Photosensitization, hydrogen atom mercury

Falconer and Cvetanovic (40) attempted to obtain a more quantitative value for the fraction of nonterminal addition in the case of propylene. They produced hydrogen atoms by mercury photosensitized decomposition of H2, using at least 100 times as much H2 as C3H6 and total pressures of 40 and of 250 mm. Under these conditions the reactions of importance were the combination and disproportionation of the iso- and n-propyl radicals and their cross reactions, the combination of the two radicals with H atoms (assumed to be equally probable), and a very small amount of decomposition of hot n-propyl radicals. Disproportionation to combination ratios were taken as 1.64 for two iso-propyl, 1.14 for two w-propyl, and hence 1.39 was taken as the mean of the two values for one iso- and one n-propyl radical. Using these values and the analysis of the products, the nonterminal addition of H atoms to C3H6 and C3D6 was found to amount to 6 1%. 

Studies by Gunning et al. of the mercury-photosensitized decompositions of cyclopentane and cyclohexane indicate that the main primary steps are now the hydrogen-atom elimination reactions... 

Jennings and Cvetanovic [11] obtained hydrogen atoms by the mercury-photosensitized decomposition of n-butane and measured their concentration from the rate of formation of H2 isolated at low temperatures. The measured rate coefficient was log k = 7.86 at 297°K, and is lower than most other values. 

The importance of photosensitization derives f rom the f act that reaction is produced in the presence of the sensitizer in circumstances where the direct photochemical dissociation is not possible. The example just cited is a case in point. Radiation of wavelength 253.7 nm was absorbed by a mercury atom. The excited mercury atom dissociated a molecule of hydrogen by transferring the excitation energy in a collision. The mercury atom had 471.5 kJ of this 432.0 kJ were needed for the dissociation 39.5 kJ are left over and go into additional translational energy of the two hydrogen atoms and the mercury atom. If the attempt is made to dissociate H2 directly by the process... 

The possibility of mercury-photosensitized reactions was first predicted in 1922 by Franck (45) and experimentally verified in the sensitized decomposition of H2 by Cario and Franck (46). They found that free H atoms were produced when a mixture of Hg vapor and H2 was irradiated with the 254 nm Hg resonance line at room temperature. Bates and Taylor (23) studied the Hg-sensitized decomposition of methanol, ethanol, and ethylamine, and showed that with these compounds the rate of the sensitized reaction was faster than the direct photolysis by about two orders of magnitude. Again hydrogen was the major product. Aldehydes were formed from the alcohols. 

Callear and Robb [10] reported a somewhat lower value for log k (8.02 at 304°K) using the mercury-photosensitized decomposition of hydrogen gas as the source of atoms. The determination of the H-atom concen-... 

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