NMR experiments were mostly carried out at high magnetic fields using the protocols shown in Figure . To polarize the N nuclei we used pulse sequences with radiofrequency (rf) pulses applied to the N channel. For transferring spin order from pH 2 essentially a single long N pulse of a low intensity is used, hereafter called pseudo continuous-wave ("pseudo-cw") pulse. When the parameters of the rf field applied to the N channel are properly set, proton singlet spin order is transferred to N spins in the SABRE complex. Specifically, the effective field 𝜔 𝑒𝑓𝑓 (given in the frequency units) should be matched to a certain combination of scalar J-couplings in the AA MM system. ′ ′ The effective field is given by the vector sum of the transverse rf-field (in the rf-rotating frame) and the longitudinal field given by offset from resonance where is the rf-𝜔 𝑒𝑓𝑓 = 𝜔 1 + Δ 2 𝜔 1 field strength and the resonance offset. A more detailed Δ discussion of the definition of for the system under study is Δ given below. In the case of single-resonance experiments (excitation only on the N channel) the source of polarization is given by the population difference of the singlet and central triplet states of H 2 . As we show below, this feature is critical for the performance of the pulse sequences. The magnetization transferred to the N nuclei is parallel to the effective field. Hence, it is a purely transverse magnetization when (on-resonance excitation); whereas for the Δ = 0 Δ ≠ 0 magnetization has a longitudinal component. In SABRE, one seeks to generate polarization of the free substrate molecule via chemical exchange. Upon exchange, longitudinal polarization of the bound species is transferred to the free substrate pool. Thus, in the case of resonant excitation, an additional 90 pulse should be inserted to convert transverse into longitudinal polarization; when this may be not Δ ≠ 0 necessary. To maximize N signal enhancement, the polarization transfer cycle is repeated times. For this reason, 𝑛 the additional 90 pulse should be selective, exciting only the bound substrate, but not its free form in solution. Finally, after applying a 90 pulse, the N Free Induction Decay (FID) signal is Please do not adjust margins Please do not adjust margins acquired (its Fourier transform gives the NMR spectrum). Two of the pulse sequences shown in Figure exploit resonant rfexcitation (Fig. ), whereas the other two make use of offresonant excitation (Fig. ). The sequence in Fig. b is known as LIGHT-SABRE (Low-Irradiation Generation of High Tesla-SABRE). Two of the pulse sequences (Fig. ) in Figure comprise a modification which would be meaningless if we were dealing only with pure rotation-invariant singlet order: this is a 90 pulse applied to the proton channel. It turns out that this pulse strongly affects the performance of the polarization transfer experiment if some hyperpolarized oH 2 is generated. In this work, we analyse the effect of proton pulses with arbitrary nutation angles on the N NMR signal enhancement. The pulse sequences given in Fig. c and d are known as SLIC-SABRE (SLIC = Spin-Locking Induced Crossing ). In addition to high-field NMR experiments, we also performed field-cycling NMR studies, in which polarization was allowed to build up by SABRE at an ultralow magnetic field (in the range 10 nT < B 0 < 1 µT). Subsequently the hyperpolarized sample is transferred to an 400 MHz NMR spectrometer ( T.) To 𝐵 0 = 9.4 run such experiments, we used a home-built device for sample shuttling with a set of coils inside the magnetic shield, as described before . after applying a 90 pulse. The delay between the cycles is equal to 𝑡 𝑑 and the duration of the 15 N pseudo cw pulse is denoted as . 𝑡 𝑐𝑤 All NMR spectra were recorded using a 400 MHz Bruker NMR spectrometer. In all high-field SABRE experiments the paracomponent of H 2 was enriched to 85% by using a commercial Bruker parahydrogen generator. In ultralow field experiments we have used 95% enriched pH 2 , obtained by cooling down H 2 in a helium cryostat CFA-200-H2CELL (CryoPribor). The pH 2 bubbling pressure was equal to 2 bar. The signal enhancement factor (ratio of the hyperpolarized N NMR signal and thermal 𝜀 signal both measured at 9.4 tesla) gives a measure of polarization.