Broadband multiple-quantum NMR spectroscopy

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 degrees 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 of Q((1)), Q((2)), and Q((3)) groups, which are characterized by a large chemical-shift anisotropy spanning more than 200 ppm. With these experiments, P-31 high-resolution double-quantum NMR spectroscopy is introduced as a tool for the direct investigation of dipolar connectivities between like or different Q((n)) units in phosphates. P-31 dipolar connectivities in two samples, Mg2P2O7 and MgP4O11 were established. Therefore, double-quantum spectroscopy of this kind has potential for the investigation of disordered solids, for instance, phosphorus glasses. (C) 1996 Academic Press, Inc.