Carbonyl photochemical properties

Figure 13 illustrates two models used to explain the photochemical properties of rhenium carbonyl complexes. The MLCT absorption band often... 

In this chapter, we focus on these imique photochemical properties of rhenium(I) diimine carbonyl complexes (Fig. 1), especially photochemical reactions and photocatalysis. 

The photochemical properties of organometallic compounds are investigated, but none to the depth to which metal-carbonyl complexes are subjected. Photosensitivity of the complexes often is cited, but there is little emphasis on synthetic utility. This section examines the synthetic potential of the photochemical transformations. Studies that are concerned solely with mechanistic aspects are excluded, as well as synthetic applications of the mixed compounds, such as h -CjHjCr(CO)3 and h -CjHjMnfCOjj, where the primary photochemical reaction is CO loss these are discussed in 13.2.4. 

Solvent shifts are useful as a criterion to identify n,7t transitions in absorption spectra because hydrogen bonding of protic solvents with the carbonyl oxygen stabilizes the np lone pair and gives rise to a hypsochromic shift of the n,7t absorption bands see the positions of the n,7t absorption band of acetone in heptane and water (Figure 6.5, top). This contrasts with 7t,7t transitions that tend to be shifted bathochromically in polar solvents. Also, the photophysical and photochemical properties often serve to identify the nature of the lowest excited state. Lone-pair interaction in biacetyl splits the two np-orbitals giving rise to two n,7t transitions at v = 2.23 and 3.54 im In the spectrum of 1,4-naphthoquinone in methanol, the n,7t band is barely detectable as a shoulder on the red edge of the 71,71 absorption. 

The reaction enthalpies for the photo-reduction of ketones and azoalkanes in n,ji states by the model hydrogen donors methanol and dimethylamine have been calculated. These data, together with a consideration of excited-state electrophilicity and the antibonding character of transition states, have been used to rationalize the nucleophilic character of singlet n,ji excited azoalkanes and the electrophilic character of correspondingly excited ketones. A theoretical model for the photo-reduction of carbonyl compounds, based on the assumption that hydrogen transfer involves electron transfer followed by proton transfer, has been developed. The model was able to describe the available kinetic data for the photo-reduction of o-benzoquinones and fluorenone in the presence of para substituted iV,iV-dimethylanilines and those for the photo-reduction of p-chloro-anil and 2,6-dichloro-l,4-benzoquinones in the presence of polymethylben-zenes. The photochemical properties of the triplet ji, n state of 5,12-naph-... 

General Photochemical Properties of Metal Carbonyls 2.1. SPECTROSCOPY... 

What important photochemical property do carbonyl compounds share with... 

Carbonyl compounds have close-lying mi and titi states with different photochemical properties. If the ordering of the two states can be controlled externally, then one could obtain, at wiU, products from the two states. One approach is to make use of a polar medium. If the ordering of the two states is different in polar and nonpolar solvents, different products would be obtained in these solvents. Even olefins have two states (jiji and doubly excited 7i 7i ) of different polarizabihty. When these two states are nearby, the photochemistry could be solvent dependent. 

Chemical Properties. Hydantoins can react with electrophiles at both nilrngen atoms and at C-5. The electrophilic carbonyl groups can be attacked by nucleophiles, leading to hydrolysis of the ring or to partial or lotal reduction of the carbonyl system. Other reactions are possible, including photochemical cleavage nf the ring. 

Arnold, D. R., R. L. Hinman, and A. H. Glick Chemical properties of the carbonyl n,7t state. The photochemical preparation of oxetanes. Tetrahedron Letters 1964, 1425. 

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