Ortho-Para Conversion

The ortho-para conversion of molecular hydrogen is catalyzed by NiO. A supported catalyst is available with a specific surface area of 305 m2/g and a void volume of 0.484 cm3/g. A spherical catalyst pellet has an apparent density of 1.33 g/cm3 and a diameter of 0.5 cm. If the system is not far from equilibrium, an apparent first-order rate constant (kr) can be defined in the following manner. 

The enthalpy of copper at nitrogen temperature is H77K = 6 J/g, so the total entropy of the sphere will be about 6 x 106 J. The time needed to cool from 77 K down to 4K is of the order of 4h. The total helium consumption from room temperature to 4.2 K would be about 6001. The temperatures reached in a test run are reported in Table 16.2. The expected final sphere temperature is about 20 mK. A comparison of MiniGRAIL and Nautilus cool down is made in Table 16.2. The high power leak on the sphere has been attributed to a time-dependent heat leak caused by the ortho-para conversion (see Section 2.2) of molecular hydrogen present in the copper of the sphere (see Fig. 16.5) (the Nautilus bar instead is made by Al). A similar problem has been found in the cool down of the CUORICINO Frame (see Section 16.6). 

Fig. 16.5. An overview of the minimum temperature of the different elements of the system. An estimate is made on the heat flows Q due to conduction between the different stages that are all connected with stainless steel rods or tubes. The total heat leak on the mixing chamber is estimated to be 45pW. This heat leak decreases in time and comes from the sphere and copper masses. We will see further on that this can be explained by ortho-para conversion of 70 ppm hydrogen impurities in the copper (courtesy of Leiden Cryogenics).

One of them is the ortho-para conversion of hydrogen trapped in the frame copper during the production process (see ref. [100-102] and Section 2.2). 

The theoretical minimum work for hydrogen liquefaction depends on the pressure of the hydrogen feed, the rate of ortho-para conversion and the temperature difference between ambient temperature and the temperature of the liquid hydrogen. The following formula is valid for ambient input and output pressures ... 

There is no change in the chemical composition of the reacting species in reactions (a) and (b) nevertheless, it is possible to measure the rate of these reactions by studying the process of the change in the spin state of the nuclei (ortho-para conversion). Theoretically, reactions (a)-(d) are of special interest because for them rather accurate non-empirical calculations of the potential energy surface, as well as detailed, up to quantum mechanical, calculations of the nuclear dynamics during an elementary reaction act can be carried out. 

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. 

Fig. 1. Rate of ortho/para conversion of hydrogen over dextrose charcoal as a function of temperature...

Although a temperature-dependent equilibrium determines the orthopara ratio, in a sample of H2, sudden temperature changes will not cause a rapid shift in this ratio unless the ortho-para conversion is catalyzed also. Obviously, methods to break and then remake the II—H bond offer a path to equilibrium. The passage of electric discharge through the sample results in such bond breaking and redistribution likewise, addition of a small amount of atomic hydrogen (abbreviated H ) results in similar action ... 

M. A. Strahemechny and R. J. Hemley, New ortho-para conversion mechanism in dense solid hydrogen. Phys. Rev. Lett., 2000, 85(26), 5595-5598. 

K. Motizuki and T. Nagamiva, Theory of hie ortho-para conversion in solid hydrogen.. Phys. Soc. Jpn., 1956,11, 93-104. 

In Section V below we show that spillover can induce catalytic activity on the support. The nature of the active site created on the support may result from the surface reduction, or the adsorbed hydrogen may be a center and site for reaction (123). On the other extreme, spiltover hydrogen has been shown to inhibit ortho-para conversion over sapphire and ruby surfaces... 

As the ortho-para conversion involves the forbidden triplet-singlet transition in the nuclear spin state, spontaneous transformation is very slow. Thus, samples of hydrogen that have been cooled exhibit the proportion of the two forms corresponding to the equilibrium composition at room temperature. The conversion of the ortho molecules to para molecules and vice versa requires the simultaneous breaking of both the spin and orbital symmetries, and the equilibrium ratio is therefore established slowly. 

The equilibrium ortho-para conversion studies of Rosenbaum and Hogness originally appeared inconsistent with the work of Bodenstein since the equilibration of the nuclear spins took place much more rapidly than the Bodenstein rates. Sullivan has re-interpreted the conversion data in the light of parallel molecular and free radical mechanisms and shown it to be equivalent to the results of his experiments which were run far from chemical equilibrium. 

The oldest example of the application of the transition state method is the reaction H + Hg = H2 + H and the similar reactions with deuterium. They have been measured partly directly, partly by the ortho-para-conversion. The calculation of the corresponding energy surface is the simplest example of its kind. Although in my opinion, the agreement between experiment and theory is not complete, the disagreement amounts to less than 50 per cent, with the most suitably chosen set of constants. A similar example is Cl -f- H2 = CIH H, where the agreement with the measurement of Rodebush is within... 

Since atoms retain their nuclear spins unaltered in all processes except those involving ortho-para conversions, there is no change in the nuclear spin entropy. It is consequently the common practice to omit the nuclear spin contribution, leaving what is called the practical entropy or virtual entropy. ... 

H2/D2 exchange involves type (/) species, but their exact role is still under debate. A kinetic isotope effect operates such that HD is produced — 1.5 times faster from a 2H2 D2 mixture than from a H2 2D2 mixture. Naito et al favour an Eley-Rideal mechanism at 200 K involving a gas-phase molecule and a type (/) chemisorbed atom. Kokes et a/. initially favoured a similar mechanism involving a type Hi) chemisorbed molecule instead of one in the gas phase. However, in a later paper they show that type Hi) species are only important in allotropic ortho-para) conversion and that the exchange reaction involves a Bonhoeffer-Farkas mechanism, using type (/) adatoms, at all temperatures. Richard et al., on the basis of the variation of the first-order rate constant with pressure, deduced that the mechanism must involve atomic H on surface pair sites that most probably were adjacent ion pairs thus, a Rideal type of mechanism was favoured. 

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