Dipolar spectroscopy and spin alignment in electron paramagnetic resonance

Two single-frequency techniques for refocusing (SIFTER) dipolar couplings between electron spins are introduced. The experiments are based on the solid-echo and Jeener-Broekaert sequences, well established in dipolar NMR spectroscopy of solids, and open up new routes to high-resolution two-dimensional EPR spectroscopy with only moderate requirements on the spectrometer. For distances between paramagnetic centres larger than 3 nm, SIFTER provides better resolution than double electron-electron resonance (DEER). Good agreement between distances from SIFTER measurements and force-field computations is found for shape-persistent biradicals with distances up to 5.1 nm corresponding to a dipolar frequency of 390 kHz. (C) 2000 Elsevier Science B.V. All rights reserved.