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Spin isomers of hydrogen
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Each hydrogen molecule (H2) consists of two hydrogen atoms linked by a covalent bond. If we neglect the traces of deuterium and tritium which could 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. The spins of the two hydrogen nuclei can couple to form a triplet state (I = 1, α1α2, (α1β2 + β1α2)/21/2, or β1β2 for which MI = 1, 0, -1 respectively – this is orthohydrogen) or to form a singlet state (I = 0, (α1β2 – β1α2)/21/2 MI = 0 – this is parahydrogen).
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Each hydrogen molecule (H2) consists of two hydrogen atoms linked by a covalent bond. If we neglect the traces of deuterium and tritium which could 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. The spins of the two hydrogen nuclei can couple to form a triplet state (I = 1, α1α2, (α1β2 + β1α2)/21/2, or β1β2 for which MI = 1, 0, -1 respectively – this is orthohydrogen) or to form a singlet state (I = 0, (α1β2 – β1α2)/21/2 MI = 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. 99.95% at 20 K). Other molecules and functional groups containing two hydrogen atoms, such as water and methylene, also have ortho and para forms, although their ratios differ from that of the dihydrogen molecule.
The permutational antisymmetry of the H2 wavefunction (protons are fermions) imposes restrictions on the possible rotational states the two forms of hydrogen can adopt. Orthohydrogen, with symmetric nuclear spin functions, can only have rotational wavefunctions that are antisymmetric with respect to permutation of the two protons. Conversely, parahydrogen with an antisymmetric nuclear spin function, can only have rotational wavefunctions that are symmetric with respect to permutation of the two protons. Because of this symmetry-imposed restriction, orthohydrogen has residual rotational energy at low temperature (it can not fall into the lowest, symmetric rotational level) and possesses nuclear-spin entropy due to the triplet state's threefold degeneracy. Orthohydrogen is therefore unstable at low temperatures and spontaneously converts into parahydrogen, but the process is slow in the absence of a magnetic catalyst to facilitate interconversion of the singlet and triplet spin states. The conversion from ortho to para state is exothermic (releasing heat). The presence of a magnetically ordered substance in liquid hydrogen can induce rapid heating - an undesirable occurrence when one wants hydrogen to remain liquid. At room temperature, hydrogen contains 75% orthohydrogen, a proportion which the liquefaction process preserves. One must therefore use a catalyst like ferric oxide, activated carbon, platinized asbestos, rare earth metals, uranium compounds,
chromic oxide, or some nickel compounds to accelerate the conversion of the liquid hydrogen into parahydrogen, or supply additional refrigeration equipment to absorb the heat that the liquid hydrogen will give off as it spontaneously converts itself to pure parahydrogen.
The first synthesis of pure parahydrogen was achieved by Paul Harteck and Karl Friedrich Bonhoeffer in 1929.
When used during hydrogenations, parahydrogen gives rise to abnormally high signals in the NMR spectrum. This effect is called PHIP or PASADENA effect and was simultaneously discovered at two laboratories in Los Angeles (USA) and Bonn (Germany) in 1995. It was subsequently utilized to study the mechanism of hydrogenation reactions.
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