Multiple-pulse NMR experiments

Abstract

The behavior of a spin system disturbed by a train of closely spaced pulses can, under the proper limiting circumstances, be described in terms of a time-independent effective Hamiltonian. The necessary conditions on pulse spacing are discussed and a recipe is given for constructing the effective Hamiltonian. The method is applied to the previously employed trains of 180° or 90° pulses, obtaining the well-known limiting behavior for small pulse spacing. An analysis is given of a recently reported experiment using a train of phase-alternated 90° pulses. Two new experiments involving complex pulse sequences are proposed, one of which permits the arbitrary scaling of the effective chemical shifts in a high-resolution NMR spectrum, and the other of which annihilates dipole-dipole and quadrupole effects in solids, leaving the chemical shifts and scalar couplings.