For allyl pyruvate, adiabatic transfer of polarization becomes more challenging. This stems from the properties of the spin system of the molecule, namely, the extremely weak couplings between the C nucleus and the protons stemming from pH2 and only moderate couplings with the CH2 protons that relay polarization transfer. The result is a slow buildup of polarization and an increase of the sweep duration (Fig. .b). Even a sweep as long as 50 s does not produce a distinct maximum of transfer efficiency. To compare simulations with experimental data, we normalized the former to a P( 13 C) = 8 %. This corresponds to the experimentally achievable polarization, ignoring relaxation losses. A theoretical consideration gives a value of P( 13 C) = 56 % for a 3 minute long sweep, which greatly exceeds the theoretical values for a fixed field (P( 13 C) = 6.4 %) and for coherent transfer (P( 13 C) = 6.4 %). However, the experimentally observed polarization level in adiabatic approach is only P( 13 C) = 6.8 %, well below the predicted value.