NMR spectra of some simple spin systems studied by two-dimensional fourier transformation of spin echoes

Abstract

Phase-modulated spin echoes have been generated by a 90–180° pulse sequence applied to some strongly coupled proton spin systems (AB, ABX, and ABC), and the modulation has been analyzed by two-dimensional Fourier transformation. Projection of the resulting two-dimensional spectra onto the F 1 axis leads to a “ J spectrum”, whereas projection onto the F 2 axis gives the conventional NMR spectrum. Analytical solutions are presented which correctly predict the form of the AB and ABX J spectra, and an iterative numerical analysis is used to fith the ABC case. Projections onto the F 1 axis are predicted to have exact symmetry about F 1 = 0, and this is borne out by experiment, whereas the spin multiplets of the conventional high-resolution spectrum in general lack symmetry. Certain peaks in the two-dimensional spectra are predicted to have negative intensities, although no spin population inversion is involved. Experimental spectra from AB, ABX, and ABC spin systems illustrate these effects, and strong lines of negative intensity are detected unequivocally.