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H. Diamond coauthored research on isomerization.^[ref]

Interconversion of isomers without changing the overall molecular formula. Conformational interconversion can be kinetically slow when rotation is hindered.

Hugh Stott Taylor coauthored measurements of hydrogen spin-isomer conversion at paramagnetic and diamagnetic surfaces. Ortho-para conversion changes nuclear-spin and rotational populations without changing the molecule's elemental composition.

A surface can provide a conversion pathway that is slow in the isolated gas. The resulting time-dependent composition matters when interpreting the heat capacity or cooling of hydrogen.^[ref]

A chemist.^[ref]

The 1935 primary paper The Spin-Isomerization of Hydrogen at Paramagnetic and Diamagnetic Surfaces, pp. 1251–1252, describes evacuated catalyst samples cooled before admitting molecular hydrogen. Gas samples were analyzed in a glass cell in liquid air: a heated platinum wire and Wheatstone bridge converted changes in thermal conductivity into electrical resistance readings, calibrated against normal hydrogen.^[ref]

Its comparison of oxide surfaces supplied a control: specimens exhibiting strong paramagnetism converted the gas much faster than the comparison exhibiting diamagnetism despite all adsorbing molecular hydrogen. Contact time, pressure, and temperature were recorded. This supports a role for surface magnetic properties beyond adsorption alone; it does not identify every microscopic surface site or directly measure nuclear-spin orientations.^[ref]

The inference chain is preparation, contact with a specified surface, then a calibrated bulk transport readout of composition. It is distinct from a direct calorimetry measurement of heat capacity: transport and heat capacity can both depend on the spin populations, but they are different measured quantities. Changing the catalyst's own surface structure, introducing magnetic impurities, or changing the detector's gas pressure can confound an uncalibrated comparison. The free-rotor partition functions predict equilibrium populations and stored energy, not the conversion rate on a particular surface.