Para-ortho hydrogen interconversion

A few solutions have been found to possess the unusual property of activating molecular hydrogen so as to bring about reduction of organic or inorganic compounds or to cause H2-D2 exchange or ortho-para-hydrogen interconversion. Such catalytic systems have been termed homo-... 

Isotope exchange reactions can be used in the same way as the ortho-para hydrogen interconversion, and the results may be interpreted more readily. The obvious reaction to compare with the ortho-para exchange is... 

A fourth assay measures the rate of interconversion of ortho and para hydrogen. Since any type of rupture of the hydrogen-hydrogen bond that permits random recombination will be detected by this assay, it was suggested that this is the most specific measurement of hydrogenase. However, the finding that a D2O medium substitutes the exchange reaction (HD formation) for the ortho-para conversion indicates that... 

The hydrogen nucleus is classified as a Eermi particle with nuclear spin I = 1/2. Because of Pauli exclusion principle, hydrogen molecule is classified into two species, ortho and para. Erom the symmetry analysis of the wave functions, para-hydrogen is defined to have even rotational quantum number J with a singlet nuclear spin function, and ortho-hydrogen is defined to have odd J with a triplet nuclear spin function. The interconversion between para and ortho species is extremely slow without the existence of external magnetic perturbation. 

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. 

The origins of symmetry induced nuclear polarization can be summarized as follows as mentioned above molecular dihydrogen is composed of two species, para-H2, which is characterized by the product of a symmetric rotational wave-function and an antisymmetric nuclear spin wave function and ortho-H2, which is characterized by an antisymmetric rotational and one of the symmetric nuclear spin wavefunctions. In thermal equilibrium at room temperature each of the three ortho-states and the single para-state have practically all equal probability. In contrast, at temperatures below liquid nitrogen mainly the energetically lower para-state is populated. Therefore, an enrichment of the para-state and even the separation of the two species can be easily achieved at low temperatures as their interconversion is a rather slow process. Pure para-H2 is stable even in liquid solutions and para-H2 enriched hydrogen can be stored and used subsequently for hydrogenation reactions [54]. 

To confirm the mechanism proposed for the interconversion of para and ortho hydrogen in the gas phase and to calculate rate constants. 

A good example of a change initiated by free atoms is the spontaneous interconversion of ortho- and para-hydrogen which occurs in the gas phase at a red heat. The mechanism depends upon a dissociation followed by an exchange of atomic partners thus ... 

The proportions x and y depend on the type of hydrogenase and the pH (13). Hydrogenases also catalyze the interconversion of ortho- and para-hydrogen, which differ in the relative orientations of the proton spins (13). The exchange reactions do not require electron transfer to an external electron acceptor. They show that the enzymes catalyze heterolytic cleavage of H2 into a hydron (proton), H+ and hydride, H . 

Figure 4.2 represents the simplest construction consistent with this information, and is the mechanism most widely accepted for Rh asymmetric hydrogenation. It still needs to be treated with some caution because of the dynamic nature of the ground state, and the possibility for direct interconversion of enamide diastereomers without dissociation. For example, the kinetics and spectroscopic observations cannot rule out an alternative in which hydrogen added to a part-dissociated enamide complex, which reverted to the preferred intermediate by rapid olefin dissociation-recombination prior to internal hydride transfer. If hydrogen adds reversibly to the solvate complex (which is present at low concentration) to produce a transient / -intermediate without interconversion of ortho- and para-Hj, then this must react irreversibly with substrate in the rate-determining step to accord with the observed kinetics. These alternative possibilities can only be discriminated by further experiment. 

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