Parahydrogen conversion, mechanism

Analogous parahydrogen conversion and deuterium exchange reactions, catalyzed by NH2, have been observed in liquid ammonia (Wilmarth and Dayton, 61). The kinetics are of the same form as those of the OH -cat-alyzed reaction in water and the mechanism is open to similar interpretations. The NH2 -catalyzed reaction is much faster, its rate constant at —50° being 10 times that of the OH -catalyzed reaction at 100°. The assumption of equal frequency factors for the two reactions leads to a calculated activation energy for the NH2 -catalyzed reaction of about 10 kcal. This low value has been attributed to the much greater base strength of NH2 relative to OH . The results provide some support for the hydride ion mechanism. 

Similar mechanisms can be provided for other reactions, such as parahydrogen conversion, hydrogen exchange reactions, dehydrations, etc. Reactions of the exchange type, where the energy levels in the two species are close, if not identical, can proceed in the van der Waals layer at low temperatures by virtue of the quantum mechanical exchange interaction. However, other classes of reactions are unlikely to proceed exclusively in other than the chemisorbed state. 

A. Farkas (69) found the parahydrogen conversion was catalyzed by a tungsten wire and described his results in terms of the dissociation and recombination of hydrogen in a loosely bound chemisorbed layer. Roberts results on the stability of the chemisorbed 61m showed that this mechanism was unlikely to hold for the specihc catalyst and conditions concerned (70). The rate of combination of chemisorbed atoms at room temperature is immeasurably slow and cannot account for the extremely rapid conversion observed. In reply, Farkas suggested (71) that most of the chemisorbed hydrogen was inactive, the conversion proceeding by his mechanism for a few active spots, not detectable in Roberts work. 

Hydroxide ion catalyzes the conversion of parahydrogen in aqueous solution and the exchange of hydrogen with the deuterium in heavy water. This reaction was first observed in 1936 by Wirtz and Bonhoeffer 66), and its kinetics have been examined in detail by Wilmarth, Dayton, and Fluornoy 67) and by Miller and Rittenberg 68). The rate-law given in Table I has been confirmed over the temperature range 80-190°, and OH concentrations of 10 to lAf. The two mechanisms which have been considered for the reaction involve, either intermediate formation of a solvated hydride ion,... 

At 100 the rate of the OH -catalyzed conversion of parahydrogen is, at most, twice that of the D2-H2O exchange. The small magnitude of this isotope effect suggests that proton transfer in the rate-determining step occurs by a classical mechanism rather than by tunneling. This is also indicated by the normal value (8 X 10 l.m. sec. 0 of the frequency factor of the reaction. 

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. 

Establishment of the rate equation is an experimental problem. One can, however, postulate a mechanism, or series of steps, by which the conversion proceeds. For example, the gas phase conversion of ortho or parahydrogen on a solid catalyst in a continuous flow converter probably involves the following steps ... 

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