A. Llor et al., COHERENT ISOTROPIC AVERAGING IN ZERO-FIELD NUCLEAR-MAGNETIC-RESONANCE.1. GENERAL-THEORY AND ICOSAHEDRAL SEQUENCES, The Journal of chemical physics, 103(10), 1995, pp. 3966-3981
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.