Photosensitization with Mercury

A particularly large amount of research has been devoted to mercury-sensitized photochemical reactions of hydrogen compounds (H2, hydrocarbons, etc), especially to decomposition and oxidation reactions. The secondary process of interaction between an excited mercury atom and a reactant molecule may be involved in these reactions by proceeding two paths Hg + HH Hg -f- R + H and Hg + RH - HgH + R. 

HgH -j- H and the relatively low dissociation energy (35.5 kJ) of the HgH molecule formed, dissociation of the latter is very probable. For this reason, both paths of the mercury-sensitized hydrogen dissociation can be considered as resulting eventually in the dissociation of the H2 molecule to two free atoms. 

It has been suggested [402] that most reactions photosensitized with atomic Hg ( Pi) occur via complexes decomposing either to HgH -]- or (with the emission of light) to Hg + the initial molecule. 

Out of the large number of mercury-photosensitized photochemical reactions of hydrocarbon decomposition take as an example the decomposition of ethane studied by several authors (see [452] Chapter V). The composition of products and the reaction kinetics close to room temperature seem to be most accurately defined by 

Whilst the interaction of excited atomic mercury with hydrogen or hydrogen-containing molecules yields atomic hydrogen, the reaction Hg + N2O - Hg + N2 + O gives atomic oxygen. This reaction is used for the production of 0( P) atoms (see e.g. [10, 410]). 

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Intermolecular photocycloadditions of alkenes can be carried out by photosensitization with mercury or directly with short-wavelength light.179 Relatively little preparative use has been made of this reaction for simple alkenes. Dienes can be photosensitized using benzophenone, butane-2,3-dione, and acetophenone.180 The photodimerization of derivatives of cinnamic acid was among the earliest photochemical reactions to be studied.181 Good yields of dimers are obtained when irradiation is carried out in the crystalline state. In solution, cis-trans isomerization is the dominant reaction. 

Intermolecular photocycloadditions of alkenes can be carried out by photosensitization with mercury or directly with short wavelength light. Relatively little... 

The rate of reaction (33) is ten times that of (34). Presumably, the transition to (9, 0) of the 0 system in H8II state is affected by light of wavelength 1860 A. Flory and Johnston studied the mercury photosensitized reaction with a low-pressure Hg resonance lamp over the region 0.02-7 mm. The reaction rate was followed by the pressure change during the reaction, which was caused by the reaction of 02 with mercury. 

It must be stressed that the co-adsorption of mercury (from the vacuum line) could lead to mercury photosensitization of the adsorbed gas and great care must be taken to distinguish sensitization by the surface or other substances on the surface with mercury sensitized decomposition. The use of low pressure mercury resonance lamps which produce the 253.7 nm mercury line... 

In all the solvents studied the species responsible for the color with TFA were extremely photosensitive and were bleached in a few seconds when irradiated with unfiltered light from a medium pressure mercury lamp. 

Air direct photolysis t,/2 = 1.17 h predicted by QSPR method in atmospheric aerosol (Chen et al. 2001). Surface water photolysis t,/2 = 0.35 h near surface water, 40°N midday, midsummer and photosensitized oxygenation t,/2 = 1.5 h at near surface water, 40°N, midday, midsummer (Zepp Schlotzhauer 1979). photolysis t,/2 = 0.18 h in aqueous solution when irradiated with a 500 W medium pressure mercury lamp (Chen et al. 1996). 

To perform the dissociation of the hydrocarbon to alkyl radicals with C—C bond scission, a hydrocarbon molecule should absorb light with the wavelength 270-370 nm. However, alkanes do not absorb light with such wavelength. Therefore, photosensitizers are used for free radical initiation in hydrocarbons. Mercury vapor has been used as a sensitizer for the generation of free radicals in the oxidized hydrocarbon [206-212], Nalbandyan [212-214] was the first to study the photooxidation of methane, ethane, and propane using Hg vapor as photosensitizer. Hydroperoxide was isolated as the product of propane oxidation at room temperature. The quantum yield of hydroperoxide was found to be >2, that is, oxidation occurs with short chains. The following scheme of propane photoxidation was proposed [117] ... 

Gray [215] studied the photooxidation of methane and ethane at room temperature in the gas phase using Hg as a photosensitizer. Hydroperoxide was found as the product of oxidation. Along with hydroperoxide, ozone was also found as the product of photooxidation. It was supposed to be due to the reaction of excited mercury atoms with dioxygen. 

The antidepressant protriptyline (116) causes skin photosensitization in man. Jones and Sharpies irradiated an aqueous solution of the hydrochloride with a medium-pressure mercury lamp for 16 h and separated the products by preparative TLC. First formed was the epoxide (117) which photohydrated to the diol (118). Also isolated was the enol (119) [84], Earlier, Gasparro and Kochevar had shown that only the hydrochloride was photodegraded under nitrogen in water or ethanol. Three products were isolated and all lysed erythrocytes, but the structure of only one was suggested. This was a cyclobutyl dimer as shown by its mass spectrum and its photolysis back to protriptyline by light of 254 nm. Presumably, a [2 + 2] cycloaddition of the olefine bonds had occurred [85]. 

The antibacterial, nalidixic acid (337), is associated with a high incidence of photosensitivity reactions. Detzer and Huber irradiated a solution in 0.1 M sodium hydroxide with a high-pressure mercury lamp and identified four photodegradation products the decarboxylated derivative (338), carbon dioxide, ethylamine, and the new dione (339) [188]. 

Little experimental data on the photochemistry of compounds with B—0 bonds have been reported. Boron-hydride/oxygen mixtures are explosive. These explosions can be initiated by photochemical processes. Grimm and Porter have studied the photochemical decomposition of H2B2O3 using a low pressure mercury lamp. The UV spectrum of this compound is illustrated in Fig. 13. The rate of the reaction was increased when mercury was present as a photosensitizer. 

The photosensitivity of PMMA is significantly enhanced by the incorporation of 10 to 40 mole% 3-oximino-2-butanone methacrylate. Terpolymerization with methacrylonitrile increases that sensitivity still further, P(M-OM-CN) (69 16 15) being 85 times more sensitive than PMMA on exposure to the full output of a 1000 watt mercury lamp. Upon addition of external sensitizers, this sensitivity may be increased by an additional factor of 2 to 3. The high resolution characteristics of PMMA have been retained and the polymers in question show good plasma resistance. 

Cvetanovic67 was concerned with oxygen atom reactions with unsaturated hydrocarbons. The oxygen atoms were obtained in his experiments by mercury-photosensitized decomposition of N20. Cvetanovi6 came to the conclusion that the reaction of oxygen atoms with ethylene proceeded essentially with scission of the hydrocarbon bond, while with higher olefins this was not observed. Corresponding oxides (epoxides) and carbonyl compounds were formed in the course of the reaction. 

Duncan and Cvetanovic27 studied the reaction with isobutene of methylene generated by the mercury photosensitized decomposition of CH2CO, which is believed to produce triplet methylene. Product ratios reached high-pressure limiting ratios at 200 mm. The observed yield of... 

Methylene produced by mercury photosensitized decomposition of 011200 was found to add to cfs-butene-2 to give product ratios similar to those obtained by Frey under conditions of high dilution with argon. Addition of this type of methylene to (rans-butene-2 gave similar product ratios, with the exception that the trans isomers of 1,2-dimethylcyclo-propane and pentene-2 were favored over the cis. These results were taken as proof of the nonstereospecific addition of triplet CH2 to olefins. 

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