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EPR Free Induction Decay Coherence Observed after a Single Pulse in Saturation Recovery Experiments for Samples with Resolved Multiline CW Spectra
Authors:V Kathirvelu  H Sato  R W Quine  G A Rinard  S S Eaton  G R Eaton
Institution:(1) Department of Chemistry and Biochemistry, University of Denver, Denver, Colorado, USA;(2) Department of Engineering, University of Denver, Denver, Colorado, USA
Abstract:An electron paramagnetic resonance (EPR) spin-coherence signal has been observed following a single pulse for rapidly tumbling radicals with well-resolved nuclear hyperfine splitting in fluid solution when B 1 is large enough to excite multiple hyperfine lines. This signal, which has the shape of a spin echo, arises from constructive interference of overlapping free induction decays (FIDs) from the hyperfine lines. It has been observed for 2,6-di-t-butyl-1,4-benzosemiquinone, 2,5-di-t-butyl-1,4-benzosemiquinone, 2,3,5,6-tetramethoxy-1,4-benzosemiquinone, 2,4,6-tri-t-butylphenoxyl radical, and 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy. It occurs at a time after the pulse that is equal to the inverse of the nuclear hyperfine splitting, independent of EPR resonance frequency from 250 MHz to 9.1 GHz. As the length of the pulse is increased, separate coherence signals can be observed that correspond to the beginning and end of the pulse. This coherence is distinct from the "single-pulse echo" signals discussed in the literature. For 2,6-di-t-butyl-1,4-benzosemiquinone, which has two resolved couplings (1.24 and 0.052 G), FID oscillations with a period that corresponds to the larger hyperfine coupling are observed on the coherence signal that arises from the smaller hyperfine coupling. If phase cycling is not perfect, the coherence signal can interfere with measurements of T 1 by saturation recovery. Authors' address: Gareth R. Eaton, Department of Chemistry and Biochemistry, University of Denver, Denver, CO 80208, USA
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