ABSTRACT:We have recently demonstrated that sensitive and chemically specific NMR spectra can be recorded in the absence of a magnetic field using hydrogenative parahydrogen induced polarization (PHIP) 1−3 and detection with an optical atomic magnetometer. Here, we show that non-hydrogenative parahydrogen-induced polarization 4−6 (NH-PHIP) can also dramatically enhance the sensitivity of zero-field NMR. We demonstrate the detection of pyridine, at concentrations as low as 6 mM in a sample volume of 250 μL, with sufficient sensitivity to resolve all identifying spectral features, as supported by numerical simulations. Because the NH-PHIP mechanism is nonreactive, operates in situ, and eliminates the need for a prepolarizing magnet, its combination with optical atomic magnetometry will greatly broaden the analytical capabilities of zero-field and low-field NMR.T he past decade has witnessed increasing interest in the development of low-cost portable NMR spectrometers. 7 Such spectrometers promise to enable chemical analysis at greatly reduced cost in environments not accessible to standard high-field NMR technology. Detection of analytes at low concentrations primarily requires sensitive detectors and sufficient nuclear polarization. The weak thermal polarization of nuclear spins has given NMR the reputation of being an inherently insensitive method. For example, even in magnetic fields of superconducting magnets, the polarization obtained does not exceed 10 −4 .A variety of available hyperpolarization techniques such as dynamic nuclear polarization (DNP), 8,9 chemically induced DNP (CIDNP), 1 0 spin-exchange optical pumping (SEOP) 11−13 of noble gases, and parahydrogen induced polarization (PHIP) [1][2][3]14,15 suggest that sensitivity limitations given by the Boltzmann thermal polarization can be overcome for a large range of analytes. All these hyperpolarization techniques, DNP, 16−19 CIDNP, 20−22 SEOP 23 and PHIP, 24,25 have been shown to greatly enhance sensitivity of low-field NMR experiments where thermal polarization is even lower.Of equal importance, sensitive low-field NMR detectors of nuclear magnetization have been developed. These include systems based on inductive detectors, 7,26,27 superconducting quantum-interference devices (SQUIDs), 19,28−30 and optical magnetometers. 24,31−34 These technologies, in varying stages of maturity, permit the sensitive detection of low-frequency NMR signals, including NMR spectra in Earth's magnetic field.Combinations of hyperpolarization and novel detection schemes are thus particularly attractive in unconventional or portable NMR applications. For example, we have recently demonstrated that high-resolution and high SNR spectra can be recorded at zero field using PHIP from standard hydrogenative processes and an atomic magnetometer. 24 In the present contribution, we describe the first NMR experiments in zero field employing non-hydrogenative parahydrogen induced polarization (NH-PHIP) producing signal amplification by reversible exchange (SABRE). 4−6 Following the...

We report an observation of long-lived spin-singlet states in a 13C-1H spin pair in a zero magnetic field. In 13C-labeled formic acid, we observe spin-singlet lifetimes as long as 37 s, about a factor of 3 longer than the T1 lifetime of dipole polarization in the triplet state. In contrast to common high-field experiments, the observed coherence is a singlet-triplet coherence with a lifetime T2 longer than the T1 lifetime of dipole polarization in the triplet manifold. Moreover, we demonstrate that heteronuclear singlet states formed between a 1H and a 13C nucleus can exhibit longer lifetimes than the respective triplet states even in the presence of additional spins that couple to the spin pair of interest. Although long-lived homonuclear spin-singlet states have been extensively studied, this is the first experimental observation of analogous singlet states in heteronuclear spin pairs.

ABSTRACT:We report the acquisition and interpretation of nuclear magnetic resonance (NMR) J-spectra at zero magnetic field for a series of benzene derivatives, demonstrating the analytical capabilities of zero-field NMR. The zeroth-order spectral patterns do not overlap, which allows for straightforward determination of the spin interactions of substituent functional groups. Higher-order effects cause additional line splittings, revealing additional molecular information. We demonstrate resonance linewidths as narrow as 11 mHz, permitting resolution of minute frequency differences and precise determination of long-range J-couplings. The measurement of J-couplings with the high precision offered by zero-field NMR may allow further refinements in the determination of molecular structure and conformation.

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