Broadband Multiple-Quantum NMR Spectroscopy

Abstract

A simple broadband radiofrequency pulse sequence for the excitation of multiple-quantum coherences in the presence of fast magic-angle spinning is introduced. This sequence involves back-to-back (BABA) 90° radiofrequency pulse cycles timed to span two rotor periods. It proved to be robust and insensitive to off-resonance effects, to isotropic chemical shifts, and to chemical-shift anisotropies. This is demonstrated on crystalline phosphates, composed ofQ⁽¹⁾,Q⁽²⁾, andQ⁽³⁾groups, which are characterized by a large chemical-shift anisotropy spanning more than 200 ppm. With these experiments,³¹P high-resolution double-quantum NMR spectroscopy is introduced as a tool for the direct investigation of dipolar connectivities between like or differentQ⁽ⁿ⁾units in phosphates.³¹P dipolar connectivities in two samples, Mg₂P₂O₇and MgP₄O₁₁were established. Therefore, double-quantum spectroscopy of this kind has potential for the investigation of disordered solids, for instance, phosphorus glasses.