Mercury-photosensitized reactions, with

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. 

The latter cannot return to the ground state by emission since the transition is doubly forbidden with J = 0 - J = 0 and AS = 1. The Hg(6 Po), thus, has a very long radiative lifetime164 and the probability of its suffering a collision before radiating is very great. Examples of mercury photosensitized reactions are... 

That the mercaptan product arises exclusively from singlet sulfur atom reactions was proven in experiments with added CO2. The data in Table VI clearly demonstrate the suppressing effect of COj on the mercaptan yield, with a concomitant and equivalent rise in episulfide production. Further substantiation comes from mercury-photosensitization studies with COS in which it was found that the sulfur atoms produced by this means yield only episulfide, no mercaptans being formed with ethylene or with any of the other olefins studied. 

Work on the mercury-photosensitized reactions of acetylene has been carried out by Bates and Taylor , lungers and Taylor , Melville , Le Roy and Stea-cie , Shida et al ", Mains et Sherwood and Gunning , Tsukuda and Shida , and LeRoy . The system offers an interesting contrast to that with ethylene, in that a primary energy-transfer process appears to be imimportant the main steps are probably... 

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. 

Finally, Crabtree has reported the gas-phase mercury photosensitized reaction of methane with ammonia to yield methylene imine as the ultimate product [41]. Higher imines are also produced if the gas-phase residence time of methylene imine is prolonged. 

Preliminary work on the mercury-photosensitized reaction of trimethylsilane with vinylidene fluoride has indicated that hydrogen is produced with the quantum yield to be expected by analogy with the trimethylsilane-tetra-fluoroethylene reaction, and that hexamethyldisilane is not formed, so that initiation is proceeding as before to give trimethylsilyl radicals, which add to the olefin. Since no appreciable yields of 1 1 or 1 2 adduct were obtained, however, it appears that the MesSi-CHs CFj- radicals which are produced react by termination rather than by hydrogen abstraction. 

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. 

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]. 

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... 

Resonance Lamp.—Such lamps (sometimes called low pressure lamps) are often used as line sources in photochemical studies. These usually contain a small amount of a metal vapor (e.g., mercury, cadmium, zinc, etc.) and several mm pressure of a rare gas. They operate at relatively low current (ca. 100 ma.) and high voltages (several thousand volts). This is in contrast to a typical medium pressure lamp which may operate off a 110-220 v. power supply delivering ca. 3-5 amp. The most common example in photochemistry is the mercury resonance lamp which has strong emission of the unreversed resonance lines at 2537 A. and 1849 A. (ca. 90% or more of the total) along with other, much weaker lines ( resonance lines are those which appear both in absorption and emission). There is little continuum. Sources of this type are widely used for photosensitized reactions. 

Ozone synthesis by mercury (3Pf) photosensitization was first reported by Dickinson and Sherrill (24). They reported that at least 7 molecules of ozone are formed for each mercury atom passing through the reaction zone. Subsequently, Volman (91) found that at least 40 molecules of ozone could be formed for each mercury atom and still later reported a value of 60, (92). The above studies were made in flowing systems at atmospheric pressure. Callear et al. (18) have investigated the reaction by photometeric and thermal methods in a static system at pressures of 200 mm. or less. A quantum yield of 0.14 was reported, and evidence that oxygen was not removed by reaction with Hg(3P1) atoms and that mercury was removed by a dark reaction with ozone was obtained. In all of the above studies mercuric oxide was always formed. 

Before mercury-photosensitized decomposition of nitrous oxide could be used as a technique for generation of oxygen atoms, it was necessary to establish unambiguously the primary step in this reaction. Early investigation (70) could not distinguish with certainty between the two possible primary steps... 

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%. 

The mercury photosensitized fluorescence of thallium vapour appears to provide an exception to the rule, because the reaction with the largest cross-section corresponds to a substantial change in internal energy, notwithstanding the opportunity of electronic energy transfer to a state of almost identical internal energy. At 900 °C the reaction... 

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