Multiple-quantum ESR and distance measurements

It is shown that allowed double-quantum coherences (DQC) can now be routinely generated in disordered and oriented solids containing nitroxide biradicals and random distributions of stable radicals. The Pake doublets obtained from DQC pathways can be effectively used to determine long (∼30 Å) distances in the former case, and concentrations in the latter. The DQC signals are strong and often comparable to standard single-quantum signals. In the limit of non-selective pulses their interpretation becomes independent of complicating features which affect other ESR distance methods.