para-hydrogen
Quick Summary
Para-hydrogen is a form of molecular hydrogen where the two hydrogen nuclei spin in opposite directions, resulting in zero total angular momentum. This is in contrast to ortho-hydrogen, where the proton spins are aligned in the same direction. These two forms, ortho- and para-hydrogen, differ slightly in their physical properties, such as melting and boiling points.12
Para-hydrogen can be produced by bringing a mixture of hydrogen into contact with charcoal at the temperature of liquid hydrogen, converting all ortho-hydrogen to para-hydrogen. While ortho- and para-hydrogen are distinct modifications, they can interconvert under certain conditions, such as the presence of catalysts or electrical discharge.2
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