Parahydrogen conversion hydrides

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. 

This enzyme activates the simplest of all substrates and causes the parahydrogen conversion and hydrogen deuteride reaction. Krasna and Rittenberg (26, 27) have postulated formation of an enzyme hydride EH,... 

Farkas and Farkas (81, 82) assumed that the rate of dissociation of hydrogen molecules on the catalyst was measured by the parahydrogen conversion. They further assumed that the hydride was adsorbed by dissociation of a hydrogen atom, and that hydrogen and hydride competed for the chemisorbed layer. 

The parahydrogen conversion on this view is then, broadly, a test for the presence of chemisorbed hydrogen atoms. The effect of an increased pressure of hydride in decreasing the velocity of parahydrogen conversion is to be attributed in the first place to a displacement of hydrogen by hydride from the van der Waals layer. 

The notion that a saturated hydride RH, when it is chemisorbed, is held as R and H fragments seems fundamentally sound. So far as the present author is aware, an appreciable parahydrogen conversion is always obtained in such cases, showing the presence of chemisorbed hydrogen. This is in direct contrast to chemisorbed ethylene and carbon monoxide (see... 

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,... 

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See also in sourсe #XX -- [ ]

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