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- W1500038897 abstract "The objective was to develop solid-state nuclear magnetic resonance(NMR) homonuclear correlation experiments for half-integer quadrupolar nucleiso as to study atomic proximities and connectivities in disordered materials.Nearby nuclear spins are coupled through space via their magnetic dipole moments.Dipolar broadening is removed by magic angle spinning (MAS) forisolated spin pairs. However, the noncommutation of the electric quadrupolarinteraction with the dipolar interaction means that the latter will not be removedby MAS. This interplay between the dipolar and quadrupolar interactions,combined with the effects of multiple noncommutating homonucleardipolar couplings, was investigated by observing spin-echo dephasing curves aswell as magnetisation transfer in 2D spin diffusion experiments. Polycrystallinelithium diborate samples were synthesised to act as model compounds. Thepreparation of samples with differing 11B isotopic abundances enabled a comparisonof samples with either predominantly isolated spin-pairs or multiplecoupled nuclei.Spin diffusion experiments probed 11B–11B correlation at three magneticfield strengths, 80% and 25% 11B isotopic abundances, MAS rates from4427 Hz to 7602 Hz and under DOR. Enhanced magnetisation transfer was observedfor the higher 11B isotopic abundance and at slower spinning speeds. Thelatter dependence was reproduced by four-spin computer simulations. Secondorderquadrupolar broadened spin diffusion cross-peaks under MAS had a mixedpositive and negative appearance for the 80% 11B sample. A similar effect waspreviously observed for four dipolar-coupled I = 1/2 nuclei.Spin-echo dephasing curves were recorded for 5%, 25% and 100% 11B isotopicabundances and MAS rates of 5 kHz to 20 kHz. Depletion of 11B isotopicabundance prolonged the coherence dephasing time because of a reduction ofnoncommuting homonuclear dipolar couplings. Faster dephasing was observedfor the smaller CQ = 0.51MHz site; four-spin computer simulations showed thisis consistent with the reintroduction of the dipolar coupling being most efficientwhen the MAS rate and first-order quadrupolar interaction are of the samemagnitude. Speeding-up the MAS rate prolonged the dephasing time for theCQ = 2.56MHz site but not for the CQ = 0.51MHz site because of an interplaybetween the quadrupolar and multiple dipolar interactions. Through-bond Jcouplingsbetween 11B nuclei were not detected, setting an upper bound of 2JBB<3 Hz in polycrystalline lithium diborate." @default.
- W1500038897 created "2016-06-24" @default.
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- W1500038897 date "2009-12-01" @default.
- W1500038897 modified "2023-09-26" @default.
- W1500038897 title "Homonuclear correlation in solid-state NMR : developing experiments for half-integer quadrupolar nuclei" @default.
- W1500038897 hasPublicationYear "2009" @default.
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