Chemistry Without a Magnet: Spectra at Zero Field express
A substance's spectrum is its fingerprint. Usually, to record one, you place the sample in a strong magnetic field, like in an MRI scanner. But there is a way to do without a magnet altogether: detect the signal from atomic nuclei at zero field. Until now, this method suffered from poor sensitivity, and its complex spectra were hard to predict.
The authors of the new study took ordinary liquids, the kind sitting on a lab shelf, and for the first time recorded their spectra at zero field using carbon-13 at its natural abundance (just 1.1% of all carbon atoms). No special sample preparation or expensive signal enhancement was needed. The instrument, a compact rubidium vapor magnetometer, proved so sensitive that it even distinguished molecules with two rare carbon-13 atoms, which make up only 0.0121% of the sample. The spectral lines were narrow (less than 250 millihertz), and the instrument ran stably for over a week.
In parallel, the scientists learned to predict these spectra using quantum-chemical calculations that account for molecular vibrations. The prediction accuracy is on the order of a few hertz. Comparing calculation with experiment proved useful in itself: discrepancies showed where a molecule forms hydrogen bonds, how it hydrates in water, or forms ion pairs in a concentrated salt solution.
This approach paves the way for cheap and flexible analysis of substances: you can look inside metal containers, and you don't need a bulky superconducting magnet. And the calculations help not only identify a molecule but also understand who it interacts with in solution.
Original abstract
as written by the authors arXiv:2604.26071
Zero-field (ZF) nuclear magnetic resonance (NMR) spectroscopy probes scalar J-couplings between nuclei while dispensing with large homogeneous magnetic fields, enabling low-cost and geometrically flexible detection, including through conductive enclosures. Despite these advantages, its broader use for chemical analysis has been limited by sensitivity and by the difficulty of predicting the dense spectral multiplets that arise at zero field. Here we demonstrate natural-abundance (1.1%) 13C ZF spectroscopy on off-the-shelf liquids using a compact commercial 87Rb magnetometer for the first time, without hyperpolarization or special sample preparation. Instrumental advances yield improved sensitivity, <250-mHz linewidths and >week-long stability, enabling isotopomer-resolved fingerprint spectra across a 13-molecule library, including the ability to discern rare (0.0121%) doubly 13C-labelled species. In parallel, we demonstrate vibrationally corrected density-functional theory (DFT) based prediction of ZF NMR spectra for chemically diverse molecules with few-hertz accuracy. Comparing experiment with these calculations renders residual deviations as chemically informative, reporting on hydrogen bonding, hydration and ion pairing at high ionic strength. Together, these results contribute towards DFT-assisted ZF NMR as a general platform for field-constraint-free molecular identification and for extracting transient solution-state structure from responsive J-coupling observables.
The rubidium vapor magnetometer is so sensitive that it can detect the magnetic field of a human heart from a distance of a few centimeters.