Carbon suboxide, photolysis

The decompositions of C302, CO, C02, CS2, COS, CSe2 and COSe are dealt with in this section. Apart from carbon suboxide, this is a group of stable, un-reactive compounds. Considerable emphasis has been placed on the investigation of the photolytic decompositions of some of these compounds which are thought to provide useful sources of atoms (C, O, S and Se) and free radicals (C20). The photochemistry of carbon dioxide has particular relevance to the chemistry of planetary atmospheres, although to date the mechanism of C02 photolysis remains obscure. 

Carbon Suboxide Photoiysis. In principle, carbon suboxide (1) can be used as a precursor to atomic carbon and two molecules of carbon monoxide as shown in Eq. 2. However, this reaction is endothermic by 141 kcal/mol and can only be realized in the vacuum ultraviolet (UV) at wavelengths that destroy most organic substrates. However, photolysis of 1 at 1470 A produces C atoms in a low-temperature matrix. The short wavelength flash photolysis of 1 coupled with atomic absorption has been used to measure the rate constants for various spin states of carbon with simple substrates. 

The photolysis of carbon suboxide in the near ultraviolet has been studio I by many workers. The formation of C( 3/J) atoms is not energetically possibl< above the incident wavelength, 2066 A. Hence, the primary process must I ... 

The C( S) state lies 2.683 eV above the ground state C(3P) with a lifetime of 2 sec (32). The production of C.( S) atoms is observed in the photolysis of carbon suboxide in the vacuum ultraviolet. The C( S) atom production can be detected by absorption at 2479 or 1752 A. Rate constants of C(1S) with molecules have been measured by Meaburn and Perner (687), Husain and Kirsch (505), and Braun et al (141). The rate constants are in general much smaller (collision efficiencies 10 2 to 10 6) than those for C( D), in analogy with the results for O( D) and O( S) atom quenching rates given in Table IV-3. 

The ground state C atoms are produced by the photolysis of carbon suboxide. They can be monitored by optical absorption at 1657 A. Reactions of C(3P) atoms with molecules have been studied by Husain and Kirsch (497,498) and Braun et al. (141). 

Carbon suboxide (C302),50 an evil-smelling gas, is formed by dehydrating mal-onic acid with P2Os in vacuum at 140 to 150°C, or better, by thermolysis of diacetyltar-taric anhydride. The molecule is linear and can be represented by the structural formula 0=C=C=C=0. It is stable at -78°C, but at 25°C it polymerizes, forming yellow to violet products. Photolysis of Q02 gives QO, which will react with olefins ... 

Carbon suboxide, C3O2, 0=C=C=C=0 , also called l,2-propadiene-l,3-dione, is an evil-smelling unstable gas, bp +6.8 °C, which is obtained by the dehydration of malonic acid with P4O10 in vacuum at 140-150 °C (equation 6). Carbon suboxide polymerizes readily at room temperature to a yellow solid, and above 100 °C to a ruby-red water-soluble solid. Photolysis of C3O2 gives C2O ( C=C=0 ) as a reactive intermediate, which reacts with alkenes by carbon atom insertion (equation 7). 

Wavelengths longer than 220 nm. The absence (2) of fluorescenci or phosphorescence in carbon suboxide excited in the ultraviolet would suggest that it is efficiently photodecomposed in this region of the spectrum. From its structural similarity to ketene, it is expected that this will involve production of C2O in step 1. The work of Bayes has in fact shown that this is the most probable process in the absorption region with a maximum at 265 nm. Bayes (11) found that photolysis of 02/02 mixtures led to production of allene rather than the acetylene, which is known (12) to be formed by insertion of carbon atoms into C2H. ... 

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