Intermolecular multiple-quantum coherence transfer echoes and multiple echoes in nuclear magnetic resonance

In recent years evidence was presented that two radio frequency pulses in c ombination with suitably matched field gradient pulses produce cross peaks in two-dimensional NMR spectroscopy that reveal all the features of multipl e-quantum/single-quantum coherence transfer signals. This experiment was te rmed CRAZED by Warren et al. Quite amazingly, identical signals were predic ted on a purely classical basis. Thus the intriguing question arises whethe r this means that a classical analog to multiple-quantum coherences exists. In this study we show that this is clearly not the case. Coherence pathway s of a purely single-quantum basis are excited anyway and lead independentl y to superimposed identical signals in parallel. Moreover, the observation of CRAZED signals per se neither disproves the adequacy of the high-tempera ture approximation, which was questioned in this context, nor proves a stro ng influence of intermolecular multiple-quantum coherences. However, the in dispensable prerequisite for the description of the manifold CRAZED phenome na is that long-range dipolar interactions are taken into account either ex plicitly or in the continuous-limit form. (C) 1999 American Institute of Ph ysics. [S0021-9606(99)01008-9].