Direct ¹H NMR evidence of spin-rotation coupling as a source of para → ortho-H₂ conversion in diamagnetic solvents

At ambient temperature, conversion from 100% enriched para-hydrogen (p-H₂; singlet state) to ortho-hydrogen (o-H₂; triplet state) leads necessarily to the thermodynamic equilibrium proportions: 75% of o-H₂ and 25% of p-H₂. When p-H₂ is dissolved in a diamagnetic organic solvent, conversion is very slow and can be considered as arising from nuclear spin relaxation phenomena. A first relaxation mechanism, specific to the singlet state and involving a combination of auto-correlation and cross correlation spectral densities, can be retained: randomly fluctuating magnetic fields due to inter-molecular dipolar interactions. We demonstrate here that (i) this dipolar mechanism is not sufficient for accounting for the para→ortho conversion rate, (ii) spin-rotation interaction, an intra-molecular mechanism, behaves similarly to random-field interaction and, thus, may be involved in the singlet relaxation rate. Also, as the para→ortho conversion is monitored by proton nuclear magnetic resonance (NMR) of dissolved o-H₂ (p-H₂ is NMR-silent), one has to account for H₂ exchange between the liquid phase and the gas phase within the NMR tube, as well as for dissolution effects. Experimental evidence of the above statements is brought here in the case of two organic solvents: acetone-d₆ and carbon disulfide. The observed temperature dependence of the para→ortho conversion rate shows that spin-rotation can be the dominant contribution to the p-H₂ relaxation rate in the absence of tangible dipolar interactions. Our findings shed new light on the "mysterious" mechanism of the para→ortho conversion which has been searched for several decades.