Homonuclear NMR Correlations between Half-Integer Quadrupolar Nuclei Undergoing Magic-Angle Spinning

Abstract

This study presents a theoretical, numerical, and experimental survey on the nature of homonuclear dipolar couplings in systems of half-integer quadrupolar nuclei undergoing magic-angle-spinning (MAS). Various spin interactions that do not commute with homonuclear dipolar couplings (chemical shift effects, heteronuclear dipolar couplings, quadrupolar interactions) may lead to recoupling effects under MAS, yielding a variety of pathways for transferring magnetization between proximate quadrupole nuclei in 2D correlation experiments. The Hamiltonians underlying this anisotropy-driven recoupling of the dipolar interactions are theoretically derived and their characteristics revealed from theoretical and numerical arguments. To explore when and how these various recoupling mechanisms become relevant, a variety of 23Na and 11B 2D exchange NMR experiments were performed at different external magnetic fields and MAS frequencies on several compounds: Na2HPO4·2H2O, Na2SO3, disodium deoxycytidine heptahydrate, B2O3 and B10H14. The structural information content afforded by these experiments as well as their potential limitations are discussed.