Photochemistry of the Uranyl Ion

The most studied system consists of a solution containing the uranyl and oxalate ions. This combination has received extensive study because of its widespread use as a chemical actinometer. Photolysis of such solutions lead to the formation of both carbon monoxide and carbon dioxide. The two principal reactions that lead to product formation (Eqs. (8.6) and (8.7)] do not involve reduction of the uranyl ion.  

An accompanying reaction that does involve reduction of the uranyl ion to 1)021 makes only a minor contribution to the total decomposition of oxalate 

The quantum yield for oxalate decomposition is relatively independent of pH for acidic solutions, in addition to being independent of temperature, concentration of reactants, and light intensity. The absorption of light by the uranyl oxalate system results in the generation of LMCT excited states. Such an absorption results in the formation of carbon dioxide and the reduced UC ion  

Photochemical reactions are observed between the uranyl ion and other car-boxylate anions such as formate, acetate, lactate, and higher homologue carboxy-lates and dicarboxylates. Upon irradiation, aqueous solutions containing the uranyl ion and formic acid undergo a photoredox reaction to give and carbon 

Aparallel reaction that also occurs is the uranyl ion photosensitized decarboxylation of acetic acid to methane  

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Finally, in the light of what is known about the electronic structure, it is worth commenting on the long luminescence lifetimes and extensive photochemistry of the uranyl ion. The lifetime of uranyl ion luminescence at low temperatures in solids is close to that determined by the radiative relaxation rate, which can be estimated from the absorption intensity. Non-radiative relaxation mechanisms are therefore very inefficient. We may trace this to the nature of the first excited state, which has 77g symmetry in the isolated ion, and to the magnitude of the energy gap separating it from the ground state. 

These reaction products can be explained on the basis of the pathway shown in Scheme 8.4. This pathway involves an initial photoinduced one-electron transfer in a uranyl formate complex to give UO2 and a formate radical, followed by disproportionation to give and carbon dioxide. The photochemistry of the uranyl ion with either acetic acid or its higher homologues follows a similar type redox pathway. Such a pathway with acetic acid leads to the formation of carbon dioxide, ethane, and carbon dioxide ... 

The solution photochemistry of the actinides begins with uranium none has been reported for actinium, thorium, and protactinium. Spectra have been obtained for most of the actinide ions through curium in solution (5). Most studies in actinide photochemistry have been done on uranyl compounds, largely to elucidate the nature of the excited electronic states of the uranyl ion and the details of the mechanisms of its photochemical reactions (5a). Some studies have also been done on the photochemistry of neptunium (6) and plutonium (7). Although not all of these studies are directed specifically toward separations, the chemistry they describe may be applicable. 

Prior to the advent of the laser, photo-induced reactions sat, for the most part, in the chemical background. Photochemists of that time typically gave little thought to actinide elements other than uranium which was known for years to be a very excellent chemical actinometer when present as the uranyl ion. The expertise and specialized equipment required in the handling of the other actinides, coupled with their very limited supply,served to discourage photochemists from fundamental investigations of these elements. As a result, no report of actinide photochemistry (save that of uranium) is to be found in the open literature prior to 1969. 

In the absence of chemical quenching, uranyl compounds have long luminescent lifetimes and high luminescent quantum efficiency [21]. Often, however, the excited state reacts chemically. The photochemistry of the ion, the most famous example of which is the uranyl oxalate actinometer, has generated an enormous body of work and been the subject of comprehensive reviews [22,23]. It can occur both in solution and in the solid state. The most common reaction is the oxidation of organic substrates. Both the photochemistry and the remarkable properties of the covalent bond, demand a satisfactory interpretation in terms of the electronic structure. 

Knowledge of actinide photochemistry is limited mainly to the complexes of uranium, especially those containing the uranyl ion, The lowest excited state of this ion is... 

In all of the photochemistry of UO2 , the nature of the bimolecular product formed by association of the reactant with the excited state U02 is an important facet of the chemistry. In aqueous solution the uranyl ion exists as the hydrate, U02(H20)5, where the five water molecules are complexed in the equatorial plane. Replacement of one or more of these complexed water molecules by an incoming substrate allows for bimolecular photoreaction to occur in the vicinity of the metal center. The reaction types can be summarized by the series of reactions shown in Scheme 8.5, where the radical pair can undergo dissociation, or either forward or back electron transfer reactions. Since the potentials for the (U02 /U02) and the (UOj/U ) couples are 0.06 V and 0.55 V, respectively, the UO2 ion can either be readily oxidized back to UOi, or it can act as a one-electron oxidant to a substrate that reacts as a reducing agent. By observing the reactions... 

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 majority of the photochemical studies with actinide ions have been carried out with the uranyl (UC ion. This ion is yellow in color both in the solid and solution states. The early photochemistry of this ion has been reviewed. " Excitation of this ion results in an LMCT absorption that involves a transition from an essentially nonbonding 7r-orbital on oxygen into an empty 5/orbital on uranium. This LMCT assignment is that given to the weak visible bands in the absorption spectrum at 500 nm and 360 nm. The absorption spectrum also shows a series of bands of increasing intensity to higher energy. The positions of the absorption bands of are very sensitive to both temperature and the chemical environment... 

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