Anomolous cross-peaks in two-dimensional spectra are sometimes observed for samples that have a high density of magnetic nuclear spins. Two interpretations of these effects have been given in the literature: There is a classical interpretation involving Larmor frequency shifts generated by the nuclear demagnetizing field, and there is a macroscopic quantum interpretation invoking long-range multiple-quantum coherences. The article contrasts the two, with emphasis placed on the classical description.
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Malcolm H. Levitt (1996) studied this question.