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