Polarization transfer via field sweeping in parahydrogen-enhanced nuclear magnetic resonance

We show that in a spin system of two magnetically inequivalent protons coupled to a heteronucleus such as ¹³C, an adiabatic magnetic field sweep, passing through zero field, transfers the proton singlet order into magnetization of the coupled heteronucleus. This effect is potentially useful in parahydrogen-enhanced nuclear magnetic resonance and is demonstrated on singlet-hyperpolarized [1-¹³C]maleic acid, which is prepared via the reaction between [1-¹³C]acetylene dicarboxylic acid and para-enriched hydrogen gas. The magnetic field sweeps are of microtesla amplitudes and have durations on the order of seconds. We show a polarization enhancement by a factor of 10⁴ in the ¹³C spectra of [1-¹³C]maleic acid in a 1.4 T magnetic field.