Para-ortho H2 conversion in collisions with NO: A surprising mechanism

Molecular hydrogen exists in two kinds, para-H2 and ortho-H2, which act as distinct molecules in many physical and chemical processes. Their interconversion is extremely slow but can be catalyzed by collisions with paramagnetic molecules such as O2 and NO. Temperature-dependent rate coefficients for para-ortho H2 conversion in gas-phase collisions with O2, NO, and NO2 were measured in 1933 in a beautiful series of experiments by Farkas and Sachsse [Z. Phys. Chem. B 23, 1-18 (1933)]. A complete and quantitative theoretical study of the conversion rate coefficients in collisions with O2 was published recently [X. Zhang et al., Nat. Sci. 1, e10002 (2021)]. The present paper describes a similar, although more difficult, study of para-ortho H2 conversion in collisions with NO. Not only does it yield conversion rate coefficients in good agreement with the experimental data, but it also explains the surprising finding that collisions with NO are substantially more effective in converting para-H2 into ortho-H2 than collisions with O2. It shows, moreover, that the effectivity of NO is caused by a completely unexpected mechanism: coupling with the transient magnetic dipole of NO generated by the transition of its excited spin-orbit state to the ground state in collisions in which para-H2 near-resonantly converts into ortho-H2.