Parahydrogen-enhanced zero-field nuclear magnetic resonance

Journal name:
Nature Physics
Volume:
7,
Pages:
571–575
Year published:
DOI:
doi:10.1038/nphys1986
Received
Accepted
Published online

Abstract

Nuclear magnetic resonance, conventionally detected in magnetic fields of several tesla, is a powerful analytical tool for the determination of molecular identity, structure and function. With the advent of prepolarization methods and detection schemes using atomic magnetometers or superconducting quantum interference devices, interest in NMR in fields comparable to the Earth’s magnetic field and below (down to zero field) has been revived. Despite the use of superconducting quantum interference devices or atomic magnetometers, low-field NMR typically suffers from low sensitivity compared with conventional high-field NMR. Here we demonstrate direct detection of zero-field NMR signals generated through parahydrogen-induced polarization, enabling high-resolution NMR without the use of any magnets. The sensitivity is sufficient to observe spectra exhibiting 13C–1H scalar nuclear spin–spin couplings (known as J couplings) in compounds with 13C in natural abundance, without the need for signal averaging. The resulting spectra show distinct features that aid chemical fingerprinting.

At a glance

Figures

  1. Scheme for detecting parahydrogen-induced polarization at zero magnetic field.
    Figure 1: Scheme for detecting parahydrogen-induced polarization at zero magnetic field.

    a, The experimental set-up. A microfabricated alkali vapour cell is mounted inside a set of coils used for applying magnetic-field pulses. The alkali vapour is optically pumped with a circularly polarized laser beam, resonant with the D1 transition of 87Rb. A linearly polarized laser beam, tuned about 100GHz off resonance, is used to probe the alkali spin precession. The magnetometer is primarily sensitive to magnetic fields in the vertical (z) direction. A 7-mm-inner-diameter glass tube contains the sample, and a 1/32”-inner-diameter Teflon tube is used to bubble parahydrogen through the solution. A set of magnetic shields surrounding the magnetometer, not shown, isolates the magnetometer from external magnetic fields. b, The magnetic-field noise spectrum of the magnetometer. Above 100Hz, the noise floor is about 0.15nGHz−1/2. c, The experimental pulse sequence.

  2. Single-shot zero-field PHIP J-spectra (imaginary component).
    Figure 2: Single-shot zero-field PHIP J-spectra (imaginary component).

    a,b, Ethylbenzene-β13C (a) and ethylbenzene-α13C (b), polarized through addition of parahydrogen to labelled styrene. The inset shows the ethylbenzene molecule with the β and α positions indicated by the blue and green carbons, respectively. The blue and green traces in a and b, respectively, are the results of numerical simulations, described in the text.

  3. Zero-field J spectrum (imaginary component) of ethylbenzene, produced through parahydrogenation of styrene with 13C in natural abundance.
    Figure 3: Zero-field J spectrum (imaginary component) of ethylbenzene, produced through parahydrogenation of styrene with 13C in natural abundance.

    These data result from averaging eight transients following a pulse of magnetic field in the z direction with ηπ/2. The high-frequency components of the signals arising from the α and β isotopomers are easily recognizable from the spectra shown in Fig. 2, and are highlighted by the green and blue bands, respectively. The signal in the neighbourhood of 156Hz is due to isotopomers with 13C on the benzene ring, and is highlighted in red.

  4. Zero-field PHIP spectra for several compounds.
    Figure 4: Zero-field PHIP spectra for several compounds.

    a, Parahydrogen is added to 1-phenyl-1-propyne, labelled with 13C in the CH3 group. b, Parahydrogen is added to acetylene dimethylcarboxylate with 13C in natural abundance. c, Parahydrogen is added to 3-hexyne with 13C in natural abundance. In a and b the imaginary component is presented; in c the magnitude is presented.

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Affiliations

  1. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • T. Theis,
    • P. Ganssle,
    • G. Kervern &
    • A. Pines
  2. Department of Chemistry, University of California at Berkeley, Berkeley, California 94720-3220, USA

    • T. Theis,
    • P. Ganssle,
    • G. Kervern &
    • A. Pines
  3. Time and Frequency Division, National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA

    • S. Knappe &
    • J. Kitching
  4. Department of Physics, University of California at Berkeley, Berkeley, California 94720-7300, USA

    • M. P. Ledbetter &
    • D. Budker
  5. Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • D. Budker

Contributions

T.T. designed research, carried out experiments and wrote the paper. P.G. contributed to construction of the experiment. G.K. carried out experiments and theoretical analysis. S.K. and J.K. provided the microfabricated vapour cell. M.P.L. built the experiment, designed research, carried out experiments, simulations and theoretical analysis and wrote the paper. D.B. designed research and wrote the paper. A.P. designed research and wrote the paper.

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