Relaxation-allowed cross-peaks in two-dimensional N.M.R. correlation spectroscopy

An analysis is made of the effects of non-exponential transverse relaxation during the free precession periods of two-dimensional N.M.R. experiments. It is shown, both theoretically and experimentally, that the presence of cross-correlation between various interactions that cause relaxation, such as chemical shift anisotropy and dipole-dipole interactions, may lead to the creation of spin-ordered states that make coherence transfer processes possible in spite of the absence of scalar, dipolar or quadrupolar splittings. The principal example discussed in the context of two-dimensional correlation experiments is the occurrence of cross-peaks between two spins which are not scalar coupled to one another. The misinterpretation of these novel effects can represent a pitfall to the practising N.M.R. spectroscopist, whilst their correct interpretation may provide useful evidence about the spatial arrangement of the nuclei.