Hydrogen spin isomers

Despite its very simple electronic configuration (Is ) hydrogen can, paradoxically, exist in over 50 different forms most of which have been well characterized. This multiplicity of forms arises firstly from the existence of atomic, molecular and ionized species in the gas phase H, H2, H+, H , H2" ", H3+. .., H11 + secondly, from the existence of three isotopes, jH, jH(D) and jH(T), and correspondingly of D, D2, HD, DT, etc. and, finally, from the existence of nuclear spin isomers for the homonuclear diatomic species. 

As already mentioned, molecular hydrogen consists of two spin isomers, one of which has a total spin of 1=0 (singlet), and the other a total spin of 1=1 (triplet). The first is named parahydrogen, the latter orthohydrogen (Figure 9.8). 

All homonuclear diatomic molecules having nuclides with non-zero spin are expected to show nuclear spin isomers. The effect was first detected in dihydrogen where it is particularly noticeable, and it has also been established for D2, T2, N2, N2, Oj, etc. When the two nuclear spins are parallel (ort/jo-hydrogen) the resultant nuclear spin quantum number is 1 (i.e. 5 -b j) and the state is threefold degenerate (2S -(-... 

The separation of nuclear spin isomers and isotopes of hydrogen was the first prominent example of a practical application of capillary adsorption chromatography (Figure 1). Later, it was shown that the time required for the separation of protium and deuterium could be shortened by decreasing the column temperature to 47 K. 

Rgure 1 Chromatogram of the separation of nuclear spin isomers and isotopes of hydrogen in an open tubular adsorption glass capillary column. The 80 m long open tubular capillary column contained a 20 mm thick silica layer as an adsorbent 7= 77.4 K neon was used as the carrier gas. 1, helium 2, p-prolium 3, o-prolium 4, DH 5, o-deuterium 6, p-deuterium. 

Fig. 1-2. Chromatogram of separation for hydrogen nuclear-spin isomers and isotopes on a capillary adsorption glass column [16].

With ortho-hydrogen (0-H2 parallel nuclear spin) and para-hydrogen (p-Hj anti-parallel nuclear spin) hydrogen disposes of two nuclear spin isomers. At room temperature, the equihbrium fraction of p-Hj is 25%. This mixture is... 

As PHIP effects in the products are the direct measure of the deviation of the o p ratio of the nuclear spin isomers of H2 from the statistical 3 1 value, the influence of the substrate on the reversibility of oxidative hydrogen addition to the metal center can be studied, as demonstrated for Wilkinson s catalyst and a cationic Rh complex in the presence of phenylacetylene [12,45]. 

Pure para-hydrogen was first prepared in 1929. Although hydrogen seems to be the molecule that has been studied extensively, in principle all polyatomic molecules can exist as nuclear spin isomers. 

Another advanced NMR technique is para-hydrogen-induced polarization (PHIP) spectroscopy. Dihydrogen consists of two nuclear spin isomers, one of which has a total spin of zero (I = 0) and is called para-hydrogen. The other spin isomer has a total spin of one (/ = 1) and is called ortho-hydrogon. At room temperature, dihydrogen is a mixture of about 25% para- and 75% ortho-hydrogen. 

These two isomeric forms of R2 are physically different because of the pairing of the nuclear spins on the hydrogen nnclei in the molecule. They can be physically separated by chromatography at very low temperatures. Generally to convert one isomer to the other requires a cleavage of the R-R bond. As with the hydrogen-isotope-exchange reactions, no external acceptor or donor is required. 

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