Spin Echoes in Multi-Half-Spin Systems: Demodulation and High Resolution

Abstract

Carr‐Purcell spin‐echo (CPSE) experiments have been carried out with typical examples of homonuclear AX, homonuclear A3X, heteronuclear ABX, and homonuclear AA′XX′ systems respectively, in order to compare the recently developed theory with experiment. An examination of the theory has been undertaken to predict conditions when spin‐echo patterns and the extraction of information therefrom are simplified. These involve two markedly different rf power levels. When H 1 is high, at certain values of tcp (the time interval between adjacent 180° pulses), ``decoupling'' and ``double resonance'' of the CPSE have been demonstrated in a homonuclear AX system. For instance, in an AX system, when tcp is an integer multiple of 1 2 J AX the CPSE of the same sign show simple exponential decay instead of the usual modulated pattern. For intermediate rf power when H 1 is of the same order as the internal chemical shift, a density matrix approach has been developed to choose experimental conditions when magnetically nonequivalent nuclei in homonuclear systems may be pulsed selectively. Under these conditions of high resolution, spin‐echo experiments and theory for homonuclear A a X x systems show no modulation in both A a and X x groups. This provides a method for measuring separately T 1 and T 2 of chemically nonequivalent nuclei in the same molecule. Such experiments have been carried out on the title compounds. In dichloroacetaldehyde T 2 for the aldehyde, hydrogen is independent of tcp , whereas T 2 for the CHCl2 hydrogen varys with tcp . Furthermore, the T 1's for the two protons are different. It is concluded that proton chlorine scalar coupling in the CHCl2 group is responsible for these effects. A similar effect takes place in 1‐chloro‐1‐fluoroethylene.