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.