ZERO-FIELD NUCLEAR-MAGNETIC-RESONANCE IN HIGH-FIELD

We present a complete description of techniques for obtaining nuclear magnetic resonance (NMR) spectra of solids that have the appearance of zero field spectra but are obtained entirely in high field. With these techniques, we gain the resolution and simplicity of zero field NMR while preserving the full sensitivity and isotopic selectivity of high field NMR. Combinations of rapid sample rotation and the synchronous application of sequences of resonant radio frequency pulses are used to average nuclear spin couplings in solids from their usual orientation-dependent forms in high field to scalar, zero field forms. We describe the theoretical basis for the techniques, the derivation of specific combinations of sample rotations and pulse sequences that produce the desired scalar average couplings, and the experimental implementation of the techniques. We use exact dynamical simulations to assess the effects of experimental nonidealities on the spectra and present new sequences designed to be less sensitive to the dominant nonidealities. Experimental proton NMR spectra of selectively deuterated, polycrystalline benzene samples are presented. The spectra show sharp lines with splittings that depend only on internuclear distances as in zero field spectra, rather than the inhomogeneously broadened "powder pattern" lines of traditional high field NMR spectra. Applications of the techniques to structural studies of polycrystalline and noncrystalline solids are discussed. © 1990 American Institute of Physics.