J-Coupling Nuclear Magnetic Resonance Spectroscopy of Liquids in nT Fields

Abstract

In ultralow magnetic fields, liquid state nuclear magnetic resonance (NMR) spectra of homonuclear spin systems exhibit line widths dominated by their natural lifetime. Chemical shifts become negligible, and heteronuclear NMR spectra show predominantly the electron-mediated field-independent J-coupling. However, weak polarization and Larmor frequencies down to a few hertz require special detectors, such as Superconducting Quantum Interference Devices (SQUID), that also enable the simultaneous detection of broad band spectra of heteronuclear spin systems. We acquired spectra of 2,2,2-trifluoroethanol and trimethyl phosphate at detection fields varying from 444 nT to 3.34 muT after prepolarizing the sample in a field of 250 muT. Down to a 1H Larmor frequency of 40 Hz, the spectra of trifluoroethanol exhibited four clearly resolvable peaks. The numerical simulation agreed well with the measured spectra. Trimethyl phosphate exhibited two major groups of nonresolved proton lines. At 1H Larmor frequencies below 150 Hz, the separation of the two groups decreased, reflecting the transition from weakly to strongly coupled spin systems. Direct determination of 3J(H,P) from the peak separation is possible only at Larmor frequencies above 150 Hz. The experimental setup allowed an easy adjustment of the detection field over several octaves. This enabled us to adapt the detection field to the best-suited measurement window providing the maximum spectral information. Low-field NMR may open new applications, such as monitoring heteronuclear reactions, low-field imaging, simultaneous NMR/magnetoencephalography measurements, or quantum computing.