Zero-Field Nuclear Magnetic Resonance

Techniques for studying molecular structure, such as x-ray diffraction and nuclear magnetic resonance (NMR), often require the use of single crystals' or oriented samples. ' NMR, for example, is sensitive to interatomic distances and angles through spectral splittings caused by internucl. ear dipol. e-dipole couplings in high magnetic field. Where only polycrystalline powders or amorphous materials are available, the distribution of molecul. ar orientations (and thus internuclear vectors) with respect to the direction defined by the x-ray beam or by the magnetic fie1.d gives rise to a powder pattern" in which most structural information is lost. ' Consider the notion of NMR with zero magnetic fie1d, i.e. , spectroscopy of pure dipole-dipo1. e interactions. Without an externally imposed direction in space, all orientations are equivalent and the spectrum of a powder should be "crystal1.ike" with al1. equivalent molecules yielding identical splittings. We present in this Letter a novel experiment, Fourier-transform zero-field NMR, which provides just such spectra, opening the way for crystal1ography and molecu1. ar structure determination in pol. ycrystall. ine or disordered materials. Figure 1 il.lustrates the idea for the simple case of two coupled proton spins [Ba(CIO,), ~ H, Ot, showing increased resol. ution due to the removal. of orientational broadening in zero field. Figure 1(a) is the normal high-field powder spectrum. Figure 1(b) is a normal high-field single-crystal spectrum where the doublet splitting depends on crystaI. orientation with respect to the fiel.d. Figure 1(c) is the zero-field powder spectrum in which the splittings assume only the largest values of the powder pattern in 1(a), resulting from the untruncated dipol. ar Hamiltonian XD. While zero-field spectroscopy is known and