Cyclical asynchronicity in two-dimensional (2D) correlation spectroscopy

The so-called Noda’s rules, which determine the relative directions and the sequential order of spectral intensity variations purely based on the signs of cross peaks appearing in 2D correlation spectra, seem to work surprisingly well for monotonically changing spectral data. Although qualitative in nature, such information provides valuable insights into the mechanistic understanding of the underlying phenomena captured by the spectral data. Occasionally, however, we have observed an apparent breakdown of the Noda’s rules, suggesting that metachronic error of placing an event after its real time may be occurring. In an extreme case, even inconsistent or self-contradictory sequential order assignments seem to arise from a large set of cross peaks. They are in fact caused by the occurrence of cyclical order of events. Fortunately, the occurrence of such complicated asynchronicity problem is usually rare. The proper interpretation is not only possible for the seemingly confusing cross peak signs but also safely anticipated for certain types of spectral signals to be analyzed by 2D correlation method.