3-SPIN SYSTEMS IN ROTATING SOLIDS AND THE X-APPROXIMATION

Three-spin systems in which two of the nuclei have a relative chemical-shift difference comparable to the magnitude of their internuclear spin interactions, whereas the relative chemical shift of the third spin is considerably larger, are considered for rotating solids. Using the X approximation, which is commonly made for solution-state nuclear magnetic resonance to simplify spectral analysis, it is shown that such systems may be described by stationary eigenstates using average Hamiltonian theory in the high-spinning-frequency approximation. It is demonstrated that the spectrum of the weakly coupled third spin is dependent on the relative shielding tensor orientation and the magnitude of the homodipolar coupling in the strongly coupled spin pair. Furthermore, this dependence leads to broadening of the spectrum of the weakly coupled nucleus. © 1992.