Hydrogen spin isomerism

As in the case of hydrogen and tritium, deuterium exhibits nuclear spin isomerism (see Magnetic spin resonance) (14). However, the spin of the deuteron [12597-73-8] is 1 instead of S as in the case of hydrogen and tritium. As a consequence, and in contrast to hydrogen, the ortho form of deuterium is more stable than the para form at low temperatures, and at normal temperatures the ratio of ortho- to para-deuterium is 2 1 in contrast to the 3 1 ratio for hydrogen. 

Ortho-Para Tritium. As in the case of molecular hydrogen, molecular tritium exhibits nuclear spin isomerism. The spin of the tritium nucleus is S, the same as that for the hydrogen nucleus, and therefore H2 and T2 obey the same nuclear isomeric statistics (16). Below 5 K, molecular tritium is... 

Most physical properties are but little affected by nuclear-spin isomerism though the thermal conductivity of P-H2 is more than 50% greater than that of 0-H2, and this forms a ready means of analysing mixtures. The mp of P-H2 (containing only 0.21% (3-H2) is 0.15 K below that of normal hydrogen (containing 75% 0-H2), and by extrapolation the mp of (unobtainable) pure... 

At one time Taylor considered7 that if adsorption on a solid surface would catalyse the interconversion of ortho and para hydrogen, it was an indication that chemisorption with dissociation into atoms occurred the temperature at which such interconversion took place would (if this were true) have been some indication of the activation energy for chemisorption. No doubt chemisorption does promote this spin isomerization but it is now known that adsorption of the van der Waals type can result in speedy attainment of the equilibrium mixture of ortho and para hydrogen, provided that the surfaces are of some paramagnetic substance the close proximity to a paramagnetic surface can catalyse the interconversion,8 without dissociation of the adsorbed molecules. 

Ah diatomic molecnles for which the constituent nnclei have spin exhibit the phenomenon of spin isomerism. The nnclear spins can be parallel (the ortho isomer) or opposed (the para isomer). For most diatomic molecules, which might be expected to exhibit, spin isomerism the energy separation of the rotational states is small compared to kT, even at low temperatures. However, in the case of hydrogen molecules, which have the smallest moment of inertia of any diatomic molecnles, the energy difference between the rotational energy levels is relatively large and only the lowest states are popnlated at room temperature. 

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. 

Enzymatically active NADH has been selectively produced by visible light photoreduction of NAD using [Ru(bipy)3]S04 and [Ru(bipy)3]2(S04)3 as sensitizers and triethanolamine as electron donor (Wienkamp and Steckhan). There is continuing interest in the photogeneration of co-ordinatedly unsaturated species from metal carbonyls etc. which can act as or give rise to catalysts, e.g., for cis-trans isomerization and hydrogenation of alkenes. Ger-rity et al. have used chromium hexacarbonyl to make the first quantitative measurements of the distribution of atomic excited states produced by multiphoton dissociation of a metal carbonyl. The distribution of states turns out to be statistical rather than spin- or polarity-difierentiated. The use of perfluoromethylcyclohexane as solvent has enabled Simon and Peters to observe naked Cr(CO)s as a transient from Cr(CO)6. 

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