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It is well known that the association of parahydrogen (pH2) with anunsaturated molecule or a transient metalorganic complex can enhance theintensity of NMR signals; the effect is known as parahydrogen-inducedpolarization (PHIP). During the last decades, numerous methods were proposedfor converting pH2-derived nuclear spin order to the observable magnetizationof pro-tons or other nuclei of interest, usually 13C or 15N. Here, we analyzethe constraints imposed by the topological symmetry of the spin systems on theamplitude of transferred polarization. In asymmetric systems, heteronuclei canbe polarized to 100%. However, the amplitude drops to 75% in A2BX systems andfurther to 50% in A3B2X systems. The latter case is of primary importance forbiological applications of PHIP using sidearm hydrogenation (PHIP-SAH). If thepolarization is transferred to the same type of nuclei, i.e. 1H, symmetryconstraints impose significant boundaries on the spin-order distribution. ForAB, A2B, A3B, A2B2, AA'(AA') systems, the maxi-mum average polarization foreach spin is 100%, 50%, 33.3%, 25%, and 0, respectively, when A and B (or A')came from pH2 We also discuss the effect of dipole-dipole induced pH2spin-order distribution in heterogeneous catalysis or nematic liquid crystals.Practical examples from the literature illustrate our theoretical analysis.

Symmetry constraints on spin order transfer in parahydrogen-induced polarization (PHIP)