Dipolar spectroscopy and spin alignment in electron paramagnetic resonance

Two single-frequency techniques for refocusing (SIFTER) dipolar couplings b etween electron spins are introduced. The experiments are based on the soli d-echo and Jeener-Broekaert sequences, well established in dipolar NMR spec troscopy of solids, and open up new routes to high-resolution two-dimension al EPR spectroscopy with only moderate requirements on the spectrometer. Fo r distances between paramagnetic centres larger than 3 nm, SIFTER provides better resolution than double electron-electron resonance (DEER). Good agre ement between distances from SIFTER measurements and force-field computatio ns is found for shape-persistent biradicals with distances up to 5.1 nm cor responding to a dipolar frequency of 390 kHz. (C) 2000 Elsevier Science B.V . All rights reserved.