Uranyl ions, reactions

The reason for the decrease in product yield from a 10 mmolar to a 2.5 mmolar uranyl ion reaction system is unknown, but may be due to the need to have a high enough concentration of product to permit nucleation for subsequent precipitation. Another possibility is that there is not enough PANa and/or uranyl ion present to encourage formation of the crosslinking chelations. It is not easy to differentiate between these choices since recovery of PANa, PAA or the uranyl ion from the reaction system is difficult and may affect the actual structures of the particular species. Previous studies with uranyl polyesters showed that even dimeric units precipitated from reaction mixtures at concentrations of 10" molar. 

In the second example we present a study where theoretical helped unravel an unexpected result in the laboratory a reaction expected to lead to the thermodynamically most stable species (uranium oxo), instead generated a uranium imido species. As mentioned earlier, the uranyl ion possesses U-O bonds with high thermodynamic stability and extreme kinetic inertness. As a result, the majority of uranyl ion reaction chemistry involves substitution of equatorially coordinated ligands while leaving the U-O bond unaffected. Its isoelectronic bis(imido) [U(NR)2] + analog, while possessing many of the bonding features found in UO2+ [46] exhibits reactivity quite distinct from UO. ... 

Partial regeneration of the product of the electrode reaction by a disproportionation reaction is characteristic for the reduction of the uranyl ion in acid medium, which, according to D. H. M. Kern and E. F. Orleman and J. Koutecky and J. Koryta, occurs according to the scheme... 

Khan and Martell [J. Am. Chem. Soc., 91 (4668), 17, 1969] have reported the results of a kinetic study of the uranyl ion catalyzed oxidation of ascorbic acid. The stoichiometric equation for this reaction mav be represented as... 

The most convenient methods to be used with silica having no pores or very wide ones are the reaction with thionyl chloride, the esterification with methanol or w-butanol and higher alcohols, and ion exchange, e.g., with uranyl ions (however, the general applicability of the latter reagent... 

H-atom abstraction has been demonstrated to be the mechanism of action of excited uranyl ions, and in this case negligible base oxidation is found. Nucleo-base (especially guanine) oxidation is the principal reaction caused by singlet oxygen and this reactive species can be generated by a number of the complexes (e.g. many Ru(II)polypyridyls and porphyrins). It is worth pointing out, however, that the yield of may be lower when the sensitiser is bound to DNA, and it is the authors view that some of the reactions claimed to proceed via 62 may be caused by direct reaction of the photo-oxidised sensitiser with the DNA. 

One last point. In the reaction of uranium(IV) where it is convenient to do a tracer experiment because there is only one metal ion product, we have actually determined the number of oxygens transferred to the uranyl ion product from the chlorite, and this number corresponds to 1.3 oxygen per chlorite transferred to the uranium. This is consistent with the results we reported some years ago (5) on the oxidation of uranium (IV) with Pb02 and Mn02, where indeed more than one oxygen is transferred. In conclusion, we feel that we have some direct evidence for two-electron transfer in these reactions and the formation of a chlorine(I) intermediate followed by the formation of chlorate. 

Uranyl ions form complexes in solutions with most anions. Uranyl sulfate and carbonate complexes are especially strong and are used in extracting uranium from its ores. Of great practical importance are the complexes of the uranyl ions with nitrate that are soluble in organic liquids such as alcohols, ethers, ketones, and esters. One of the most important of these reactions is that involving the extraction of uranyl nitrate into TBP (the Purex process) ... 

A review (91 references) on electrophilic and nucleophilic reactions of trivalent phosphorus acid derivatives, reactions of two-coordinate phosphorus compounds, and miscellaneous reactions has appeared.228 Earlier in this review we looked at the heavy-atom isotope effects on reactions of Co(III)-bound /vnitrophenyl phosphate,186 the uranyl ion hydrolysis of /vnitrophcnyl phosphodiesters (218)-(220),190 and the Th(IV) hydrolysis of these.191... 

Hemmi et al. [ 11 ] has described a differential pulse polarographic procedure for the determination of nitrate in environmental samples such as silage, grass, plants, snow and water. This method utilizes the catalytic reaction between nitrate and uranyl ion in the presence of potassium sulfate. The differential pulse polarographic peak is proportional to the nitrate ion concentration from 1 to 50 pmol/1. The detection limit for nitrate in water is 8 x 10 7 mol/1. Using this procedure, between 1 and 70 mg/g nitrate was found in vegetation samples. 

A large number of other metal complexes have received long and detailed attention, but activity in recent years has revealed few new principles appropriate for discussion here and some systems have been treated in detail elsewhere.2 Included among these are oxalato complex photochemistry where oxidation of the oxalato ligands is coupled to the central metal reduction Ag(I) photochemistry related to imaging systems uranyl ion photochemical reactions coupled to organic oxidations and aquo ion photoredox reactions. Two specific topics have recently emerged as... 

The uranyl ion luminesces in fluid solutions at room temperature2 201 202 thus providing a tool for the study of bimolecular excited state processes. In several cases, however, this study is complicated by the fact that the uranyl ion forms complexes with a variety of chemical species, so that it is often difficult to distinguish between intramolecular photochemical processes involving uranyl ion-ligand complexes and intermolecular photochemical processes involving reaction between an electronically excited UO + species and the substrate2,201,202 ... 

In like manner, inclusion of mineral acid in the photo-sensitised grafting of styrene in methanol to cellulose enhances the polymerisation yields significantly (Figure 2). The results obtained with phosphoric and sulfuric acids were consistent with earlier data (13,14). Dioxan is also a useful solvent for these reactions when uranyl ion is used as sensitiser. However dioxan is not as efficient as methanol for the grafting reaction. The grafting in dioxan exhibits a Trommsdorff peak at 60% monomer concentration. The results in Figure 2 also show that the unsensitised process does occur in dioxan as in other solvents, but... 

Excited uranyl ion, U022+, is a strongly oxidising species (E° = + 2.35 0.1 V vs. SCE [82]) and has been shown to be capable of oxidising a variety of substrates [78]. In the absence of any deliberately added solution phase organic compound, it has been suggested that UV photo-excitation of UC>22+ in aqueous sulphuric acid solution initiates the following sequence of reactions to produce the observed primary products of U02+ and OH [83,84] ... 

In this text only the reactions of uranyl ions will be described. 

Reactions of uranyl ions UO + To study these reactions use a 0- 1m solution of uranyl nitrate, U02(N03)2.6HzO or of uranyl acetate, U02(CH3C00)2.2H20. 

The first reaction describes the excitation of uranyl ions. The excited sensitizer can lose the energy A by a non-radiative process (12b), by emission (12c) or by energy transfer in monomer excitation to the triplet state (12d). Radicals are formed by reaction (12e). The detailed mechanism of step (12e) is so far unknown. Electron transfer probably occurs, with radical cation and radical anion formation these can recombine by their oppositely charged ends. The products retain their radical character. Step (12g) corresponds to propagation and step (12f) to inactivation of the excited monomer by collision with another molecule. The photosensitized initiation and polymerization of methacrylamide [69] probably proceeds according to scheme (12). Ascorbic acid and /7-carotene act as sensitizers of isoprene photoinitiation in aqueous media [70], and diacetyl (2, 3-butenedione) as sensitizer of viny-lidene chloride photopolymerization in a homogeneous medium (N--methylpyrrolidone was used as solvent) [71]. 

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