Monday, March 09, 2009

Magnetic Resonance in Chemistry (MRC), up to March 9, 2009

Magnetic Resonance in Chemistry (MRC), up to March 9, 2009

Using hyperbolic secant pulses to assist characterization of chemical shift tensors for half-integer spin quadrupolar nuclei in MAS powder samples

by Thomas T. Nakashima, Roshanak Teymoori, Roderick E. Wasylishen

online March 9, 2009

Determination of the NMR anisotropic magnetic shielding parameters from magic angle spinning, MAS, powder samples containing half-integer spin quadrupolar nuclei is achieved by analysis of the difference spectrum obtained with and without application of a hyperbolic secant pulse. Application of a hyperbolic secant pulse to any spinning sideband associated with the central transition, mI = 1/2 to mI = - 1/2, results in 'saturation' of the entire central transition manifold. Similarly, if one spinning sideband associated with the mI = 3/2 to mI = 1/2 and mI = - 1/2 to mI = - 3/2 satellite transitions is perturbed, the entire satellite manifold associated with these transitions is 'saturated' while the central transition is enhanced by population transfer. Three 'difference spectrum' techniques are employed to selectively yield the spinning sidebands associated predominantly from the central transition. The success of these difference techniques is first demonstrated by examining 51V NMR spectra of three metavanadate salts and 59Co NMR spectra of Co(acac)3. The vanadium and cobalt chemical shift tensors in these compounds have spans between 400 and 1400 ppm. Because the hyperbolic secant techniques proposed here yielded results that are in good agreement with earlier reports, they have been applied to characterize the 51V chemical shift tensor of the dimer of bis(N, N-dimethylhydroxamido)-hydroxooxovanadate, {V(O)(ONMe2)2}2O, whose chemical shift tensor has not been previously reported.


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