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Molecular hydrogen occurs in two isomeric forms, one with its two proton spins aligned parallel (orthohydrogen), the other with its two proton spins aligned antiparallel (parahydrogen).〔P. Atkins and J. de Paula, Atkins' ''Physical Chemistry'', 8th edition (W.H.Freeman 2006), p. 451–2 ISBN 0-7167-8759-8〕 Parahydrogen is in a lower energy state than is orthohydrogen. At room temperature and thermal equilibrium, thermal excitation causes hydrogen to consist of approximately 75% orthohydrogen and 25% parahydrogen. After hydrogen is liquified, there is a slow spontaneous transition to a predominantly para ratio, with the released energy having implications for storage. ==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. Each proton has an associated magnetic moment, which is associated with the proton's spin of 1/2. In the H2 molecule, the spins of the two hydrogen nuclei (protons) couple to form a triplet state known as orthohydrogen, and a singlet state known as parahydrogen. The triplet orthohydrogen state has total nuclear spin I = 1 so that the component along a defined axis can have the three values MI = 1, 0, or −1. The corresponding nuclear spin wavefunctions are and (in standard bra–ket notation). Each orthohydrogen energy level then has a (nuclear) spin degeneracy of three, meaning that it corresponds to three states of the same energy, although this degeneracy can be broken by a magnetic field. The singlet parahydrogen state has nuclear spin quantum numbers I = 0 and MI = 0, with wavefunction . Since there is only one possibility, each parahydrogen level has a spin degeneracy of one and is said to be non degenerate. The ratio between the ortho and para forms is about 3:1 at standard temperature and pressure – a reflection of the ratio of spin degeneracies. However if chemical equilibrium between the two forms is established, the para form dominates at low temperatures (approx. 99.8% at 20 K).〔Rock, Peter A., ''Chemical thermodynamics; principles and applications'' (Macmillan 1969) Table p.478 shows (No/Np)H2 = 0.002 at 20K ISBN 1891389327〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「spin isomers of hydrogen」の詳細全文を読む スポンサード リンク
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