Multiple-quantum ESR and distance measurements

Ž . 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 ̊ Ž . pathways can be effectively used to determine long ;30 A 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. q 1999 Elsevier Science B.V. All rights reserved.