COHERENT ISOTROPIC AVERAGING IN ZERO-FIELD NUCLEAR-MAGNETIC-RESONANCE.1. GENERAL-THEORY AND ICOSAHEDRAL SEQUENCES

We present a general theory of coherent isotropic averaging in nuclear magnetic resonance (NMR). In a zero external field, magnetic-field pu lses can selectively average the internal spin Hamiltonians, while pre serving the intrinsic invariance of the spectrum with respect to the s ample orientation. The theory predicts the limits of the scaling facto rs for tenser interactions of different ranks. Time reversal is found to be possible for first- and second-rank tensors with scaling factors of -1/3 and -1/4, respectively. Explicit sequences, based on icosahed ral symmetry, are given for a number of optimal scaling factors. To il lustrate the theory, an experiment is also presented in the special ca se of rank-selective decoupling. As in high-field NMR, applications ca n be expected from the introduction of coherent averaging schemes for zero-held techniques: for example, decouplings (by rank or nuclear spe cies), time reversal, and multipolar experiments (zero-field analog of multiple-quantum NMR). (C) 1995 American Institute of Physics.