Zero-Field Nuclear Magnetic Resonance in High Field: The Untruncation of Dipole-Dipole Couplings by Coherent Averaging

It is shown experimentally that the orientation dependence of nuclear magnetic dipole-dipole couplings in high-field NMR can be removed by rapid rotation of the sample and application of a synchronized rf pulse sequence. The sample rotation and rf pulses coherently average the usual truncated, high-field couplings to the form of untruncated, zero-field couplings. The resulting NMR spectra of polycrystalline solids show sharp lines with splittings that depend only on internuclear distances, rather than the usual broad powder-pattern lines. The derivation of the pulse sequence is described.