Central-Transition Double-Quantum Sideband NMR Spectroscopy of Half-Integer Quadrupolar Nuclei: Estimating Internuclear Distances and Probing Clusters within Multi-Spin Networks,

A. Brinkmann*, and M. Edén,
Phys. Chem. Chem. Phys. 16, 7037-7050, (2014).
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We introduce a strategy to estimate the size of clusters of recoupled homonuclear half-​integer quadrupolar nuclei under magic-​angle spinning (MAS) conditions, by combining double-​quantum (2Q) sideband NMR experiments with an approximate numerical analysis based on the summation of all spin-​pair present over a given radius of the structure. The experiment relies solely on the evolution of homonuclear 2Q coherences (2QC) among the central-​transitions (CT) of half-​integer spins and is suitable for probing clusters in network structures, such as those encountered in large groups of oxide-​based materials. Experimental 11B, 23Na and 27Al NMR results are presented on of bis(catecholato)diboron, Na2SO4 and Al2O3, respectively; in each case, the growth of the spin-​cluster size was monitored from a series of experiments that employed progressively lengthened 2QC excitation intervals. Our new approach is the first option for probing larger constellations of half-​integer spins; it provides similar information as "multiple-​quantum spin counting", which is well-​established for spin-1/2 applications but has hitherto not been demonstrated in for half-​integer spins undergoing MAS. We also discuss various options for determining the internuclear distance within a (nearly) isolated pair of half-​integer spins by comparing the experimental 2Q sideband NMR spectra with results from numerical simulations involving various degrees of approximation.