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Karl Friedrich Bonhoeffer and Paul Harteck prepared parahydrogen at low temperatures in 1929. The work made the distinction between orthohydrogen and parahydrogen experimentally accessible and connected the physical properties of molecular hydrogen with its nuclear-spin states.^[ref]

A chemist.^[ref]

Normal hydrogen denotes the $3:1$ orthohydrogen-to-parahydrogen mixture, the high-temperature equilibrium limit. Near room temperature the equilibrium ratio is already close to this limit. Cooling an isolated sample need not change its composition on the measurement timescale: ordinary collisions can redistribute rotational populations within each spin species much faster than they interconvert the spin species. A kinetically frozen composition can therefore coexist with rotational thermal equilibrium separately within each species.

The distinction was historically decisive. The institutional account Die Entdeckung des para-Wasserstoffs, in its discussion of the rotational heat capacity, explains that assuming an equilibrating composition missed the observed behavior, whereas freezing the high-temperature ratio reproduced it. It also records Karl Friedrich Bonhoeffer and Paul Harteck's 1929 preparation using a catalyst and thermal conductivity detection.^[ref]

Paul Harteck and Karl Friedrich Bonhoeffer prepared parahydrogen at low temperatures in 1929. The experiment distinguished forms of molecular hydrogen associated with different nuclear-spin states, connecting a measurable sample property with the quantum description of spin isomers of hydrogen.^[ref]

A chemist.^[ref]