Hydrogen Spin Isomers

Nuclear spin states of H₂

Each hydrogen molecule (H₂) consists of two hydrogen atoms linked by a covalent bond. If we neglect the small proportion of deuterium and tritium which may be present, each hydrogen atom consists of one proton and one electron. The proton has an associated magnetic moment, which is associated with the proton's spin. In the H₂ molecule, the spins of the two hydrogen nuclei (protons) couple to form a triplet state (I = 1, α₁α₂, (α₁β₂ + β₁α₂)/(2^1/2), or β₁β₂ for which M_I = 1, 0, −1 respectively — this is orthohydrogen) or to form a singlet state (I = 0, (α₁β₂ – β₁α₂)/(2^1/2) M_I = 0 — this is parahydrogen). The ratio between the ortho and para forms is about 3:1 at standard temperature and pressure - a reflection of the spin degeneracy ratio, but if thermal equilibrium between the two forms is established, the para form dominates at low temperatures (approx. 99.8% at 20 K). Other molecules and functional groups containing two hydrogen atoms, such as water and methylene, also have ortho and para forms (e.g. orthowater and parawater), although their ratios differ from that of the dihydrogen molecule.