The hydrogenation reaction was then monitored in a pressurizable 5 mm glass NMR tube at three different hydrogen pressures by acquiring a spectrum every 5 s while continuously bubbling para-enriched hydrogen gas into a 400 μl reaction solution at 50 ml/min. The time dependences of the dimethyl maleate and dimethyl succinate peak integrals are plotted in Fig. , and show signal build-up and decay for both reaction products. The signals decay as the hyperpolarized nuclear spins relax to thermal equilibrium; the relaxation time constants were measured from the peak widths to be 0.7 s for maleate, and 0.5 s for succinate. Given the proximity of the 1 H and 13 C spins in space, the relatively rapid relaxation is likely dominated by the fluctuating intra-pair dipole-dipole coupling. The maleate signals additionally decay as the species is further hydrogenated into succinate. It should be noted that the application of the magnetic field pulse in order to observe the reaction products destroys the signals, and so neglecting relaxation effects, the signal seen in each scan corresponds only to the product formed after the previous pulse.