Branching-chain propagation of parahydrogen-derived nuclear spin order on a catalyst surface

When a parahydrogen molecule dissociates on a surface of a heterogeneous catalyst (e.g., of a metal nanoparticle), the correlation of the nuclear spins initially inherited by the two surface H atoms may be shared with other surface hydrogens as they diffuse and combine with random H atoms to produce H2 molecules which subsequently dissociate. This branching-chain-type propagation of nuclear spin order leads to its gradual dilution but at the same time is accompanied by an increase in the number of H atoms that share nuclear spin order. These conclusions, confirmed by the spin density matrix calculations, may be relevant in the context of parahydrogen-induced polarization (PHIP) in heterogeneous hydrogenations catalyzed by supported metal catalysts, observation of which apparently contradicts the accepted non-pairwise mechanism of hydrogen addition to an unsaturated substrate over such catalysts. The potential consequences of the reported findings are discussed in the context of PHIP effects and beyond.