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

TL;DR

This paper investigates how the symmetry of spin systems constrains the maximum achievable polarization transfer in parahydrogen-induced polarization (PHIP), with implications for optimizing NMR signal enhancement in various molecular systems.

Contribution

It provides a theoretical analysis of symmetry-imposed limits on spin order transfer in PHIP, including specific maximum polarization values for different spin system topologies.

Findings

Maximum polarization for heteronuclei can reach 100%.

Symmetry constraints limit proton polarization transfer to 50% in A2B systems.

In A3B systems, the maximum polarization drops to 33.3%.

Abstract

It is well known that the association of parahydrogen (pH2) with an unsaturated molecule or a transient metalorganic complex can enhance the intensity of NMR signals; the effect is known as parahydrogen-induced polarization (PHIP). During the last decades, numerous methods were proposed for converting pH2-derived nuclear spin order to the observable magnetization of pro-tons or other nuclei of interest, usually 13C or 15N. Here, we analyze the constraints imposed by the topological symmetry of the spin systems on the amplitude of transferred polarization. In asymmetric systems, heteronuclei can be polarized to 100%. However, the amplitude drops to 75% in A2BX systems and further to 50% in A3B2X systems. The latter case is of primary importance for biological applications of PHIP using sidearm hydrogenation (PHIP-SAH). If the polarization is transferred to the same type of nuclei, i.e. 1H, symmetry constraints impose significant boundaries on the spin-order distribution. For AB, A2B, A3B, A2B2, AA'(AA') systems, the maxi-mum average polarization for each 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 pH2 spin-order distribution in heterogeneous catalysis or nematic liquid crystals. Practical examples from the literature illustrate our theoretical analysis.