Friday, August 17, 2007

Hiyam Journal Update

J. Am. Chem. Soc., 129 (33), 10195 -10200, 2007.

Differential Line Broadening in MAS Solid-State NMR due to Dynamic Interference
Veniamin Chevelkov, Katja Faelber, Anna Schrey, Kristina Rehbein, Anne Diehl, and Bernd Reif

Many MAS (magic angle spinning) solid-state NMR investigations of biologically relevant protein samples are hampered by poor resolution, particularly in the 15N chemical shift dimension. We show that dynamics in the nanosecond-microsecond time scale in solid-state samples can induce significant line broadening of 15N resonances in solid-state NMR experiments. Averaging of 15NH/ multiplet components due to 1H decoupling induces effective relaxation of the 15N coherence in case the N-H spin pair undergoes significant motion. High resolution solid-state NMR spectra can then only be recorded by application of TROSY (Transverse Relaxation Optimized Spectroscopy) type techniques which select the narrow component of the multiplet pattern. We speculate that this effect has been the major obstacle to the NMR spectroscopic characterization of many membrane proteins and fibrillar aggregates so far. Only in very favorable cases, where dynamics are either absent or very fast (picosecond), high-resolution spectra were obtained. We expect that this approach which requires intense deuteration will have a significant impact on the quality and the rate at which solid-state NMR spectroscopic investigations will emerge in the future.

Chem. Mater., 19 (17), 4166 -4173, 2007. 10.

DFT Modeling of NMR Contact Shift Mechanism in the Ideal LiNi2O4 Spinel and Application to Thermally Treated Layered Li0.5NiO2
Cédric Chazel, Michel Ménétrier,* Dany Carlier, Laurence Croguennec, and Claude Delmas

LiNi2O4 spinel-type phases were prepared by thermal treatment of electrochemically deintercalated layered Li~0.5NiO2. The phase transformation was followed by 7Li NMR, showing a gradual change of the signal from the layered compound. The characteristic signal of the latter (related to local Li/vacancy and Ni3+/Ni4+ ordering) vanishes after heating to 150 C and is replaced by a new signal showing faster exchange kinetics (originating from Ni3+/Ni4+ hopping around Li), which progressively transforms into a broad distribution of signals. Around 200 C, a set of three positively shifted signals is observed, corresponding to the appearance of the spinel phase as seen from XRD; these signals disappear after heating to 240 C, corresponding to the beginning of decomposition of the spinel into a disordered Rm type phase with oxygen evolution as previously shown by Guilmard et al. (Chem. Mater. 2003, 15, 4476 and 4484). In an ideal LiNi2O4 spinel, only one 7Li NMR signal is expected. DFT (GGA) calculations were carried out and show that the mechanism for the electron spin density transfer from NiO6 octahedra to corner-sharing LiO4 tetrahedra with close to 120 Ni-O-Li configuration is a delocalization one, although the p orbitals on oxygen do not present ideal orientation, leading to a much weaker transfer compared to cases where both Ni and Li are in octahedral coordination with 180 Ni-O-Li configuration. The complex but well-defined experimental NMR signals consistently observed show that the material is far from the ideal spinel structure. However, it could not be correlated to the actual stoichiometry of the compound. It was therefore tentatively assigned to structural defects resulting from incomplete migration of Ni ions from their site to the Li layer in the pristine compound, such as partial occupation of tetrahedral sites.

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