Photolysis of hydrogen iodide

The proportions of ground-state and (2P ) excited iodine atoms produced in a photolysis using monochromatic radiation can be approximately calculated from Fig. 1. To conserve momentum, essentially all the energy from the primary process in excess of that used in bond dissociation (HI - H+I) = 

A steady-state treatment of the kinetic scheme yields the differential equation 

Bonhoeffer and Farkas estimated k3/k2 100 and claimed that at 15 % decomposition the photolysis is completely self inhibited. More recent work by Ogg and Williams9,10 showed that for the photolysis with 2537 A radiation, k3/k2 is independent of HI pressure (50-150 torr), independent of temperature and has a value 3.5+0.3. The effect of cyclohexane as an inert diluent11 was to increase k3/k2 to 7.0+0.4 at 155°, which value remained constant at high cyclohexane hydrogen iodide ratios. This result was attributed to collisional thermalisation of the hot H atoms produced by 2537 A radiation and this limiting high-pressure value of k3/k2 = (k3/k2)aa was considered to be that for thermally equilibrated H atoms. 

These experiments were extended by Hamill et a .12, who used helium, argon and hydrogen as inert diluents. Addition of these gases also produced an increase in k3fk2, which approached a limiting value, (k3jk2) J, at high inert gas pressures. 

The vacuum ultraviolet photolysis of HI has been studied by Martin and Willard16 using the 1849 A mercury lines as the exciting radiation. They estimated the molar extinction coefficient at 1849 A to be between 110 and 150 in fair agreement with Romand s3 experimental value of 125. The hydrogen atoms produced have sufficient energy to cause the reaction 

---

Later, Holmes and Sundaram210 studied the gas phase photolysis of hydrogen iodide in the presence of nitric oxide. NO was found to inhibit the reaction by reducing the quantum yields of hydrogen and iodine to an equal extent. The mechanism proposed was... 

More recently. Holmes and Sundaram studied the photolysis of hydrogen iodide in the presence of NO at 25 °C, using the Hg emission at 3130 and 3660 A. They estimated fci3o/fci26> using Sullivan s value for the rate coefficient of... 

In a subsequent study by Holmes and Sundaram the photolysis of hydrogen iodide in the presence of nitric oxide was run at 6 °C and —20 °C. The products at these two temperatures include important amounts of N2, which is insignificant at 25 °C, as well as I2, H2, and H2O. The major difference between the 25 °C results and those at lower temperatures is attributed to the much lower vapor pressure of I2 at 6 and —20 °C. Since runs were made at NO/HI > 4 and the experiments at 25 °C indicate all the H atoms are scavenged by NO at these concentration levels. Holmes and Sundaram assume that all the H2 arises from the reaction of HNO with HI in the products. 

Evidence has also been presented for the importance of hot radical reactions in the liquid and solution phase photolysis of methyl iodide, and in the photolysis of methyl iodide in the presence of hydrogen halides . Majer and Simons , however, have discussed the possibility of the formation of an energised CH3I-CH3 radical complex. 

O Neal and Benson S studied the photolysis of acetone at 3130 A in the presence of hydrogen iodide. The main products of the reaction were methane and acetaldehyde CO could be detected only at high temperatures. The dependence of the CH3CHO/CO ratio on pressure was considered to be a result of the pressure-dependent decomposition of CH3CO. The experimental results concerning the radical decomposition obeyed the relation predicted by the Hinshelwood-Linde-mann theory. The values reported for the limiting high-, and low-pressure rate coefficients were... 

The rate of combination of trifluoromethyl radicals to form hexafluoro-ethane has been measured by the flash photolysis of trifluoromethyl iodide coupled with rapid-scan i.r. spectroscopy in the absence of an inert diluent (At, N2, or COt) carbon tetrafluoride and tetrafluoroethylene were also formed, presumably via fluorine atom abstraction from trifluoroiodomethane by hot trifluoromethyl radicals (c/. ref. 27). Photolysis of trifluoroiodomethane has been used in studies on (i) the direction of radical attack on 1,3,3,3-tetrafluoropropene [- CFj CHI CHFCF, (75%) + (CFa)jCH CHFl (25%)] (ii) the rates of hydrogen abstraction from ammonia, ammonia-ethylene oxide, silane, trimethylsilane, tetramethylsilane, and cycloalkanes and (iiQ the competitive addition of the CFj- radical to ethylene and vinylidene fluoride. Radicals formed by photolysis of the fluoroalkyl iodides CFJ, C FJ, n-C,F,I, (CF,)jCFI, (CFaljCHI, (CFalaCDI, (CFa)jCClI, and (CFa)iCPhI (the last was synthesized by treatment of CFs CPh CTj with CsF and iodine in DMF) have been... 

The photolysis of polyhalogenated compounds forms the basis for another monochrome system. Iodoform can undergo photolysis to produce hydrogen iodide, which subsequendy reacts with a di(2-furfuryl) derivative (3) and aromatic amines to produce a colored dye adduct (4) (29). The photolysis scheme and subsequent reactions can be shown by the following ... 

Campbell JM, Schulte-Frohlinde D, von Sonntag C (1974) Quantum yields in the UV photolysis of 5-bromo-uracil in the presence of hydrogen donors. Photochem Photobiol 20 465-467 Chapman JD (1984) The cellular basis of radiotherapeutic response. Radiat Phys Chem 24 283-291 Chatterjee M, Mah SC, Tullius TD, Townsend CA (1995) Role of the aryl iodide in the sequence-selective cleavage of DNA by calicheamicin. Importance of thermodynamic binding vs kinetic activation process. J Am Chem Soc 117 8074-8082 Chaudry MA, Weinfeld M (1995) The action of Escherichia coli endonuclease III on multiply damaged sites in DNA. J Mol Biol 249 914-922... 

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. 

The flash photolysis of hexaarylbiimidazole produces imidazolyl radicals " which have been shown to be more nearly planar than the parent dimers ort/io-substituents in the aryl rings decrease the radical stability. The radicals oxidize electron-rich substrates by rapid electron abstraction from tertiary amines, iodide, and metal ions, and by hydrogen abstraction from phenols, mercaptans, secondary amines, and active methylene com-pounds." " Studies have been made of the photooxidation of /euco-triphenyl-methane dyes by these radicals." " ... 

Irradiation of primary alkyl bromides or iodides affords typically a mixture of reduction and elimination products. For example, a radical pair produced by photolysis of the halooctanes 469 undergoes in the initial stage either competing diffusion and hydrogen abstraction from the solvent molecules (to give 470) or electron transfer giving rise to elimination product (471) a favoured process in case of alkyl iodides (Scheme 6.229).1336 Formation of the nucleophilic substitution product 472 is not observed, because solvent does not participate in carbocation generation. 

---