Zero- to Ultralow-Field NMR

This article presents the basic principles and methodology used in modern implementations of zero- to ultralow-field NMR (ZULF NMR), with emphasis on the case where spin evolution is detected directly in the ZULF environment. In contrast to conventional high-field NMR, ZULF NMR allows for measurement of spin–spin interactions ‘in their natural environment’ free of truncation by dominant coupling to applied magnetic fields. However, the absence of a large applied magnetic field means that spin precession frequencies and equilibrium spin polarization—related to the sensitivity of inductive detection and to the magnitude of the measurable magnetization, respectively—are dramatically lower than in the high-field case. ZULF NMR thus requires the use of alternative detectors such as atomic magnetometers, along with the production of nonequilibrium spin polarization as prepared by, for example, prepolarization in a permanent magnet or parahydrogen-induced polarization. Nevertheless, ZULF NMR permits particularly high-resolution measurement of spin–spin couplings due to the high absolute magnetic field homogeneity and the absence of certain relaxation pathways such as chemical shift anisotropy or susceptibility-induced gradients. Furthermore, ZULF NMR is capable of directly detecting spin interactions that do not commute with the Zeeman Hamiltonian and are thus unobservable with high-field NMR.