Measurement of Untruncated Nuclear Spin Interactions via Zero- to Ultra-Low-Field Nuclear Magnetic Resonance

TL;DR

This paper demonstrates the measurement of residual dipolar couplings in molecules using zero- to ultra-low-field NMR, revealing interactions hidden in high-field NMR and expanding the technique's applications.

Contribution

It introduces a novel method to observe dipolar couplings in zero-field NMR, enabling detection of previously unobservable spin interactions.

Findings

First measurement of dipolar couplings as a perturbation on J-coupling at zero field.

Observation of dipole-dipole terms invisible in high-field NMR.

Potential for enhanced chemical analysis and material characterization.

Abstract

Zero- to ultra-low-field nuclear magnetic resonance (ZULF NMR) provides a new regime for the measurement of nuclear spin-spin interactions free from effects of large magnetic fields, such as truncation of terms that do not commute with the Zeeman Hamiltonian. One such interaction, the magnetic dipole-dipole coupling, is a valuable source of spatial information in NMR, though many terms are unobservable in high-field NMR, and the coupling averages to zero under isotropic molecular tumbling. Under partial alignment, this information is retained in the form of so-called residual dipolar couplings. We report zero- to ultra-low-field NMR measurements of residual dipolar couplings in acetonitrile-2-$^{13}$C aligned in stretched polyvinyl acetate gels. This represents the first investigation of dipolar couplings as a perturbation on the indirect spin-spin $J$-coupling in the absence of an applied magnetic field. As a consequence of working at zero magnetic field, we observe terms of the dipole-dipole coupling Hamiltonian that are invisible in conventional high-field NMR. This technique expands the capabilities of zero- to ultra-low-field NMR and has potential applications in precision measurement of subtle physical interactions, chemical analysis, and characterization of local mesoscale structure in materials.