Fundamental Aspects of Parahydrogen Enhanced Low-Field Nuclear Magnetic Resonance

Abstract

We report new phenomena in low-field 1 H nuclear magnetic resonance (NMR) spectroscopy using parahydrogen induced polarization (PHIP), enabling determination of chemical shift differences, , and the scalar coupling constant J. NMR experiments performed with thermal polarization in millitesla magnetic fields do not allow the determination of scalar coupling constants for homonuclear coupled spins in the inverse weak coupling regime ( < J). We show here that low-field PHIP experiments in the inverse weak coupling regime enable the precise determination of and J. Furthermore we experimentally prove that observed splittings are related to in a nonlinear way. Naturally abundant 13 C and 29 Si isotopes lead to heteronuclear J-coupled 1 H-multiplet lines with amplitudes significantly enhanced compared to the amplitudes for thermally prepolarized spins. PHIP-enhanced NMR in the millitesla regime allows us to measure characteristic NMR parameters in a single scan using samples containing rare spins in natural abundance. DOI: 10.1103/PhysRevLett.110.137602 PACS numbers: 76.60.Àk, 33.25.+k, 82.56.Ub The established approach to liquid-state NMR spectroscopy in high fields is based on resolving NMR lines probing chemical shifts, J-coupling constants, and multiplicity. Those parameters are easily extracted from the spectra, because high-field experiments are typically performed in the weak coupling regime, where ) J is valid. The clearly separated spectral lines allow identification of molecular structure. In the presence of rare spins additional heteronuclear J-coupled multiplets arise [1,2].In the last decades various hyperpolarization technologies [3][4][5][6] and sensitive detection schemes [7][8][9][10] have rekindled the interest in low-field NMR. High resolution NMR spectroscopy with hyperpolarized molecules has been demonstrated in the Earth's magnetic field and close to zero field [10][11][12][13]. PHIP, where singlet state order (parahydrogen) is transferred into large observable nuclear polarization, offers an attractive means of hyperpolarization [14][15][16][17][18]. NMR spectroscopy with hyperpolarized J-coupled spins at zero and close to zero field [19][20][21] has been demonstrated. In these cases the presence of rare spins (e.g., 15 N) in the molecule is required to yield observable transitions in J-coupled spin systems. Close to zero field there are still ambiguity problems for molecules with more than two chemical groups and the chemical shift information is lost [22]. The drawbacks of low-field NMR spectroscopy for pure 1 H spin-systems with thermal polarization are the low signal-to-noise ratio (SNR) and the loss of J-coupling and chemical shift information.We show striking differences between the spectra obtained by PHIP and thermal prepolarization in low magnetic fields. In the following we detail how low-field PHIP spectra provide access to J and in addition to benefitting from the inherent SNR enhancement that allows for single-shot acquisition of compounds containing rare spins in natural abundan...

Measurement of untruncated nuclear spin interactions via zero- to ultralow-field nuclear magnetic resonance

“…However, the only terms of the spin-coupling Hamiltonians that may be observed in high-field NMR are those that commute with the Zeeman Hamiltonian, which effectively truncates many interaction Hamiltonians that possess different symmetry. Recently, however, NMR experiments have been carried out in the opposite regime of very small magnetic fields [2][3][4][5], taking advantage of advances in hyperpolarization [6][7][8][9] and new detection modalities [10][11][12][13][14][15][16], which offer a significant time savings compared to earlier field-cycling techniques [17,18]. In zero-to ultra-low-field NMR (ZULF NMR), the strongest interactions are the local spin-spin couplings, which involve coupling tensors that are of different symmetry from the Zeeman Hamiltonian and are many orders of magnitude smaller in amplitude, thus permitting the direct observation of nuclear spin interactions that vanish at high magnetic fields.…”

Nuclear magnetic resonance at millitesla fields using a zero-field spectrometer

“…At present, the electromagnetic coils within the shielded chamber supply DC magnetic fields up to several millitesla, providing a means to study spin phenomena around the "crossover zone" between the regimes of dominant spin-spin coupling and Larmor precession (1-100 µT), including relaxation dispersion [36,37,38] and also parahydrogen induced hyperpolarization, [5,6,39,40,41,42,43,44,45], which is strongly influenced by field-dependent level anticrossings and could be advantageously used in ZULF NMR. In future, we expect many opportunities for multidimensional experiments that correlate spin phenomena between the two regimes and make use of the large catalog of existing high-field pulsed NMR methods.…”

Scalable NMR spectroscopy with semiconductor chips

“…For example, portable systems can be used in an industrial production line to inspect the product quality (34) may also facilitate, and lower the cost of, multimodal analysis where NMR spectroscopy is combined with and complemented by other analytical methods such as liquid chromatography (12), capillary electrophoresis (13), mass spectrometry, and Fourier transform infrared spectroscopy. The NMR sensitivity of these miniaturized systems can be further enhanced by techniques such as parahydrogen polarization transfer (36).…”