The Experts below are selected from a list of 17805 Experts worldwide ranked by ideXlab platform

Malcolm H Levitt - One of the best experts on this subject based on the ideXlab platform.

  • long lived nuclear spin states in methyl groups and quantum rotor induced polarization
    Journal of the American Chemical Society, 2013
    Co-Authors: Benno Meier, Jeannicolas Dumez, Gabriele Stevanato, Joseph T Hillcousins, Soumya Singha Roy, Par Hakansson, Salvatore Mamone, Richard C D Brown, Giuseppe Pileio, Malcolm H Levitt
    Abstract:

    Substances containing rapidly rotating methyl groups may exhibit long-lived states (LLSs) in solution, with Relaxation Times substantially longer than the conventional spin-Lattice Relaxation Time T1. The states become long-lived through rapid internal rotation of the CH3 group, which imposes an approximate symmetry on the fluctuating nuclear spin interactions. In the case of very low CH3 rotational barriers, a hyperpolarized LLS is populated by thermal equilibration at liquid helium temperature. Following dissolution, cross-Relaxation of the hyperpolarized LLS, induced by heteronuclear dipolar couplings, generates strongly enhanced antiphase NMR signals. This mechanism explains the NMR signal enhancements observed for 13C-γ-picoline (Icker, M.; Berger, S. J. Magn. Reson. 2012, 219, 1–3).

  • an nmr thermometer for cryogenic magic angle spinning nmr the spin Lattice Relaxation of 127i in cesium iodide
    Journal of Magnetic Resonance, 2011
    Co-Authors: Riddhiman Sarkar, Maria Concistre, Ole G Johannessen, Peter Beckett, Mark Denning, Marina Carravetta, M K Almosawi, C Beduz, Y Yang, Malcolm H Levitt
    Abstract:

    The accurate temperature measurement of solid samples under magic-angle spinning (MAS) is difficult in the cryogenic regime. It has been demonstrated by Thurber et al. (J. Magn. Reson., 196 (2009) 84-87) [10] that the temperature dependent spin-Lattice Relaxation Time constant of 79Br in KBr powder can be useful for measuring sample temperature under MAS over a wide temperature range (20–296 K). However the value of T1 exceeds 3 min at temperatures below 20 K, which is inconveniently long. In this communication, we show that the spin-Lattice Relaxation Time constant of 127I in CsI powder can be used to accurately measure sample temperature under MAS within a reasonable experimental Time down to 10 K.

Balazs Dora - One of the best experts on this subject based on the ideXlab platform.

  • nuclear spin Lattice Relaxation Time in tap and the knight shift of weyl semimetals
    Physical Review B, 2019
    Co-Authors: Zoltan Okvatovity, H Yasuoka, M Baenitz, F Simon, Balazs Dora
    Abstract:

    We first analyze the recent experimental data on the nuclear spin-Lattice Relaxation rate of the Weyl semimetal TaP. We argue that its nonmonotonic temperature dependence is explained by the temperature-dependent chemical potential of Weyl fermions. We also develop the theory of the Knight shift in Weyl semimetals, which contains two counteracting terms. The diamagnetic term follows $\ensuremath{-}ln[W/max(|\ensuremath{\mu}|,{k}_{B}T)]$ with $W,\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}$, and $T$ being the high-energy cutoff, chemical potential, and temperature, respectively, and is always negative. The paramagnetic term scales with $\ensuremath{\mu}$ and changes sign depending on the doping level. Altogether, the Knight shift is predicted to vanish or even change sign upon changing the doping or the temperature, making it a sensitive tool to identify Weyl points. We also calculate the Korringa relation for Weyl semimetals which shows an unusual energy dependence rather than being constant as expected for a noninteracting Fermi system.

  • unusual hyperfine interaction of dirac electrons and nmr spectroscopy in graphene
    Physical Review Letters, 2009
    Co-Authors: Balazs Dora, F Simon
    Abstract:

    A theory of nuclear magnetic resonance (NMR) in graphene is presented. The canonical form of the electron-nucleus hyperfine interaction is strongly modified by the linear electronic dispersion. The NMR shift and spin-Lattice Relaxation Time are calculated as a function of temperature, chemical potential, and magnetic field, and three distinct regimes are identified: Fermi-, Dirac-gas, and extreme quantum limit behaviors. A critical spectrometer assessment shows that NMR is within reach for fully 13 C enriched graphene of reasonable size.

Manasi Ghosh - One of the best experts on this subject based on the ideXlab platform.

  • understanding the correlation between structure and dynamics of clocortolone pivalate by solid state nmr measurement
    RSC Advances, 2020
    Co-Authors: Shovanlal Gayen, Manasi Ghosh
    Abstract:

    Structural characteristics of clocortolone pivalate are unique in the topical corticosteroid field having high penetration power through the stratum corneum of skin as well as low corticosteroid-related adverse effects. The molecule was thoroughly studied by 13C 2DPASS CP MAS NMR and spin–Lattice Relaxation Time measurements. Molecular correlation Time at different carbon nuclei positions was calculated by assuming that the chemical shift anisotropy interaction and heteronuclear dipole–dipole interaction play vital roles in the 13C spin–Lattice Relaxation mechanism. The CSA parameters are substantially varied at different carbon nuclei sites. This suggests that the electronic distribution surrounding the carbon nuclei varies widely when the same carbon atom is placed in different chemical surroundings of the molecule. The spinning CSA sideband pattern for C11, C17, C26 nuclei is axially symmetric. The asymmetry parameter is very small (≤0.3) for C2, C5, C10, C22, C23, C24 nuclei, and it is reasonably high (≥0.9) for C3, C4, C6, C18, C19, C21 nuclei. The anisotropy parameter is very high for double bonded C14, C15, C18, C19, C21, C22, and C23 nuclei. Spin–Lattice Relaxation Time and molecular correlation Time are also varied substantially for carbon nuclei situated at various positions of the molecule. The spin–Lattice Relaxation Time is slow for carbon nuclei residing at the carbon ring, and it is very fast for C12, C17, C16, C26 carbon nuclei situated at the side portion of the molecule. Molecular correlation Time is of the order of 10−4 s for those carbon nuclei attached with neighbouring carbon or oxygen atoms by double bonds like C14, C15, C18, C19, C21, C22, and C23. It implies that the molecular correlation Time is very high for those carbon nuclei associated with high values of the chemical shift anisotropy parameter. In contrast, the molecular correlation Time is of the order of 10−8 s for C12, C16, and C17 carbon nuclei. From these studies, it is clear that the various portions of the molecule exhibit different degrees of motion and the dynamics is related with the structural characteristics of the molecule. These investigations on important drug clocortolone pivalate by solid state NMR will help researchers to understand the structure and dynamics of the molecule, which will give a direction to develop advance corticosteroids.

  • understanding the effect of deacetylation on chitin by measuring chemical shift anisotropy tensor and spin Lattice Relaxation Time
    Chemical Physics Letters, 2020
    Co-Authors: Manasi Ghosh
    Abstract:

    Abstract The structure and dynamics of the functional biopolymer chitosan were investigated by measuring spin-Lattice Relaxation Time, chemical shift anisotropy parameters, and compare those results with that of the chitin. The Relaxation Time and the correlation Time of the carbon nuclei reside at the polysaccharidic backbone of chitosan are decreased due to depolymerization of polysaccharide chain. The effect of the substitution of different functional group at the C2 position of the monomeric sugar unit is portrayed by this type of comparative and comprehensive, which will enlighten the way of finding biodegradable and biocompatible polymers with enormous application in biomedicine.

F Simon - One of the best experts on this subject based on the ideXlab platform.

  • nuclear spin Lattice Relaxation Time in tap and the knight shift of weyl semimetals
    Physical Review B, 2019
    Co-Authors: Zoltan Okvatovity, H Yasuoka, M Baenitz, F Simon, Balazs Dora
    Abstract:

    We first analyze the recent experimental data on the nuclear spin-Lattice Relaxation rate of the Weyl semimetal TaP. We argue that its nonmonotonic temperature dependence is explained by the temperature-dependent chemical potential of Weyl fermions. We also develop the theory of the Knight shift in Weyl semimetals, which contains two counteracting terms. The diamagnetic term follows $\ensuremath{-}ln[W/max(|\ensuremath{\mu}|,{k}_{B}T)]$ with $W,\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}$, and $T$ being the high-energy cutoff, chemical potential, and temperature, respectively, and is always negative. The paramagnetic term scales with $\ensuremath{\mu}$ and changes sign depending on the doping level. Altogether, the Knight shift is predicted to vanish or even change sign upon changing the doping or the temperature, making it a sensitive tool to identify Weyl points. We also calculate the Korringa relation for Weyl semimetals which shows an unusual energy dependence rather than being constant as expected for a noninteracting Fermi system.

  • unusual hyperfine interaction of dirac electrons and nmr spectroscopy in graphene
    Physical Review Letters, 2009
    Co-Authors: Balazs Dora, F Simon
    Abstract:

    A theory of nuclear magnetic resonance (NMR) in graphene is presented. The canonical form of the electron-nucleus hyperfine interaction is strongly modified by the linear electronic dispersion. The NMR shift and spin-Lattice Relaxation Time are calculated as a function of temperature, chemical potential, and magnetic field, and three distinct regimes are identified: Fermi-, Dirac-gas, and extreme quantum limit behaviors. A critical spectrometer assessment shows that NMR is within reach for fully 13 C enriched graphene of reasonable size.

Tokuko Watanabe - One of the best experts on this subject based on the ideXlab platform.

  • chemical shift chemical shift anisotropy and spin Lattice Relaxation Time in 89y mas and static nmr of yttrium compounds
    Bulletin of the Chemical Society of Japan, 1997
    Co-Authors: Toshie Harazono, Tokuko Watanabe
    Abstract:

    The chemical shift, the range of chemical shift anisotropy, and the spin-Lattice Relaxation Times (T1) in Y2O3, Y3Al5O12, and Y2O2S have been measured by solid state 89Y (nuclear spin 1/2)-MAS and -static NMR. The static NMR data and T1 are reported for the first Time. The range of chemical shift anisotropy was 1500—2400 Hz. This range was influenced more by the nature of the atom bound to Y than by the coordination number or the crystallographic symmetry of Y. Very long T1 values were obtained for Y2O2S (6.61 h) and Y2O3 (3.92 h at 24d site and 3.81 h at 8b site). On the other hand, the T1 value (1.10 h) of Y3Al5O12 is much shorter compared with those of Y2O2S and Y2O3. The next-nearest neighboring atom of Y in Y3Al5O12 is identified as Al, which has the nuclear spin 5/2 of 100% natural abundance. The most likely origin of significantly shorter T1 of Y3Al5O12 is a dipole–dipole interaction between 89Y and 27Al.