<p>A: Each type of hydrogen molecule consists of 2 hydrogen atoms (spheres) connected by a covalent bond (connecting bar). Light arrows indicated direction of nuclear spin; heavy arrows penetrating spheres show direction of spin axis. In orthohydrogen, the less stable type at lower temperatures, the spin axes point in the same direction; in parahydrogen the axes are directly opposed.
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B: Catalyst is a porous ferromagnetic material, hydrous ferric oxide, through which normal hydrogen (75 percent ortho, 25 percent para) is made to pass. The magnetic field within the small pores, one of which is prepresented here (dotted lines are lines of magnetic force), varies rapidly from one point to another. The inhomogenous field interacts with the magnetic moments of the hydrogen atoms and produces internal "strains" which disturb the balance within the unstable ortho molecules.One is shown here (at left) being symbolically "twisted" by the action of the field. The actual process is governed by quantum rules and the transition to the stable para structure is sudden. Another possible mechanism, for which the evidence is more definite at higher temperatures, is shown symbolically at the right. Here the ortho molecules dissociate as they are absorbed on the catalyst surfave, and then the atoms can recombine into a stable paramolecule as they are desorbed.
National Institute of Standards and Technology
National Institute of Standards and Technology