Before comparing the performance of the pulse sequences of interest, we optimized the experimental parameters, such as the delays and and the number of cycles . The 𝑡 𝑑 𝑡 𝑐𝑤 𝑛 dependence of the enhancement on these parameters is presented in ESI. In the experiments presented below we always set ms, ms and , which provide 𝑡 𝑑 = 500 𝑡 𝑐𝑤 = 39 𝑛 = 50 substantially improved signal enhancements. To optimize the performance of the pulse sequence, it is necessary to set the optimum resonance offset for the Δ pseudo-cw pulse. As pointed out above, the optimization is Please do not adjust margins Please do not adjust margins different for a single pulse and for a pulse followed by an additional 90 pulse. This is indeed the case, see Figure . When no extra pulses are used, the resulting longitudinal N polarization vanishes for (the spins are polarized parallel Δ = 0 to the effective field, and therefore do not have any longitudinal component). When an additional 90 pulse is used to convert the transverse polarization into longitudinal polarization, the resulting polarization is maximal for . The -dependence Δ = 0 Δ of polarization shows positive and negative extrema, see Figure , corresponding to matching of the energy levels of the AA′ MM spin system in the rotating frame, as explained before . To ′ make the pulse sequences work one should also optimize the value : when is very small, spin mixing is inefficient, 𝜔 1 𝜔 1 whereas if is much larger than the relevant J-couplings, the 𝜔 1 matching conditions can no longer be fulfilled (this is the reason for using a low-intensity cw rf pulse). In the experimental 𝜔 1 dependence of the enhancement, see Figure , the peak corresponds to the matching condition.