On combination lines in the NMR spectra of strongly coupled spin systems

Strongly coupled spin systems provide many curious and interesting effects in NMR spectra, one of which is the presence of unexpected (from a first-or der viewpoint) lines. A physical reason is given for the presence of these combination lines. The X part of the spectrum of an ABX spin system is anal ysed as an example. For an ABX system, it is well known that the AB nuclei give a spectrum consisting of two AB-type spectra, corresponding to the two orientations of the X nucleus. It can also be shown that the X part of the spectrum corresponds to the X nucleus undergoing a transition in the prese nce of an AB-like spin system. For weakly coupled systems, the four observe d lines correspond to the four different orientations of the A and B nuclei . For a strongly coupled system, two additional lines may appear, the combi nation lines. The resulting six lines correspond to the four spin orientati ons, plus the two zero-quantum transitions. It is shown that these six Line s are such that there is no net excitation of the AB-like spin system assoc iated with the X transitions. There is no AB coherence created directly by a pulse applied to X. AB coherence is created as the system evolves, and th is is responsible for many of the curious effects. This is shown to be true for all spin sub-systems, which are weakly coupled to a strongly coupled s ub-system.