Biomedical subjects

Jeremy J Titman

Publications and source records attributed to Jeremy J Titman.

8 recordsLinked to original sources

PubMed2006

CAESURA: measurement of slow molecular dynamics by solid-state nuclear magnetic resonance chemical shift anisotropy modulation amplification.

An alternative magic angle spinning (MAS) exchange NMR experiment based on chemical shift anisotropy (CSA) amplification is described. The CSA amplification experiment correlates a standard MAS spectrum in the omega(2) dimension with a sideband pattern in omega(1) in which the intensities are identical to those expected for a sample spinning at some fraction 1N of the actual rate omega(r). In common with 2D-PASS, the isotropic shift appears only in the omega(2) dimension, and long acquisition times can be avoided without loss of resolution of different chemical sites. The new CSA amplification exchange experiment provides information about the time scale and geometry of molecular motions via their effect on the sideband intensities in a one-dimensional pattern. The one-dimensional patterns from different chemical sites are separated across two frequency dimensions according to the isotropic shifts.

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PubMed2005

Theoretical study of the 13C NMR spectroscopy of single-walled carbon nanotubes.

The 13C NMR spectroscopy of armchair and zigzag single-walled carbon nanotubes has been investigated theoretically. Spectra for (4,4), (5,5), (6,6), (6,0), (9,0), and (10,0) nanotubes have been simulated based on ab initio calculations of model systems. The calculations predict a dominant band arising from the carbon atoms in the "tube" with smaller peaks at higher chemical shifts arising from the carbon atoms of the caps. The dominant band lies in the range of 128 and 138 ppm. Its position depends weakly on the length, width, and chirality of the tubes. The calculations demonstrate how structural information may be gleaned from relatively low-resolution nanotube 13C NMR spectra.

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PubMed2005

Chemical shift anisotropy amplification with high amplification factor and improved sensitivity.

An improved version of the recently proposed chemical shift anisotropy amplification experiment is described. The original experiment correlates a fast magic angle spinning spectrum in the omega2 dimension with a sideband pattern in omega1 in which the intensities mimic those for a sample spinning at a fraction of the rate omegar/N. Advantages of the experiment include the use of standard methods to extract the principal tensor components from the omega1 sideband patterns and the small number of t1 increments required. The improved version described here permits large amplification factors N to be obtained without resort to prohibitively long sequences of pi-pulses and allows sensitivity to be maximized by eliminating the need to store the magnetization along the z-axis for t1. Amplification factors up to 32 are demonstrated experimentally.

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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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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

Chemical shift anisotropy amplification.

A new NMR experiment which allows a measurement of the chemical shift anisotropy (CSA) tensor under magic angle spinning (MAS) is described. This correlates a fast MAS spectrum in the omega2 dimension with a sideband pattern in omega1 in which the intensities mimic those for a sample spinning at a fraction of the rate omega r/N, and these sidebands result from an amplification by a factor N of the modulation caused by the CSA. Standard methods can be used to extract the principal tensor components from the omega1 sideband patterns, and the nature of the experiment is such that the use of a large number of t1 increments can be avoided without compromising the resolution of different chemical sites. The new experiment is useful for accurately measuring narrow shift anisotropies.

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PubMedDate not supplied

Measurement of orientation distributions using chemical shift amplification.

A new three-dimensional magic angle spinning (MAS) experiment is proposed, based on a combination of the two-dimensional rotor-synchronized MAS experiment of Spiess and co-workers and a new chemical shift anisotropy amplification method. The new experiment is demonstrated on a macroscopically ordered sample of ultra-high molecular weight poly(ethylene).

Anisotropy↗