COHERENT ISOTROPIC AVERAGING IN ZERO-FIELD NUCLEAR-MAGNETIC-RESONANCE.2. CUBIC SEQUENCES AND TIME-REVERSAL OF SPIN COUPLINGS

We present a special case of the theory of coherent isotropic averagin g in zero-field NMR, given in part I of this work. In a zero external field, combinations of the magnetic-field pulses restricted to pi/2 ro tations along the three coordinate axes can selectively average intern al spin Hamiltonians while preserving the intrinsic invariance of the spectrum with respect to the sample orientation. Compared with the gen eral case, the limits of the allowed scaling factors of first- and sec ond-rank interactions are slightly reduced. For instance, time reversa l is possible for second-rank tensors with a -1/5 scaling factor, inst ead of -1/4 in general. Finite pulse compensations are analyzed and ex perimental illustrations are given for two optimum time-reversal seque nces. The cubic sequences, though less efficient than the icosahedral sequences, are technically more feasible and may be used in zero-field experiments such as decoupling (by rank or nuclear species), time rev ersal or multipolar experiments (the zero-field equivalent of multiple -quantum NMR). (C) 1995 American Institute of Physics.