Quantum yield with iodine

Dissociation of Halogen-Containing Compounds. Halogenated compounds, particularly those containing bromine or iodine, are likely to undergo photoelimination of the halogen atom. p-Bromobutyrophenone (7) yields butyrophenone, HBr, and butyrophenones with unsaturated alkyl side chains when irradiated in toluene.25,44 The quantum yield for butyrophenone... 

Nevertheless, even from steady-state measurements of iodine photodissociation quantum yields, Noyes [270] found that the quantum yield increased as the wavelength of photoexciting light decreased, presumably due to the greater separation of the iodine atoms when formed with greater... 

The bromination of cinnamic acid dissolved in carbon tetrachloride or other inert solvent.offers a convenient system for study. The dibromocinnamic acid produced remains in the carbon tetrachloride solution. The thermal reaction is so slow that it can barely be measured at room temperature and it is entirely negligible in comparison with the photochemical reaction at ordinary intensities. The quantum yield is so large that considerable reaction occurs even if the intensity of light is much reduced by the monochromator or other device for confining the light to a narrow range of frequencies. Furthermore, the reaction is easily and accurately followed by titration with sodium thiosulfate. Potassium iodide is added and the iodine liberated is a measure of the remaining bromine. 

This work was useful in establishing that the low quantum yield in the photolysis of pure methyl iodide was due to back reactions and that the initial decomposition into a methyl radical and iodine atom occurred with approximately 100% efficiency. In 1942, Blaedel et al.22 showed that the quantum yield for the decomposition of methyl iodide was, in fact, somewhat less than unity in the presence of oxygen because Bates and Spence11 had used an incorrect value for the quantum yield of the chloracetic acid actinometer. They suggested the reactions... 

Kinetics in the irradiated system HI-NO have been studied by Holmes and Sundaram . They used 3130-3660 A radiation and a reaction cell temperature of 25 or 45 °C. Uranyl oxalate actinometry was employed. The photolysis of HI in this wavelength region produces hydrogen and iodine atoms which in turn react with either HI or NO. Holmes and Sundaram found that at 25 °C additions of NO significantly reduced the initial quantum yield of Hj. As the NO/HI ratio increased, the quantum yield fell to a limiting value. Additions of nitrogen to pure HI had no effect on the quantum yield. At 45 °C the reaction products were the same but the actinometry was irreproducible due to formation of ammonium iodide on the cell windows which reduced incident light intensities. 

An approximate picture of the wavelength dependence of the primary quantum yields can be obtained by comparing the quantum yields of the decomposition at room temperature. Table 9 lists the values determined by Leighton and Blacet at 30 °C. If one assumes that (j)a represents the sum of (f>i and then the agreement with the iodine-inhibition experiments is very good. However, bearing in mind that the stoichiometry of the decomposition is complex at lower temperatures, too much stress should not be laid on the agreement in the numerical values. 

According to Blacet and Calvert , CH4, CgHg and /i-C6Hi4 are not formed in the presence of iodine. The values of co CaHs decrease, while CaUyi increases with increasing iodine concentration limiting values are attained above about 2 torr I2 added. The limiting quantum yields (Table 12) were found to be practically independent of temperature. 

Gorin ° determined the values of 1.0 (2537 A) and 0.85 (3130 A) for primary quantum yield i. Later investigations also confirmed the value at 2537 A however, a considerably lower value, < ch3I 0-2, was reported -at 3130 A and at temperatures of 100 °C and above. At 3130 A, the primary quantum yield is temperature dependent in the presence and absence of iodine Since the primary decomposition quantum yield is known to be near unity at 3130 A and above 100 °c - , one is inclined to interpret the low quantum yields, determined in the presence of iodine, as indications of the quenching effect of iodine. It is the triplet state of acetone which is likely to be quenched by Ij. Some of the results are, however, inconsistent with such a conclusion nevertheless, the quantum yields, determined in the iodine inhibition experiments, should be accepted with reserve. 

Photoreactions in the presence of small concentrations of iodine donors occur with high quantum yields (106) ... 

Reactions of Halo Compounds. - Calculations have been carried out to investigate the decomposition paths for methyl fluoride and methyl chloride. Methyl chloride undergoes photodissociation on irradiation at 157.6 nm. Photodissociation of methyl iodide at 266 nm has been studied. The methyl radical recombination has been followed by time-resolved photothermal spectroscopy. Methyl iodide also undergoes photochemical decomposition on a GaAs(llO) surface. " Photolysis of methyl iodide at 236 nm in the gas phase brings about liberation of iodine atoms with a quantum yield of 0.69. ... 

Figure 2 Experimental results for the production o/I( P ) from the photodissociation o/ICN. The solid curve and circles is the absorption spectrum of ICN, with e the molar extinction coefficient. The short dashed curve and open circles is the product of t x FP ), where 4> is the quantum yield for excited iodine atom formation. The long dashed curve and triangles is E — e (FP ), and thus represents the contribution to the absorption spectrum of dissociation leading to ground-state iodine atom production...

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