Photochemistry of uranium

Marks and co-workers (10, 10a) also studied the photochemistry of uranium alkyls in toluene solution at ambient (10°C) and low (-196°C) temperatures. As with the thorium alkyls, mixtures of the corresponding alkane and alkene were produced in near quantitative yield [e.g., Eq. (6)]. 

Dai, S. and Toth, L.M., Photochemistry of uranium(V) hexachloride complex in acidic room temperature molten salts, Proc. Electrochem. Soc., 98-11, 261, 1998. 

For example, in 1963 the photochemistry of magnesium phthalocyanine with coordinated uranium cations was studied in pyridine and ethanol and indicated the occurrence of PET to the uranium complex . A rapid photoinduced electron transfer (2-20 ps) followed by an ultrafast charge recombination was shown for various zinc and magnesium porphyrins linked to a platinum terpyridine acetylide complex . The results indicated the electronic interactions between the porphyrin subunit and the platinum complex, and underscored the potential of the linking para-phenylene bisacetylene bridge to mediate a rapid electron transfer over a long donor-acceptor distance. 

As the above results indicate, the photochemistry of these uranium-alkyl complexes is not fully resolved. The olefin formation during Marks s siudy of UCP3R in toluene solution suggests that photoinduced /3-hydride elimination occurs to some extent, presumably via a mechanism similar to that of Scheme 1. However, the other data appear to indicate a predominant radical mechanism via photoinduced M—R bond homolysis. 

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. 

The first attention given to actinide photochemistry was for the purpose of identifying any photochemical activity which might alter the efficiency of the extraction or exchange processes. Subsequently, the identification of photochemically active species of uranium and plutonium gave some indication that the photoreactions could be turned to a useful end and, perhaps, offer a cleaner way to separate actinides from each other and from the other elements accompanying them in nuclear fuel elements. 

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. 

Sostero, S. Traverso, 0. Bartocci, C. DiBernardo, P. Magon, L. Carassiti, V. "Photochemistry of Actinide Complexes. III. The Photoproduction Mechanism of Uranium(V) Oxochloro Complexes," Inorg. Chim. Acta., 1976, 19, 229. 

Miller, S. S. DeFord, D. D. Marks, T. J. Weitz, E. "Infrared Photochemistry of a Volatile Uranium Compound... 

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... 

The photochemistry of U(C H ) R (R=CH, n-C H ) derivatives was first described by Giannotti, Marquette-Ellis, and Fischer. They observed that photolysis of uranium alkyl complexes in toluene and THF solutions at 60 C apparently results in the cleavage of the alkyl-uranium bonds and in the formation of a (C H ) U THF adduct ... 

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... 

For the purposes of photophysics and photochemistry it is therefore sufficient to keep in mind the simple picture of an atom as a heavy, positively charged nucleus around which move light, negatively charged electrons. In the smallest atom, that of hydrogen, there is a single electron, whereas in the uranium atom, which is the heaviest natural element known on Earth, there are 92 electrons. It is the motion of these electrons which determines the chemical properties of an atom or a molecule so that it is now necessary to consider in a qualitative way the structure of these elementary particles of matter. 

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