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Abstract:
The inner lines of the 13C NMR quartet of a free radical solution in α-13C-toluene have been observed to be more negatively enhanced in a DNP experiment than the outer ones, which are, in fact, even positively enhanced at low free radical concentration. The results are interpreted in terms of the three-spin effect. The unusual appearance of the DNP-enhanced 13C quartet originates from the dipolar relaxation within a methyl group undergoing nearly free rotation.