Hg-sensitized photolysis

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. 

Wetmore and Taylor (47) investigated the methylamlne photolysis and found the decomposition rate of the Hg-photosensitized reaction equal to that of the direct one where a plug of gold wire was interposed between the vacuum system and the photolysis cell to exclude Hg. It seems possible though that Hg was present in both cases. Photolyses carried out with the light of low-pressure Hg arc lamps were, at least in part, Hg-sensltlzed if the contamination of the photolysis vessels by the Hg vapor present in the vacuum system was not rigorously excluded. This holds true especially for the early work. 

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Only a few photochemical reactions of carboranes have been reported in the literature. Plotkin and Sneddon synthesized a carborane dimer by the Hg-sensitized photolysis of C2B5H7 (Fig. 6). 

At present, there is some controversy regarding the chain step when RNO is HNO. Nitrogen is formed, but its yield is always much smaller than for other RNO species.92,93,407 Consequently, HNO is not an efficient chain carrier. Srinivasan398 photolyzed 1SNH3 in the presence of 14NO and found that up to 4% of the N2 produced was 14N 14N presumably it comes from reaction of HNO with NO. Strausz and Gunning407 produced HNO from the Hg-sensitized photolysis of H2 in the presence of NO, as well as by the direct photolysis of CH20 in the presence of NO. In both systems, the N2 formed never exceeded the HNO produced. In place of the chain step they postulated... 

Strausz and Gunning407 produced HNO both from the Hg-sensitized photolysis of H2-NO mixtures and the direct photolysis of CHaO in the presence of NO. They found N2 and NaO as principal products, as well as CO in the CH20 studies. From an analysis of the product variations with conditions, they deduced reaction (9) for the N2-forming step and the following sequence for NaO formation... 

Fig. 8-6. Formation of DNO and N20 and depletion of NO during the Hg-sensitized photolysis of D2 in the presence of NO (from Kohout and Lampe264 with permission of the American Chemical Society).

The Hg-sensitized photolysis of SijH, yields SiH, and higher silanes ... 

The Hg-sensitized photolysis of GeH yields some Ge H, and the X-irradiation of GejHj-CjH mixtures yields ElGe Hj, EtGejH, etc., in processes that may involve formation of new Ge-H bonds. 

Hg-sensitized photolysis of compounds such as open-chain saturated ethers as well, where the C-H bond is exclusively cleaved (252). Energy transfer, however, may operate in other... 

The Hg-sensitized photolysis of several ethers in the liquid phase was briefly studied by Tsao (264). The gas phase photosensitized decomposition of ethers has found much more interest. In Steacie s group before 1950 there were investigated diethyl ether (265), ethylene oxide (266), and dimethyl ether (267). Cvetanovic and co-workers studied ethylene oxide (268) and trans-2,3-epoxybutane (269). Subsequent work on dimethyl ether was done by Pottle et al. (259), Takezaki et al. (270), Loucks and Laidler (271), and Payette et al. (252). The Hg-sensitized... 

Most authors agree that the initial chemical event in the Hg-sensitized photolysis of saturated straight-chain ethers in the decomposition of the ether molecule into the 1-alkoxylalkyl radical and the H atom, for example, reaction 75. In the initial stages of the photolysis the main products are hydrogen and the... 

TABLE 23. Quantum yields in the Hg-sensitized photolysis of some straight-chain ethers. 

TABLE 24. Quantum yields of products from the Hg-sensitized photolysis (25°) of some epoxides. 

The data available suggest that the Hg-sensitized photolysis of acyclic aliphatic amines (47,237,235) yields the same products that are found In the direct photolysis (see above) but perhaps in different proportion. Hydrogen, ammonia, and a cyclic trlmer of N-methylmethyleneimine were obtained from methylamine (237). Baum et al. (235) suggested that the Hg-sensitized photolysis of primary and secondary amines is of preparative value for the synthesis of N-alkylalkylideneimines. 

TABLE 26. Isotopic composition %) of various products from the Hg-sensitized photolysis of (CD2)2NH (276). 

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