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Analysis of 1H–14N polarization transfer experiments in molecular crystals

A general theoretical description of polarization transfer processes in multi-spin systems containing dipole as well as quadrupole spins is formulated on the background of the Liouville–von Neumann equation. The density operator formalism is used to describe the evolution of an arbitrary spin system due to quadrupole, Zeeman and dipole–dipole interactions. This approach is applied to interpret previously published 1H–14N cross-relaxation NMR experiments for measuring the 14N quadrupole coupling constants of paranitrotoluene (PNT) and trinitrotoluene (TNT) (Nolte et al 2002 J. Phys. D: Appl. Phys. 35 939) and new experiments on urea and urotropine. It is demonstrated that according to the complexity of the analysed spin system an appropriate number of spins has to be taken into consideration for a correct description of the cross-relaxation spectra. The work is a part of an extended project aiming for a method which should permit detection of TNT explosive in anti-personnel landmines.