MULTIPLE-QUANTUM NUCLEAR-MAGNETIC-RESONANCE OF SOLIDS - A CAUTIONARY NOTE FOR DATA-ANALYSIS AND INTERPRETATION

The conventional method of data analysis and interpretation of time-resolved multiple quantum (MQ) nuclear magnetic resonance (NMR) spectra of solids is closely examined. Intensity profiles of experimental H-1 MQ NMR spectra of polycrystalline adamantane and hexamethylbenzene serve to test the Gaussian statistical model approach. Consequences of this model are explored with a least-squares fitting procedure, transformation of data to yield linear plots, and a scaling analysis. Non-Gaussian behavior of the MQ NMR spectral intensity profiles, as a function of order of coherences, is demonstrated with all these methods of analysis. A heuristic argument, based on the multiplicative properties of dipolar coupling constants in the equation of motion of the density operator, leads to the prediction of exponentially decaying MQ NMR spectral intensity profiles. Scaling analysis and semilog plots of experimental time-resolved MQ NMR spectra of adamantane and hexamethylbenzene support this deduction. Dynamical scale invariance in the growth process of multiple spin coherences is revealed with this new approach. The validity of spin counting in solids with MQ NMR is discussed in light of the present results.