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Signal Enhancement Techniques for Unreceptive Half-Integer Quadrupolar Nuclei

Many half-integer quadrupolar nuclei have low magnetogyric ratios, low natural abundance, large nuclear quadrupole moments, or even some combination of the above, making them very "unreceptive" to study by NMR.  Some projects (fundamental and applied) will be listed below that address these problems.

R. W. Schurko, I. Hung and C. M. Widdifield, 2003. Signal Enhancement in NMR Spectra of Half-Integer Quadrupolar Nuclei via DFS-QCPMG and RAPT-QCPMG Pulse Sequences, Chem. Phys. Lett. 379, 1-10.

Abstract: Two signal enhancement schemes capable of providing large signal-to-noise gains in static and magic-angle spinning NMR spectra of the central transition of half-integer quadrupolar nuclei are presented. Amplitude-modulated double-frequency sweeps (AM-DFS) and rotor assisted population transfer (RAPT) pulse sequences are used as preparatory sequences in combination with the quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) pulse sequence to obtain large signal enhancements ranging from one to two orders of magnitude. Specific applications to two spin-3/2 nuclei (87Rb and 39K) and one spin-5/2 nucleus (85Rb) are presented.

Top: Various QCPMG pulse sequences used for signal enhancement.
Bottom: 85Rb Hahn-echo, QCPMG and DFS-QCPMG spectra of RbClO4.

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I. Hung and R. W. Schurko, 2003. A Solid-State Mg-25 and C-13 NMR Study of Bis(cyclopentadienyl)magnesium, Solid State Nucl. Magn. Reson. 24, 78-93.

Abstract: Application of the "quadrupolar Carr-Purcell Meiboom-Gill" (QCPMG) sequence, permits the first solid-state 25Mg NMR study of an organometallic magnesium compound, bis(cyclopentadienyl)magnesium. Analytical and numerical simulations of both static and magic angle spinning QCPMG NMR spectra beget an axially symmetric 25Mg electric field gradient (EFG) tensor (quadrupolar asymmetry parameter, 0Q = 0.01(1)) with a nuclear quadrupole coupling constant of CQ = 5.80(5) MHz. Restricted Hartree-Fock and hybrid density functional theory (B3LYP) calculations produce good agreement with experimental EFG values and predict a chemical shielding anisotropy of about 40-50 ppm, which we attempt to elucidate by numerical simulations. The parameters and orientation of the 25Mg EFG tensor are rationalized from examination of the crystal structure and molecular symmetry. The NMR properties of the cyclopentadienyl rings are examined by 13C{1H}CPMAS NMR, RHF and hybrid-DFT (B3LYP) calculations, and simulations of the effects of chemical exchange on the carbon-13 powder pattern.

Top: 25Mg QCPMG MAS NMR spectra of Cp2Mg. Bottom: Static 25Mg QCPMG NMR spectra of Cp2Mg.

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More coming soon!

Last Updated: September 1, 2003
Copyright Rob Schurko, 2003