Ultraviolet photolysis, carbon

Loraine GA (1993) Short Wavelength Ultraviolet Photolysis of Aqueous Carbon Tetrachloride, Hazard. Waste Hazard. Mat. 10 185-194. 

Homolytic rupture of the carbon-halogen bond appears to be the dominant primary process. Lewis and Owen have studied the continuous ultraviolet photolysis of the triphenylmethyl halides in cyclohexane solution. They have proposed a mechanism which involves the formation of the triphenylmethyl radical in an excited electronic state. 

Ultraviolet photolysis of carbon monoxide occurs through absorption in narrow lines, corresponding to specific rotational and vibrational levels of several electronic transitions. The absorption wavelengths are isotope-dependent due to the effect of isotopic mass on the vibrational and rotational energies. As a consequence, the absorption lines due to C 0, the most abundant isotopologue, do not overlap those due to C O and Because of the... 

Carbajo, P.G., Smith, S.C., Holloway, A.-L., Smith, C.A., Pope, F.D., Shallcross, D.E., Orr-Ewing, A.J. Ultraviolet photolysis of HCHO absolute HCO quantum yields by direct detection of the HCO radical photoproduct. J. Phys. Chem. A 112, 12437-12448 (2008) Carlon, N.R., Papanastasiou, D.K., Fleming, E.L., Jackman, C.H., Newman, P.A., Burkholder, J. B. UV absorption cross sections of nitrous oxide (N2O) and carbon tetrachloride (CCLi) between 210 and 350K and the atmospheric implications. Atmos. Chem. Phys. 10, 6137-6149 (2010)... 

No information was found on the transformation of diisopropyl methylphosphonate in the atmosphere. Based on the results of environmental fate studies of diisopropyl methylphosphonate in distilled water and natural water, photolysis (either direct or indirect) is not important in the transformation of diisopropyl methylphosphonate in aquatic systems (Spanggord et al. 1979). The ultraviolet and infrared laser-induced photodegradation of diisopropyl methylphosphonate in both the vapor or liquid phase has been demonstrated (Radziemski 1981). Light hydrocarbon gases were the principal decomposition products. Hydrogen, carbon monoxide (CO), carbon dioxide (C02), and water were also detected. 

E. Hayon and J. McGarvey, Flash photolysis in the vacuum ultraviolet region of sulfate, carbonate, and hydroxyl ions in aqueous solutions. J. Phys. Chem. 71, 1472-1477 (1967). 

Photolysis of chlordecone in the atmosphere does not appear to be an important degradation pathway for this compound. While nonvolatile products of photolysis were not monitored, only 1.8% of the chlordecone adsorbed on silica gel and exposed to ultraviolet light (wavelength >290 nm) was photolyzed to carbon dioxide or other volatile compounds (Freitag et al. 1985). 

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. 

In spite of its long-assumed intermediacy in several reactions, no carbon-substituted silylene was directly observed for many years. In 1979, however, Drahnak et al. detected a broad ultraviolet (UV) absorption band (3-niax = 450 nm) after the photolysis of dodecamethylcyclohexasilane (6) in 3-methylpentane. This band was assigned to dimethylsilylene (5). Many different approaches to this intermediate, either photochemicaUy or thermally, were examined (Scheme 14.6). ... 

The C( lS) 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 suboxidc in the vacuum ultraviolet. The C( S) atom production can be detected by absorption at 2479 or 1752 A. Rate constants of C( S) 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 h) 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 C1(Xl L ) is a primary product of acetylene photolysis. The r/ ll state is formed from the photolysis of bromoacetylene in the vacuum ultraviolet. It is also formed in flame and discharges through carbon containing compounds. The Swan system is a major feature of emission spectrum from the heads of comets. 

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 ... 

Photolysis of this polymer gives radicals on which side chains can be formed, giving graft polymerization 122, 123, 153). Similarly the polymerization of styrene (152) or vinyl acetate (157) in the presence of bromotrichloromethane gives telomers carrying terminal bromine atoms and trichloromethyl groups. By ultraviolet irradiation (3500 A) in the presence of methyl methacrylate the carbon-bromine links are broken and block copolymers are formed. The telomerization of acrylonitrile and acrylic acid with bromoform is based on the same technique the end groups of both polyacrylonitrile and polyacrylic acid were photolyzed in the presence of acrylamide and afforded polyacrylamide blocks linked to polyacrylonitrile or polyacrylic acid blocks (164, 165). 

For comparative study, various silenes were isolated in 3-methylpentane matrix at 77 K by photolysis of the corresponding silyldiazo compounds and their ultraviolet spectra are measured. The results are summarized in Table 1. The introduction of trimethylsilyl group on carbon results in slight red shift compared with the parent silene (H2Si=CH2, 258 nm). As one might expect, considerable bathochromic shifts have been observed for conjugated silenes such as 6c, 6d and 2829,30. 

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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