At the low temperatures and small number densities typical of the interstellar medium, the interconversion of the water molecule nuclear spin isomers (NSI), o-H2O↔p-H2O, is a highly forbidden process. However, it occurs readily under confinement at temperatures as low as 4K in inert matrices, as well as within endofullerenes. The dependencies of the interconversion effective first-order rate constant (keff), as well as of the equilibrium constant (the ortho-to-para ratio, or OPR), on the temperature (T = 4K - 30K), on the water molecules isotopologue (H2AO, A = 16,17, and 18) as well as on the confinement media (Ne, Ar, Kr and Xe matrices, as well as C60) will be reported. A simple confined rotor model will be shown to provide an accurate description of rovibrational spectra for confined water revealing the topological features of the confinement potential that govern rotational-translation couplings and that are responsible for the mixing between ro-translational states thereby opening new interconversion pathways. Improving our understanding of confinement effects on the NSI interconversion mechanisms and rates should enable better storage strategies to be devised for water samples highly enriched in o-H2O en route towards surface NMR as well as interfacial chemical dynamics studies. It may also provide more robust interpretations of the NSI populations observed in the interstellar medium.

Confinement effect on the H2O nuclear spin isomer interconversion mechanism and rate

ACS Spring 2022

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