Heteronuclear Zero-field Nmr

In zero-field NMR [ 1.21, a sample is subjected to a time-dependent magnetic field which cycles between a high value, Bo, and zero field. The B. field is useful for preparation and high-sensitivity detection of the spins. Evolution occurs for a variable time tl in zero field. This yields sharp crystal-like NMR spectra from polycrystalline samples, where ordinarily only powder patterns are observed. Examples have been shown for groups of coupled protons in solids [ 11, as well as Fourier-transform pure NQR of 2D, 7Li 121, and 27Al 131. An interesting situation arises if the coupled spins are inequivalent; that is, their natural resonance frequencies differ widely due to chemical shifts or because the nuclei are of different species (e.g. the heteronuclear pair I = 1 H, S = lSC). Consider, in fact, a single I-S spin pair coupled by a dipolar interaction *