Acetone light absorption

Choline reineckate is used in the spectrophotometric determination of choline. Ammonium reineckate [13573-16-5] forms a water-insoluble complex with choline. The complex is soluble in acetone and a widely used method for determination of choline is by light absorption of acetone solutions... 

However, Noyes and co-workers, in a series of publications,52.s3.9e.97 have reported studies on the fluorescence of acetone that, by showing the complexity of the steps between light absorption and decomposition of the acetone molecule, revealed that complications were to be expected when oxygen was added to the system. 

We have omitted the decompositions of secondary species and products caused by light absorption. This is a reasonable approximation so long as the absorption coefficients of these species are of the same order of magnitude as acetone itself, or a smaller order of magnitude, since the photolysis reactions seldom proceed to more than a few per cent completion. When, however, reactions are permitted to go to completion or intermediates or products have absorption coefficients much larger than the parent species, there may well be appreciable photochemical decomposition of these products. 

The quantum yields of Table 5 are all relative, and sensitizer concentration and light absorption are not controlled. Acetone, for example, is present as the solvent [82a] singlet as well as triplet energy transfer is feasible in this case (especially given the high concentration, 0.01 M, of sulfonium salt). 

The proven, and thus preferred, general structure for sensitizers is ML2(X>2, where M can be Ru or Os, L is 2,2 -bipyridyl-4,4 -dicarboxylic acid, and X represents a halide, cyanide, thiocyanate, acetyl acetonate, thiocarbamate, or water subsistent group [29]. The stfuctures of metal complexes used as sensitizers can also be mononuclear metal complexes (Figs. 38.2, 38.3, and 38.4a) [6,41, 18], binuclear metal complexes [Ru-Ru (Fig. 38.4b) [30], and Ru-Os (Fig. 38.4c)] complexes [30]. Polynuclear complexes have been employed in order to increase absorption coefficients. However, these bulky sensitizers require more space on the Ti02 surface and penetrate less easily in the small cavities of the nanocrystalline TiOj than the mononuclear complexes [34]. Hence, for polynuclear complexes, the increased absorption coefficients in solution do not necessarily lead to enhanced light absorption on the Ti02 electrode because of the reduced surface concentration of the bulkier sensitizer molecules on the nanoporous Ti02. 

The diaryliodonium salts are highly stable in the absence of light, but decompose when irradiated at 313 or 365 nm with quantum yields of 0.2-0 3 (3). The rate of photodecomposition is independent of the structure of the counter anion, and is insensitive to temperature and to atmospheric oxygen (3). No thorough investigation of solvent effects has been reported, but decomposition rates appear to be identical in acetone and acetonitrile when quartz tubes are used and the irradiation source is a water-cooled Hanovia 450 W medium pressure mercury lamp (2). Decomposition in nitromethane is slower, probably as a result of the increased light absorption by this solvent. 

Fohc acid (1) is found as yellow, thin platelets which char above 250°C. The uv spectmm of L-foUc acid at pH 13 shows absorptions at A = 256 nm (e = 30, 000), 282 nm (e = 26,000), and 365 nm (e = 9800). FoHc acid has a specific rotation of [a] = +19.9 (c = 1, 0.1 NNaOH). Solutions of fohc acid are stable at room temperature and in the absence of light. It is slightly soluble in aqueous alkaU hydroxides and carbonates but is insoluble in cold water, acetone, and chloroform. Table 3 Hsts some physical properties of selected fohc acid derivatives. 

HOMO (top) and LUMO (bottom) of acetone change occupancy upon absorption of light. 

The combined acetone extracts were extracted six times with one-fourth volume of ethylene dichloride and the ethylene dichloride extract was evaporated under vacuum to leave the steroid residue. This steroid residue was taken up in a minimum of methylene chloride and applied to the top of a column packed with 30 grams of silica which had been previously triturated with 21 ml of ethylene glycol. Then various developing mixtures, saturated with ethylene glycol, were passed over the column. Cuts were made as each steroid was eluted as determined by the lowering of the absorption of light at 240 mp on the automatic chromatographic fraction cutter. 

In 1993, Blatter and Frei [34] extended the Aronovitch and Mazur [28] photo-oxidation into zeolitic media, which resulted in several distinctive advantages as described below. Irradiation in the visible region (633 nm) of zeolite NaY loaded with 2,3-dimethyl-2-butene, 16, and oxygen resulted in formation of allylic hydroperoxide, 17, and a small amount of acetone. The reaction was followed by in situ Fourier-transform infrared (FTlR) spectroscopy and the products were identified by comparison to authentic samples. The allylic hydroperoxide was stable at - 50°C but decomposed when the zeolite sample was warmed to 20°C [35]. In order to rationalize these observations, it was suggested that absorption of light by an alkene/Oi charge-transfer complex resulted in electron transfer to give an alkene radical cation-superoxide ion pair which collapses... 

In contrast to the chlorophylls, which absorb light in two regions of the visible spectrum, the carotenoids exhibit intense absorption in just one, 350-500 nm. Figure E8.4 compares the absorption spectra of four common carotenoids. As with the chlorophylls, the absorption maxima of the carotenoids vary with polarity of the solvent. /3-Carotene in diethyl ether has a kmax of449.8 nm, but in the more polar acetone, the A.max is 454 nm. 

Propanone (acetone) vapor undergoes a photodissociation reaction with 313-nm light with somewhat less than unity. Absorption of light by 2-propanone results in the formation of an excited state that has sufficient energy to undergo cleavage of a C-C bond (the weakest bond in the molecule) and form a methyl radical and an ethanoyl radical. This is a primary photochemical reaction ... 

Carbonyls, compounds with carbon-oxygen double bonds, also absorb light in the UV region. For instance, acetone has a broad absorption peak at 280 nm. In this example, the electron can be excited from an unshared pair into a nonbonding -orbital, (ji... 

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