Anomalous nuclear overhauser effects in carbon-substituted aziridines: Scalar cross-relaxation of the first kind

Anomalous NOESY cross-peaks that cannot be explained by dipolar cross-relaxation or chemical exchange are described for carbon-substituted aziridines. The origin of these is identified as scalar cross-relaxation of the first kind, as demonstrated by a complete theoretical description of this relaxation process and by computational simulation of the NOESY spectra. It is shown that this process relies on the stochastic modulation of J-coupling by conformational transitions, which in the case of aziridines arise from inversion at the nitrogen center. The observation of scalar cross-relaxation between protons does not appear to have been previously reported for NOESY spectra. Conventional analysis would have assigned the cross-peaks as being indicative of a chemical exchange process occurring between correlated spins, were it not for the fact that the pairs of nuclei displaying them cannot undergo such exchange. Scalar relaxation: Apparently anomalous cross-peaks in 2D ¹HNOESY spectra of carbon-substituted aziridines are shown to arise from scalar cross-relaxation of the first kind (SRFK; see picture), due to modulation of scalar coupling constants between protons. Such effects will likely be seen in NOESY spectra of other small molecules experiencing dynamic exchange modulation of scalar ¹H-¹H coupling constants.