Paramagnetic para-hydrogen conversion

Because of the scarcity of electronic paramagnetic resonance data, and because of the frequent unreliability of the data from paramagnetism, boiling point elevation, spectrophotometry, and ortho-para hydrogen conversion, most published radical dissociation constants can be accepted only with reservations. An error of 50 % is not at all improbable in many cases. We are therefore not yet in a position to explain, or rather to test our explanations of, small differences in dissociation constants. Table I shows the values of K corresponding to various hexaarylethanes in benzene at 25°. Because of the order of magnitude differences in Table I, however, it is likely that some of the expected large effects, such as steric and resonance effects, exist. 

Any substance that has paramagnetic properties will catalyse the ortho-para hydrogen conversion. This therefore represents a method for the detection of free radicals and atoms. Virtually pure para-112 is metastable up to 500° C. The con-... 

For some reactions, such as ortho-para hydrogen conversion, it is sometimes possible to show that the active site is a paramagnetic surface species. When such sites are counted, the site density is obtained (, ). In 1968 Shigehara and Ozaki (32)... 

The conversion is catalyzed by a number of surface-active or paramagnetic species such as Pd, Pt, or active Fe203. The presence of both ortho- and para-hydrogen molecules is seen in experiments as an alternation in the intensities of... 

The effects of an uncompensated electron are (1) to split the molecule s spectral lines into doublets, or in the case of certain diradicals, into triplets, (2) to make the molecule paramagnetic, (3) to catalyze the conversion of para and ortho hydrogen molecules, and (4) to cause paramagnetic resonance absorption. 

It has been known since the early work of Farkas and Sachsse (6) that the ortho-para conversion may be catalyzed by paramagnetic species. That the mechanism for this kind of conversion is nondissociative is shown by the absence of hydrogen-deuterium equilibration at a comparable rate under similar conditions. But proof that the nondissociative... 

If in the elementary step a change of total spin occurs, the reaction is forbidden, e.g. in the ortho/para conversion of the hydrogen molecule or the decomposition of N20 into nitrogen and oxygen (see section on this reaction). Materials containing paramagnetic centres could act as catalysts for this type of reaction, and many examples are actually known. 

The interesting aspect of reactions (1) and (2) is that on ortho/para conversion at a paramagnetic centre no H—H bond is broken, and consequently no H/D exchange occurs. On the other hand, when H/D exchange does occur, a hydrogen to hydrogen bond has been broken in the process, and O/P concer-sion occurs as a consequence. Therefore comparison of the rate of the two processes provides valuable information. 

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