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Duncan H Gregory

Publications and source records attributed to Duncan H Gregory.

10 recordsLinked to original sources
PubMed2005

Through-space contributions to two-dimensional double-quantum J correlation NMR spectra of magic-angle-spinning solids.

A routinely used assumption when interpreting two-dimensional NMR spectra obtained with a commonly used double-quantum (DQ) magic-angle-spining (MAS) pulse sequence referred to as the refocused incredible natural abundance double-quantum transfer experiment (INADEQUATE) [A. Lesage, M. Bardet, and L. Emsley, J. Am. Chem. Soc. 121, 10987 (1999)] has been that correlation peaks are only observed for pairs of nuclei with a through-bond connectivity. The validity of this assumption is addressed here by theory, experiment, and computer simulations. If the isotropic chemical shifts of the two nuclei are different and the MAS frequency is far from rotational resonance, the theoretical description demonstrates that DQ correlation peaks are indeed indicative of a J coupling. However, if the isotropic chemical shifts are the same, it is shown that DQ peaks can appear for pairs of nuclei even in the absence of a through-bond J coupling. These peaks appear in the specific case of a pair of nuclei with a nonzero through-space dipole-dipole coupling and chemical shift anisotropy tensors having different principal magnitudes or orientations, provided that the MAS frequency is comparable to or smaller than the chemical shift anisotropies. Experimental 31P spectra recorded on a sample of TiP2O7 and computer simulations show that the magnitude of these anomalous peaks increases with increasing B0 magnetic field and that they decrease with increasing MAS frequency. This behavior is explained theoretically.

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PubMed2005

Flexible ligands and structural diversity: isomerism in Cd(NO3)2 coordination polymers.

The ligands 1,4-bis(2-pyridylmethylsulfanylmethyl)benzene (L1) and 2,5-bis(2-pyridylmethylsulfanylmethyl)pyrazine (L2) were treated with Cd(NO3)2.4H2O in metal-to-ligand ratios of 1:1 and 2:1, respectively; L2 was also treated with CdCl2.2.5H2O in a 2:1 ratio. All products were found to be coordination polymers. The crystal structures of {[Cd(L1)(NO3)2].CH2Cl2}infinity (1a), {[Cd(L1)(NO3)2].4/3CH3CN}infinity (1b), {[Cd2(L2)(NO3)4].2CH3CN}infinity (2.2CH3CN), and {[Cd2(L2)Cl4].2CH2Cl2}infinity (3.2CH2Cl2) were determined. Compounds 1a and 1b were found to be conformational supramolecular isomers. The structure of 1b displayed topological isomerism with two isomeric polymer chains, 1b(1) and 1b(2), in the one crystal forming a single supramolecular array. The structure of 2.2CH(3)CN showed Cd2(L2) units linked together by nitrates bridging between the Cd(II) centers in a mode previously not seen in Cd(II) compounds. The overall structure of 3.2CH2Cl2 was found to be similar to that of 2.2CH3CN despite the presence of different anions and solvent molecules. Powder X-ray diffraction was used to investigate the nature of bulk preparations of compounds 1-3.

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PubMed2004

Evolution of structure, transport properties and magnetism in ternary lithium nitridometalates Li(3-x-y)M(x)N, M = Co, Ni, Cu.

The structures, magnetism and ion transport properties of the ternary nitrides Li(3-x-y)M(x)N (M = Co, Ni, Cu; y= lithium vacancy) were examined by powder X-ray diffraction, solid-state NMR and SQUID magnetometry. Doping levels are achieved up to x approximately = 0.4 for M = Cu and Co, but much higher substitution levels (x approximately =1) are obtained in the Li-Ni-N system. Transition metals substitute for Li at the Li(1) interplanar site and the ensuing lithium vacancies are disordered within the [Li(2)N] planes. High substitution levels in the Li-Ni-N system lead to the formation of ordered phases. Diffusion parameters, including activation energies, correlation times and diffusion coefficients, were obtained from variable-temperature solid-state NMR measurements in several ternary compounds. SQUID magnetometry shows significant variations of the electronic properties with dopant and x. The properties of the ternary nitrides can be rationalised in terms of the identity of the dopant and the structural modifications arising from the substitution process.

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PubMed2004

Fast lithium ion diffusion in the ternary layered nitridometalate LiNiN.

The structure, Li+ diffusion dynamics, and magnetic properties of the layered nitridonickelate(II), LiNiN, have been investigated by powder X-ray diffraction, 7Li solid-state NMR, and SQUID magnetometry and compared and contrasted with those of the Li+ fast ion conductor, Li3N. The replacement of Li+ by Ni2+ with concomitant generation of Li+ vacancies has profound effects on ionic diffusion and electronic properties. The nitridonickelate, akin to its binary parent, displays rapid Li+ ion diffusion but, by contrast, the diffusion process is confined only to the Li-N planes. Further, replacement of Li by Ni leads to a transition from semiconducting to metallic behavior, likely mediated through the creation of infinite, 1D Ni-N chains of increased covalency.

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PubMed2004

New mixed alkaline-earth nitridomolybdate(VI) nitride oxides with anion-ordered sub-structures.

New molybdenum(VI) nitride oxides were synthesised by the reaction of strontium nitride and calcium nitride with molybdenum foil at high temperature in sealed stainless steel crucibles. The reactions yielded single crystalline products determined by X-ray diffraction to form complex structures in the triclinic space group P1(no. 2). The mixed alkaline earth compounds with composition Ca38Sr13[MoN4]12N8O3 and Ca36Sr15[MoN4]12N8O3 are isostructural with the quaternary nitride oxides Sr51[WN4]12N8O3 and Ca51[WN4]12N8O3. The structures contain isolated [MoN4](6-) tetrahedra, partially disordered alkaline earth cations and an ordered sublattice of N(3-) and O(2-) anions. Oxide anions are coordinated only to the alkaline earth metals. The title compounds are the first mixed alkaline earth metal nitride oxides.

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PubMed2003

Magnesium diiodide, MgI2.

Single crystals of magnesium diiodide have been grown and the structure solved for the first time from single-crystal X-ray diffraction data. This study confirms that MgI(2) is isostructural with CdI(2) (C6 or 2H structure type). The space group is P-3m1 with the Mg atom on a site with 3m symmetry (Wyckoff site 1a) and the I atom on a site with m symmetry (Wyckoff site 2d). Trends in the 2H structures of dihalides are discussed briefly.

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PubMed2002

Lithium niobium disulfide, Li0.63NbS2.

Lithium niobium disulfide, Li(0.63)NbS(2), has been prepared by a metathesis reaction between physically separated solid reactants, i.e. separated reactant metathesis (SRM). Single-crystal data were collected at reduced temperature [150 (2) K], yielding a refined Li content of y = 0.63 (6). The Li content in the crystalline samples was also determined analytically by flame photometry. The compound crystallizes in hexagonal space group P6(3)/mmc (No. 194), with Li(+) ions situated in octahedral sites between NbS(2) layers.

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