Thursday, February 28, 2008

Joel's Journal Update

Spin-counting NMR experiments for the spectral editing of structural motifs in solids
Michaël Deschamps, Franck Fayon, Julien Hiet, Geoffroy Ferru, Marc Derieppe, Nadia Pellerin and Dominique Massiot
Scalar couplings, recoupled or full dipolar interactions can be used to characterize multinuclear structural molecular motifs in solids, by counting the neighbouring spins in solid-state NMR, opening new ways for the differentiation of overlapping spectral responses which is a limiting factor in many high resolution experiments carried out on disordered systems.

Experimental and Computational Characterization of the 17O Quadrupole Coupling and Magnetic Shielding Tensors or p-Nitrobenzaldehyde and Formaldehyde.
Gang Wu, Peter Mason, Xin Mo, and Victor Terskikh
We have used solid-state 17O NMR experiments to determine the 17O quadrupole coupling (QC) tensor and chemical shift (CS) tensor for the carbonyl oxygen in p-nitro-[1-17O]benzaldehyde. Analyses of solid-state 17O NMR spectra obtained at 11.75 and 21.15 T under both magic-angle spinning (MAS) and stationary conditions yield the magnitude and relative orientation of these two tensors: CQ = 10.7 ± 0.2 MHz, Q = 0.45 ± 0.10, 11 = 1050 ± 10, 22 = 620 ± 10, 33 = -35 ± 10, = 90 ± 10, = 90 ± 2, = 90 ± 10. The principal component of the 17O CS tensor with the most shielding, 33, is perpendicular to the H-C=O plane, and the tensor component with the least shielding, 11, lies along the C=O bond. For the 17O QC tensor, the largest (zz) and smallest (xx) components are both in the H-C=O plane being perpendicular and parallel to the C=O bond, respectively. This study represents the first time that these two fundamental 17O NMR tensors have been simultaneously determined for the carbonyl oxygen of an aldehyde functional group by solid-state 17O NMR. The reported experimental solid-state 17O NMR results provide the first set of reliable data to allow evaluation of the effect of electron correlation on individual CS tensor components. We found that the electron correlation effect exhibits significant influence on 17O chemical shielding in directions within the H-C=O plane. We have also carefully re-examined the existing experimental data on the 17O spin-rotation tensor for formaldehyde and proposed a new set of best "experimental" 17O chemical shielding tensor components: 11 = -1139 ± 80, 22 = -533 ± 80, 33 = 431 ± 5, and iso = -414 ± 60 ppm. Using this new set of data, we have evaluated the accuracy of quantum chemical calculations of the 17O CS tensors for formaldehyde at the Hartree-Fock (HF), density-functional theory (DFT), Mller-Plesset second-order perturbation (MP2), and coupled-cluster singles and doubles (CCSD) levels of theory. The conclusion is that, while results from HF and DFT tend to underestimate the electron correlation effect, the MP2 method overestimates its contribution. The CCSD results are in good agreement with the experimental data.

Proton Spin Diffusion in Polyethylene as a Function of Magic-Angle Spinning Rate. A Phenomenological Approach.
Zhenlong Jia, Lili Zhang, Qun Chen, and E. W. Hansen
Starting from the phenomenological Bloembergen-Purcell-Pound equation a relation between magic-angle spinning (MAS) rate and spin diffusion is derived. The resulting model equation was fitted to observed spin diffusion versus MAS rate data obtained at 298 K on an high-density polyethylene sample, revealing a reduction in the effective spin diffusivity by (65 + 5)% when increasing the MAS rate from 2 to 12 kHz. The same model equation enabled the rigid-lattice diffusivity to be estimated and was found to be only slightly higher, by approximately 10%, compared to the spin diffusivity observed at the lowest MAS rate applied (2 kHz). Moreover, the model equation predicts a reduction in the effective spin diffusivity by more than 90% when increasing the MAS rate to more than 30 kHz.

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