Ortho- and parahydrogen

Since interconversions between different states of symmetry (i.e., between ortho- and parahydrogen) are forbidden, the adjustment of the relative ratios of the two spin isomers to the values corresponding to the thermal equilibrium at an arbitrary temperature is normally very slow and, therefore, must be catalyzed. In the absence of a catalyst, dihydrogen samples retain their once achieved ratio and, accordingly, they can be stored in their enriched or separated forms for rather long periods (a few weeks or even a few years in favorable cases). 

A satisfactory explanation for this discrepancy was not available until the development of statistical thermodynamics with its methods of calculating entropies from spectroscopic data and the discovery of the existence of ortho- and parahydrogen. It then was found that the major portion of the deviation observed between Equations (11.24) and (11.25) is from the failure to obtain a tme equilibrium between these two forms of H2 molecules (which differ in their nuclear spins) during thermal measurements at very low temperatures (Fig. 11.4). If true equilibrium were established at all times, more parahydrogen would be formed as the temperature is lowered, and at 0 K, all the hydrogen molecules would be in the... 

It is now believed that dihydrogen initially adds to many transition metal complexes in the form of a hydrogen atoms is broken upon addition to the transition metal complex and homogeneous hydrogenation catalysts can equilibrate mixtures of ortho- and parahydrogen. 

By 1934, the only remaining trace of catalytic force lay in the low temperature catalysis of the interconversion of the newly discovered ortho- and parahydrogen by paramagnetic surface sites. 

Pure parahydrogen can be prepared at low temperatures, and the kinetics of the conversion of pure parahydrogen to an equilibrium (3 1) mixture of ortho- and parahydrogen has been studied at 923°K [31]. The rate of conversion is first order with respect to parahydrogen in a given run at constant pressure, but the observed first-order rate constant is proportional to the square root of the total hydrogen pressure. Postulate a mechanism that is consistent with the observed rate law. 

Since ortho and parahydrogen have different vapor pressures, an ortho-para analyzer was connected to sample gas from both the vapor bulb and the dewar, to measure any differences in vapor composition. 

This and the following paper are progress reports in our continuing study of the catalyzed conversion of ortho and parahydrogen. Last year we presented some converter design data based on experiments with a new hydrous ferric oxide gel catalyst. These data consisted of space velocity versus conversion curves for ortho to para conversions at various temperatures, two feeds, liquid and vapor phase, and gave a comparison between results obtained under approximately isothermal and adiabatic conditions. 

D. H. Weitzel, J. H. Blake, and M. Konecnik, "Flow conversion kinetics of ortho and parahydrogen," 1958 Cryogenic Engineering Conference Proceedings. 

We discuss first ortho- and parahydrogen. Since the proton has spin the possible nuclear spin wave functions are and corresponding to the spin wave functions a and P for an electron. We can therefore construct three symmetrical wave functions... 

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