Nuclear magnetic resonance polarization and coherence echoes in static and rotating solids

Abstract

The mechanisms of defocusing and refocusing of spin order in extended dipolar coupled nuclear spin systems are investigated by experiments on static and on rotating solids. It is demonstrated that polarization or coherence echoes are possible also under magic-angle sample spinning. The dipolar interactions, averaged by the spinning, are recovered by rotor-synchronized multiple-pulse sequences. By a simple modification of the pulse sequences, it is possible to reverse the sign of the effective dipolar Hamiltonian and to induce the refocusing of polarization or coherence. The creation of multiple-spin order in the course of polarization evolution or free precession is monitored by a modified echo experiment. Experimental results for a polycrystalline sample of calcium formate are presented.