Inherent loss of parahydrogen-induced polarization for systems with magnetically equivalent nuclei in magnetic field cycling experiments

In the present work, we elucidate the inherent loss of net magnetization (⟨Iz⟩) in parahydrogen-induced polarization (PHIP) experiments with magnetic field cycling (MFC) for spin systems containing magnetically equivalent protons. The effects are shown for propane and diethyl ether as representative examples of potential hyperpolarized MRI contrast agents, but the findings of this work are equally applicable to other multispin systems in the liquid or gas phase. These results are relevant to both adiabatic longitudinal transport after dissociation engenders net alignment (ALTADENA) experiments (where 1H nuclei are polarized) and MFC protocols used to transfer parahydrogen spin order to a heteronucleus such as 13C. The investigated effects should be incorporated for a correct evaluation of both the maximum possible NMR signal enhancement and the pairwise selectivity, which are useful in the context of mechanistic studies in the field of catalytic hydrogenation. Among signal enhancement damping factors in ALTADENA, such as T1 relaxation and insufficient adiabaticity of a field sweep, the inherent loss of net magnetization in spin systems containing magnetically equivalent protons (especially in PHIP systems commonly used for mechanistic studies such as propene or propane) has not been thoroughly considered and needs to be clarified. The maximum possible net magnetization in ALTADENA for diethyl ether and propane was shown to be ∑|⟨Iiz⟩| ≈ 0.56 for diethyl ether and ∑|⟨Iiz⟩| ≈ 0.45 for propane, respectively. The inherent loss of net heteronuclear magnetization of the same order of magnitude with an increase in the number of magnetically equivalent protons was also demonstrated for AmMnX-type spin systems.