Ortho-para conversion

Interconversion of orthohydrogen and parahydrogen, which changes the coupled nuclear-spin and rotational populations. An active conversion mechanism allows the mole fractions to follow equilibrium as temperature changes; slow conversion instead gives a frozen ortho-para mixture. Whether conversion remains active during a thermal measurement must therefore be distinguished from how the initial sample was prepared.

For hydrogen approximated as a linear rigid rotor, the $\ell =1$ to $\ell =0$ energy decrease is ${\hslash }^2/I$, with moment of inertia $I\simeq m_p{r}^2/2$. A sample of mass $M$ initially containing an orthohydrogen fraction $x_o$ therefore releases $Q\simeq x_{o}M{\hslash }^2/(m_p^2{r}^2)$ on complete conversion to the lowest parahydrogen level. This estimate neglects the electron contribution to molecular mass and changes of the rotational constant with environment. It describes an internal heat load that external insulation cannot remove.