an in situ probe of the transport of guest molecules along the one-dimensional tunnels in a crystalline urea inclusion compound, under conditions of guest exchange in which "new" guest molecules (pentadecane) are introduced at one end of the tunnel and displace the "original" guest molecules (1,8-dibromooctane). The Raman spectra, recorded as a function of position along the tunnel direction and as a function of time, demonstrate that the transport process is associated with a significant change in the conformational properties of the original (1,8-dibromooctane) guest molecules. In particular, in the boundary region between the original and new guest molecules, there is a substantial increase in the proportion of 1,8-dibromooctane guest molecules that have the gauche end-group conformation. The wider implications of this observation are discussed in relation to fundamental aspects of the molecular transport process in this material.

Crystallization↗
PubMed2006

Highly efficient one-step conversion of cyclohexane to adipic acid using single-site heterogeneous catalysts.

A solid source of 'active' oxygen (acetylperoxyborate, APB), when dissolved in aqueous solution in the presence of a single-site microporous catalyst containing redox centres (Fe(III)AlPO-31, Mn(III)AlPO-5, Fe(III)AlPO-5), converts cyclohexane with high efficiency (ca. 88%) and exceptionally high selectivity (ca. 81%) to adipic acid at 383 K; this procedure is also effective in converting styrene to styrene oxide and -pinene and (+)-limonene to their corresponding epoxides.

Journal Article↗
PubMed2005

Structural and dynamic aspects of hydrogen-bonded complexes and inclusion compounds containing alpha,omega-dicyanoalkanes and urea, investigated by solid-state 13C and 2H NMR techniques.

Solid-state 13C NMR and 2H NMR techniques have been used to investigate structural and dynamic properties of the 1,4-dicyanobutane/urea and 1,5-dicyanopentane/urea 1:1 hydrogen-bonded complexes and the 1,6-dicyanohexane/urea inclusion compound. The pure crystalline phase of urea has also been investigated. The 13C NMR studies have focused on 13C chemical shift anisotropy and second-order quadrupolar effects (arising from 13C-14N interaction) for the urea molecules and the cyano groups of the alpha,omega-dicyanoalkanes. Parameters describing these interactions are derived and are discussed in relation to the known structural properties of these materials. Comparison of 13C chemical shift anisotropies of the cyano carbons and rates of 13C dipolar dephasing suggest that 1,4-dicyanobutane and 1,5-dicyanopentane are effectively static, whereas 1,6-dicyanohexane has greater mobility. 2H NMR line shape analysis for the 1,4-dicyanobutane/urea-d4 and 1,5-dicyanopentane/urea-d4 complexes indicates that the only motion of the urea molecules that is effective on the 2H NMR time scale is a rapid libration about the C=O bond over an angular range of about 26 degrees . For the 1,6-dicyanohexane/urea-d4 inclusion compound, the 2H NMR line shape is consistent with a motion comprising 180 degrees jumps about the C=O bond at rates that are intermediate on the 2H NMR time scale. In addition, rapid libration about the C=O bond also occurs over an angular range of about 20 degrees . The dynamic properties of the urea molecules in these materials are compared with those of urea molecules in other crystalline environments.

Journal Article↗
PubMed2005

Significantly contrasting solid state dynamics of the racemic and enantiomerically pure crystalline forms of an amino acid.

Solid state dynamic properties have been investigated for the racemic (DL) and enantiomerically pure (L) crystalline forms of the amino acid serine [HO x CH2 x CH(NH3(+)) x CO2(-)] using 2H NMR line shape analysis and 2H NMR spin-lattice relaxation time measurements for samples of DL-serine and L-serine deuterated in the NH3(+) and OH groups. 2H NMR line shape analysis indicates that, for both L-serine and DL-serine, the ND3(+) group undergoes a 3-site 120 degrees jump motion, with jump frequencies in the intermediate motion regime (10(3) s(-1) to 10(8) s(-1)) in the temperature range 153-313 K. However, at a given temperature, the jump frequency is substantially higher for L-serine (e.g., at 233 K, the jump frequency is 5.0 x 10(6) s(-1) for L-serine and 6.0 x 10(4) s(-1) for DL-serine). The OD group is not dynamic on the 2H NMR time scale within the temperature range studied. The results from both 2H NMR line shape analysis (LA) and 2H NMR spin-lattice relaxation time measurements (SLR) indicate that the activation energy for the 3-site 120 degree jump motion of the ND3(+) group is significantly higher for DL-serine [38.0 +/- 1.0 kJ mol(-1) (LA); 39.7 +/- 0.8 kJ mol(-1) (SLR)] than for L-serine [23.4 +/- 0.8 kJ mol(-1) (LA); 23.8 +/- 0.3 kJ mol(-1) (SLR)]. The difference in activation energies between DL-serine and L-serine is substantially greater than any reported previously for an amino acid in different crystal forms.

Amino Acids↗
PubMed2005

Altering the polymorphic product distribution in a solid-state dehydration process by rapid sample rotation in a solid-state NMR probe.

There is currently considerable interest in the phenomenon of polymorphism in organic molecular solids, and a key issue in this field is to understand the experimental techniques and procedures that may be employed to obtain new polymorphic forms of a given molecule. This paper demonstrates that the polymorphic form of a material (sodium acetate) obtained by a solid-state dehydration process (starting from sodium acetate trihydrate) can be altered by carrying out the dehydration process under conditions of rapid (several kilohertz) sample rotation in a solid-state magic-angle-spinning NMR probe. This observation suggests a new opportunity to influence the outcome of solid-state dehydration/desolvation processes and, in particular, to alter the polymorphic form of the product obtained.

Journal Article↗
PubMed2005

Structural understanding of a molecular material that is accessed only by a solid-state desolvation process: the scope of modern powder X-ray diffraction techniques.

Many molecular materials cannot be prepared as a "pure" (nonsolvate) crystalline phase by conventional crystal growth from solution due to the facile formation of solvate structures. In such cases, it may be possible to obtain the pure phase by a solid-state desolvation process, although such processes are generally associated with loss of crystal integrity, yielding a microcrystalline powder of the pure phase. This paper demonstrates the utility of modern powder X-ray diffraction techniques for obtaining structural understanding in such cases, focusing on a particular member of a structural family that is of wider relevance within the context of crystal engineering and design.

Journal Article↗
PubMed2005

Structural rationalisation of co-crystals formed between trithiocyanuric acid and molecules containing hydrogen bonding functionality.

Crystallisation of trithiocyanuric acid (TTCA) from various organic solvents that have hydrogen bonding capability (acetone, 2-butanone, dimethylformamide, dimethyl sulfoxide, methanol and acetonitrile) leads to the formation of co-crystals in which the solvent molecules are incorporated together with TTCA in the crystal structure. Structure determination by single-crystal X-ray diffraction reveals that these co-crystals can be classified into different groups depending upon the topological arrangement of the TTCA molecules in the crystal structure. Thus, three different types of single-tape arrangements of TTCA molecules and one type of double-tape arrangement of TTCA molecules are identified. In all co-crystals, hydrogen-bonding interactions are formed through the involvement of N-H bonds of TTCA molecules in these tapes and the other molecule in the co-crystal. Detailed rationalisation of the structural properties of these co-crystals is presented.

Journal Article↗
PubMed2004

How to determine structures when single crystals cannot be grown: opportunities for structure determination of molecular materials using powder diffraction data.

Many crystalline solids cannot be prepared as single crystals of sufficient size and/or quality for structure determination to be carried out using single crystal X-ray diffraction techniques. In such cases, when only polycrystalline powders of a material are available, it is necessary instead to tackle structure determination using powder X-ray diffraction. This article highlights recent developments in the opportunities for determining crystal structures directly from powder diffraction data, focusing on the case of molecular solids and giving particular attention to the most challenging stage of the structure determination process, namely the structure solution stage. In particular, the direct-space strategy for structure solution is highlighted, as this approach has opened up new opportunities for the structure determination of molecular solids. The article gives an overview of the current state-of-the-art in structure determination of molecular solids from powder diffraction data. Relevant fundamental aspects of the techniques in this field are described, and examples are given to highlight the application of these techniques to determine crystal structures of molecular materials.

Journal Article↗
PubMed2004

Direct time-resolved and spatially resolved monitoring of molecular transport in a crystalline nanochannel system.

Confocal Raman microspectrometry has been applied successfully as an in situ probe of the transport of guest molecules through the one-dimensional channel system in a crystalline inclusion compound, yielding insights into the spatial distribution of guest molecules and, in particular, the variation in the spatial distribution of the guest molecules as a function of time during the transport process.

Journal Article↗
PubMed2004

Rationalizing the structural properties of bupivacaine base--a local anesthetic--directly from powder X-ray diffraction data.

Bupivacaine belongs to a family of 1-alkyl-2',6'-pipecoloxylidides, which has shown promise as reversible action potential blockers that can introduce prolonged local anesthetic effects. The crystal structure of the free-base form of bupivacaine has been determined directly from powder X-ray diffraction data using the Genetic Algorithm technique for structure solution, followed by Rietveld refinement. This work further emphasizes the scope and utility of ab initio structure solution directly from powder X-ray diffraction data for tackling structural problems within the biomedical field, leading to opportunities for the investigation of structure-property relationships.

Anesthetics, Local↗
PubMed2003

Direct structure determination of a multicomponent molecular crystal prepared by a solid-state grinding procedure.

A three-component molecular cocrystal material has been prepared by a solvent-free route involving mechanical grinding of the pure phases of the individual components. This material is not accessible from conventional solution-state crystallization procedures. Due to the fact that the grinding procedure intrinsically leads to a microcrystalline powder sample, the use of powder X-ray diffraction data is essential for structure determination. This work emphasizes the scope and utility of ab initio structure solution directly from powder X-ray diffraction data for carrying out structural characterization of new materials prepared via the solid-state grinding route, leading to the opportunity to establish structure-property relationships for such materials.

Journal Article↗
PubMed2003

Crystallization and preliminary X-ray diffraction data of Mycobacterium tuberculosis FbpC1 (Rv3803c).

The heterotrimeric antigen 85 complex (Ag85) is a major component of the cell wall of Mycobacterium tuberculosis and consists of three abundantly secreted proteins (FbpA, FbpB and FbpC2). These play key roles in the pathogenesis of tuberculosis and in maintaining cell-wall integrity. A homologue of the Ag85 subunits ( approximately 40% identity) was recently annotated in the M. tuberculosis genome as FbpC1. Unlike the Ag85-complex components, FbpC1 lacks mycolyltransferase activity and its function remains to be established. In order to aid functional characterization, FbpC1 has been crystallized. At room temperature, tetragonal crystals of FbpC1 were obtained belonging to space group P4(1)2(1)2 (unit-cell parameters a = b = 109.9, c = 61.8 A), yet when frozen the crystals underwent a phase transition to orthorhombic symmetry, space group P2(1)2(1)2(1) (a = 59.9, b = 108.9, c = 109.9 A). Diffraction data complete to 1.7 A resolution were recorded at 100 K at the synchrotron.

Antigens, Bacterial↗