Reduction with hydrogen isotopes

Uranium metal was used as fuel in early types of reactors. UFe, sublimating at 56 °C, is used to separate the isotopes U and U. After isotope separation, UO2 may be obtained from UFg by hydrolytic decomposition, precipitation of U as ADU and heating, and uranium metal may be produced by reduction with hydrogen to UF4 and further reduction with Ca. Handling of enriched uranium compounds requires small-scale operations with amounts of the order of 1 to 10 kg, depending on the conditions, to exclude criticality. 

Boron exists naturally as 19.78% lOB isotope and 80.22% IIB isotope. High-purity crystalline boron may be prepared by the vapor phase reduction of boron trichloride or tribromide with hydrogen on electrically heated filaments. The impure or amorphous, boron, a brownish-black powder, can be obtained by heating the trioxide with magnesium powder. 

The model compound anilinostearamide 78, labelled in the aliphatic chain only, prepared subsequently by the reduction of linoleic precursor 79 with hydrogen or deuterium (equation 30), exhibited a chromatographic isotope effect of similar magnitude. The labelled compound elutes on the re versed-phase HPLC prior to the unlabelled one. 

Coleman ML, Sheppard TJ, Durham JJ, Rouse JE, Moore GR (1982) Reduction of water with zinc for hydrogen isotope analysis. Anal Chem 54 993-995 Connolly CA, Walter LM, Baadsgaard H, Longstaffe F (1990) Origin and evolution of formation fluids, Alberta Basin, western Canada sedimentary basin II. Isotope systematics and fluid mixing. Applied Geochem 5 397 14... 

A number of mechanistic problems remain, after accepting that the encounter pair ArNH2.NO is formed by pre-association. One such problem concerns whether the rate-determining step is the proton transfer or the reaction of the nitronium ion with the free amine formed within the encounter pair. It seems likely that the latter view is true from consideration of the rate profile, the very short half-life of the free amine, and the fact that these reactions do not show a marked primary hydrogen isotope effect when the rates in H2S04 and D2S04 are compared the reduction in rate by a factor of 2-3 in the deuterated solvent (Hartshorn and Ridd, 1968) is consistent with the lower concentration of the free amine. The reaction path can then be written as shown in Scheme 7 where NOJ. ArNHJ. B is an encounter triplet . 

Primary kinetic hydrogen isotope effects in the reductions of triarylmethyl cations by formate in aqueous trifluoroacetic acid have been determined (Stewart and Toone, 1978). For 16 cations with pK + ranging from —2.25 (4-methoxy-4 -methyl) to — 7.63 (4,4 -dichloro), plots of log kH and log kD against pA"R+ show poor linear fits with the best straight lines (least squares) having slopes of 0.473 (r = 0.973) for the H-data and 0."491(r = 0.983) for the D-data. Both plots show downward curvature, with the effect being more emphatic for the H-data. Least squares fit to quadratics better... 

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