Two-Dimensional spin-echo multiple-quantum transitions in strongly coupled spin systems. Calculation of spectra

A general method for computation of two-dimensional spin-echo multiple-quantum spectra of strongly coupled spin systems is presented. It has been demonstrated that the 180° pulse can severely modify the multiple-quantum spectra of strongly coupled spins and explicit spectral details are given for the spin systems of types AB, ABX, and AB₂. In addition to the modified frequencies and additional transitions, it is found that such spectra possess additional frequency symmetry with respect to the center of each multiple-quantum order, even though the multiple-quantum spectrum without the 180° pulse may not be symmetric. The ABX spin system is treated in detail and the behavior of the homonuclear and heteronuclear multiple-quantum transitions in the presence of selective or nonselective 180° pulses is discussed. It has been demonstrated that strong coupling allows transfer of magnetization from abundant spins to a coupled rare spin even in the absence of excitation pulses on abundant spins. The amplitudes of various coherences created by selective and nonselective pulses and the nature of spin-echo zero-quantum transitions of strongly coupled spins are discussed in appendices.