Methane, decomposition photolysis

The short-wavelength radiation necessary for this decomposition is absent in the lower layers of the atmosphere it is likely that the photolysis of ethyne occurs via C-H cleavage to give radicals, which react with methane to give methyl radicals, the recombination of which affords ethane ... 

The photolysis of trifluoroacetone with light of wavelength 3130 A. has been studied by Sieger and Calvert.48 The products of decomposition were shown to be carbon monoxide, methane, ethane, 1,1,1-trifluoro-ethane, and hexafluoroethane. There was no indication of the presence of carbon tetrafluoride or methyl fluoride. The quantum yield of all products was low at low temperature and it is assumed that the excited molecule of trifluoroacetone has an appreciable lifetime and may be deactivated by collision before decomposition can occur. This contention is supported by the decrease in the quantum yields observed when foreign gases such as carbon dioxide are added, and by the fall in quantum yields with increase in trifluoroacetone concentration. 

Vanpee and Grard133 made a quantitative study of the formation of saturates (mainly ethane) in the photolysis of CH2CO with added methane (ratios CH4/CH2CO = 1 to 7) at 28 to 250 °C. and found that the results could be explained by a mechanism involving competition between CH4 and CH2CO for methylene by reactions of the first order in methylene. The rate of reaction of CH2 with CH4 was found to be 0.183 that of the reaction with ketene. Decomposition of excited ethane by the reaction... 

Decomposition of l-methyl-2-pyrrolidinone (67) was studied by vapor-phase photolysis (72JA8281). Irradiation (Hg sensitized) led, in addition to extensive polymer formation, to the following products carbon monoxide (31%), ethene (24%), water (24%), l,3,5-trimethyl-hexahydro-l,3,5-triazine (8%), 1-methylazetidine (6%), 1-methylpyrrole, and methane (<1%). The mechanism of formation of most of these products involves... 

Bell and Kistiakowsky80 have also formed CH2D2, at or near room temperature, by photolysis of diazomethane followed by the reaction, CH2 + D2 — CH2D2 ketene and H2 were also used. Decomposition of the formed methane was definitely shown to be extensive in a few runs with oxygen at 100 mm. D2. It may be estimated roughly that ka >... 

The occurrence of molecular eliminations in photolysis is something that does not occur to a significant extent in the pyrolysis of hydrocarbons. Most frequently the molecular decomposition is accompanied by free-radical decomposition, with the fraction of each often dependent on the wavelength of the light used to initiate reaction. Such a case is the photolysis of methane where the following reactions occur ... 

The increased importance of the methylene elimination to give methane also accounts for the increase in propane, since the insertion of methylene into ethane gives propane directly through stabilization or indirectly through methyl radicals if decomposition occurs. n-Butane is formed by combination of ethyl radicals in the same manner as for the 1470 A photolysis. 

The photolysis of n-butane follows a pattern similar to that of propane, with many corresponding reactions. As found for previous hydrocarbons the photolysis includes both molecular and free-radical processes. The molecular elimination of Hj and Dj from C4H10-C4D10 mixtures was first shown by Sauer and Dorfman, who concluded that at 1470 A more than 90 % of the hydrogen came from molecular processes. On the basis of a study of the decomposition of excited -butane molecules generated by electron impact , they attributed hydrogen, methane, ethylene, and other hydrocarbon products to molecular processes, and concluded that free-radical reactions were minimal. 

Sieger and Calvert reported the photolysis products of 1,1,1-trifluoroacetone at A 3130 A to be carbon monoxide, methane, ethane, 1,1,1-trifluoroethane, and hexafluoroethane. A low quantum yield for decomposition near room temperature may be explained in terms of the excited trifluoroacetone having an appreciable lifetime and therefore suffering possible collisional deactivation before decomposition can occur. Two possible primary stepts of Type 1 have been proposed... 

Later these experiments were repeated -with the conclusion that Morris findings were dubious. Zemany and Burton used equimolar mixtures of acetaldehyde and acetaldehyde- /4, at temperatures 510 and 465 °C, and found that partially deuterated methanes were formed in appreciable amounts. The ratio CHD3/ CD4 was found to be 1.2 and 1.0 at 510 and 465 °C, respectively (compared to the value of 1.6 obtained in the photolysis at 140 °C). These results clearly indicate the free radical origin of the methane. However, the fact that the CHD3/CD4 ratio is lower than the one found in the photolysis made the authors conclude that there IS some contribution from the molecular mechanism. An upper limit for the latter was estimated to be approximately 15 and 25 % of the total reaction at temperatures 510and465 °C, respectively. Zemany and Burton estimated the values for the ratios methane-rf3/ethane-d6 and methane-t /ethane-rfe, from which a chain length of 1000 can be derived, at 465 °C, for the Rice-Herzfeld type decomposition. 

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 thermal or photochemical decomposition of volatile dialkyl telluriums is of interest as a source of tellurium atoms for the production of semiconductor devices. The bond breaking enthalpy for the Te—C bond in dimethyl tellurium is 212.1 + 5.4 kJ mol , and in divinyl tellurium 226.8 +9.6kJmor . Dimethyl tellurium begins to decompose at 350° with formation of tellurium, methane, and other hydrocarbons Diethyl ditel lurium begins to decompose at an appreciable rate above 420° Photolysis of dimethyl tellurium and diethyl tellurium with 248 nm photons produced tellurium atoms in a single photon process. The liberated tellurium atoms were in the 5p 2 state. ... 

The photochemical decomposition of methanal in a solid Xe matrix has been studied. Work has also been reported dealing with the photodissociation dynamics of methanal, and ab initio calculations have been carried out on the photochemical decomposition of acetaldehyde into methane and CO. The photocatalytic decomposition of acetaldehyde to yield carbon dioxide has also been reported. The threshold for CC bond fission in propanal and the release of the CHO fragment has been shown to be at a wavelength of 326.26 nm. Chowdhury has reported the dissociation of propynal using multiphoton irradiation. Gas-phase photolysis of butyraldehyde in the 280-330 nm range has shown that the CHO radical is produced. ... 

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