title: Central-transition double-quantum sideband NMR spectroscopy of half-integer quadrupolar nuclei: estimating internuclear distances and probing clusters within multi-spin networks.
abstract: 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-pairs 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 (11)B, (23)Na and (27)Al NMR results are presented on 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 the "multiple-quantum spin counting" experiment, which is well-established for spin-1/2 applications but has hitherto not been demonstrated 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.
authors.affiliation: Measurement Science and Standards, National Research Council Canada, 1200 Montreal Road, M-40, Ottawa, Ontario K1A 0R6, Canada. Andreas.Brinkmann@nrc-cnrc.gc.ca.
authors.forename: Andreas
authors.initials: A
authors.lastname: Brinkmann
authors.forename: Mattias
authors.initials: M
authors.lastname: Edén
mesh_terms: D000537:Aluminum Oxide / Q000737:chemistry; D018360:Crystallography, X-Ray; D009682:Magnetic Resonance Spectroscopy; D011789:Quantum Theory; D013431:Sulfates / Q000737:chemistry
chemical_list: D013431:Sulfates; C012036:sodium sulfate; D000537:Aluminum Oxide