Hg sensitivity

Photolysis of N-trimethylborazine with H2 yields a crystalline product, identified as l,2-di(3, 5 -dimethylborazinyl)ethane The following radical mechanism, involving Hg-sensitization is used to explain the photochemistry... 

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). 

In a more complex system Gozzo and Camaggi,64 photolyzed mixtures of N20 and C2F4 with 1849-A radiation. Both C2F4 and N20 can be energized either by direct absorption of the radiation or by Hg sensitization. (That Hg was present was shown by the results at 2537 A see Sect. IV-B-l-d.) N20 absorption leads initially to 0(1Z)) but some of these... 

Rate-Constant Ratios for Reactions of Electronically Excited Perfluoroolefins Produced by Hg Sensitization... 

Woodall,4531 working in Gunning and Strausz s laboratory, studied the Hg-sensitized decomposition of CF2C1H in the presence of NO. The major product was CF2HNO, but the oxime was also present. The reaction sequence is presumably... 

Woodall453 also examined the Hg-sensitized decomposition of z-C4H10, n-C4H10, and 2,2-dimethylbutane. In the presence of NO, the first compound yielded (CH3)3C=NO the latter two compounds yielded oximes, presumably via isomerization of the appropriate precursor nitroso compounds. 

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).

Further evidence for this step was provided by Knight and Gunning.259 In their study of the Hg-sensitized decomposition of C2H5OH in the presence of NO, they found that no CH3CHO was formed, yet 0(C2HBONO) was only 0.25. To explain the deficiency of products from C2H50 radicals, they suggested reaction (24). 

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... 

From Condition 2, iodine is a monoisotopic element giving a very sharp line at 2062.4 A. (15). (Using the 1849 A. line of normal Hg in Hg sensitized CO produced C3O2 but no enrichment of the C isotope.)... 

Atomic hydrogen obtained (i) from dissociation in an electrical discharge, (ii) in a radio-frequency plasma discharge, (iii) photochemically in Hg-sensitized processes, or (iv) at high T reacts with phosphorus or its compounds to form phosphines. Elemental P or P40,Q and atomic H react ... 

Hydrogen atoms generated from H in an electrical discharge, by thermolysis of Hj or in a Hg-sensitized photochemical process react with Sb targets to form SbHj in low yields ... 

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. 

The Hg( / ) sensitized reaction of HNCO yields CO, Nj, and H2. The primary 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. 

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... 

TABLE 22. Hg-sensitized decomposition of saturated alcohols in the gas phase at room... 

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. 

Hg-sensitized decomposition of ethylene oxide since hydrogen formation is strongly reduced in the presence of olefins. Two routes to H atoms were proposed, reactions 80 and 81. 

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

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