Zero Field Nuclear Magnetic Resonance in High Field: The Untruncation of Dipole-Dipole Couplings.

High field NMR spectra of solids that are not single crystals typically exhibit broad powder pattern lineshapes that arise from the orientation dependence of nuclear spin couplings. In this paper, we describe the first experimental demonstration that the orientation dependence of nuclear magnetic dipole- dipole couplings can be removed in high field. The orientation dependence is removed by a combination of rapid sample rotation with the application of a synchronized sequence of radio frequency pulses. The resulting spectra exhibit sharp lines with splittings that depend only on the distances between nuclei. The spectra therefore resemble zero field NMR spectra, but are obtained with the full sensitivity, isotopic specificity, and time resolution of high field NMR. Proton NMR spectra of polycrystalline meta- C sub 6 H sub 2 D sub 4 and para- C sub 6 H sub 2 D sub 4 are presented that verify the distance dependence of the observed splittings. We describe the derivation of the pulse sequence using average Hamiltonian theory. The sample rotation and pulse sequence have the effect of averaging the orientation- dependent, "truncated" dipole-dipole couplings that occur in high field to the form of the scalar, "untruncated" couplings that occur in zero field.