A test for the number of coupled spins I = 1/2 in magic-angle-spinning solids:: Zero-quantum recoupling of multiple-quantum coherences

Current methodologies for estimating the number of coupled spins I = (1)/(2) in solids are based upon the maximum multiple-quantum order that can be observed. This strategy establishes a clear lower bound on the number of coupled spins I = (1)/(2.) However, it is difficult to ascertain the exact number of coupled spins, since the absence of a peak could be due either to the limited size of the spin system or to the experimental difficulty of exciting high-quantum orders and recovering those coherences into detectable signals. Herein, a supplementary test is proposed that allows one to determine whether a given coherence has the highest possible order in the spin system. The sample is subjected to magic-angle spinning and the behaviour of the coherence under a rotor-synchronised spin-echo sequence is compared to its behaviour under a zero-quantum recoupling sequence. A similar decay of the coherence in these two experiments is strong evidence for the coherence in these two experiments is strong evidence for the coherence order being the maximum possible. We propose applications of biomolecular solid-state NMR spectroscopy.