Photodecomposition yields

Since the last reviews of carbonyl photochemistry (61,191), there have been several studies of the primary photodecomposition processes- The earlier studies have shown that there are two main primary photodissociation pathways, radical (I) and molecular (II)  

SThis value of V3 is taken from Hardwick and Till (J. L. Hardwick and S. M. Till, J. Chem. Phys 70, 2340 (1979) and replaces the earlier value 887 cm given in reference 124. 

The most recent threshold determination for the radical process (I) utilizes the.Meinel band chemiluminescence of the O(- P) + HCO - OH + CO reaction (20). The basic photochemical questions have been and remain the determination of the true precursors for the two product channels and the energy (or excitation wavelength)-dependent quantum yields for each channel. 

DeGraff and Calvert (66) have carried out careful steady-state photolyses of H2CO and D2CO with added pressures of propylene, neopentane, and biacetyl at 334.0, 313.0, 265.4, and 253.7 nm. Propylene was used as an olefinic radical scavenger, neopentane as an inert gas for vibrational deactivation, and biacetyl as a possible triplet energy acceptor. 

Mixed isotopic molecular hydrogen can be produced by the hydrogen-atom attack on formaldehyde, for example, 

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The very small photodecomposition yields at low temperatures are to be expected in this region, as the photon energy available at 365.0 nm [78.5 kcal/mol is less than that required for dissociation by process 93 and is marginal for process 92 (AH92 = 69.5 kcal/mol, AHg = 89.3 kcal/mol)]. The temperature dependence of these yields has been best explained as a unimolec-ular decomposition of T- with an activation energy of 15 kcal/ mol (176), a value comparable to Ea values found for acetone and hexafluoroace tone. 

The absolute photodecomposition yield has been shown to be both wavelength- and pressure-dependent. Taylor and Porter (239) deduced absolute photodecompositlon yields of 0.70, 0.36, and 0.006 at 313, 334, and 366 nm, respectively, although the value at 366 nm is probably nil (203). Strachan and Thornton (233) found a zero-pressure quantum yield of 0.10 at 366 nm, and a unit decomposition yield at 313 nm which is pressure-independent at pressures lower than 250 torr. 

A theoretical calculation shows that 80 kcal/mol is required to form oxirene from ketene (73). The fact that hot ground-state ketene has no low-energy decomposition pathway available may make the explanation of the pressure dependence of photodecomposition yields plausible. The thresholds for 3g and A production have been measured as 75.7 1.0 kcal/mol and 84.0 0.6 kcal/ mol, respectively (225). These high barriers should therefore make the decomposition relatively slow for a molecule this size at low excitation energies, and thus subject to pressure quenching at moderately high pressures (10-100 torr). 

The quantum yield of the initiation process (<, ) is quite low 8 x 10, indicating the great stability of the chelate ring toward photolysis. However, the quantum yield of photodecomposition 4>d) under similar condition is 2 X 10, which is higher than It is clear, therefore, that not every molecule of Mn(acac)3 that is decomposed initiates polymerization apparently, ex-... 

The photopolymerization of this monomer with a mercury arc89,9°) produces small yields of low molecular-weight products. In the presence of oxygen an induction period is noted and the polymers contain an appreciable amount of peroxide units in the chains9 ). The photolysis of 2-vinylfuran was briefly described by Hiraoka92 cyclopentadiene and CO were reported as products. It is not certain if free radicals are involved in this photodecomposition, but presumably they are. 

Diacyl peroxides have continuous weak absorptions in the UV to ca 280 nm (e ca 50 M cm 1 at 234 nm),147 Although the overall chemistry in thermolysis and photolysis may appear similar, substantially higher yields of phenyl radical products are obtained when BPO is decomposed photochemically. It has been suggested that, during the photodecomposition of BPO, (3-scission may occur in... 

Arylamine Photodecomposition. A number of researchers have alluded to the fact that the products produced from photolysis of aromatic carbamates (i.e., la) also degrade upon irradiation (10), 17). Indeed, we found that the aryl amine 2b and the photo-Fries products 2c and 2d (resulting from photolysis of 2a) decomposed with respective disappearance quantum yields of 0.035, 0.004, and 0.003 when irradiated at 280 nm. These latter results agree with those of Schwetlick et al. (17), who found the rates of disappearance of lc and Id to be quite small. 

The quantum yield of the selected azoalkanes photodecomposition are given in Table 3.18. The extinction coefficient s depends on the wavelength of light, and for AIBN in benzene solution (room temperature) has the values as given below [205] ... 

The inefficiency of photodecomposition of S02 in this spectral region is then ascribed to the small quantum yield of 3S02. 

Because of the paucity of rate data for the reactions of F and FO one can only speculate on the course of the reaction subsequent to the initial dissociation. The results of photochemical studies398,399 give some guidance. The quantum yield F2o of photodecomposition is 1.0 at 3650 A, independent of temperature in the range 15-45 °C, pressure of F20 and pressure of oxygen398 the primary step is almost certainly as in (2)398,3". Thus, at room temperature at least, any contribution from... 

While reactions (16)—(19) and (9) are adequate to explain the small quantum yields for the photodecomposition in solution ( Cio2 2 in CC14 for X < 4100... 

Table 2 Relative ratio and yield( ) of driect and sensitized photodecomposition products of phenylazide in cyclohexene...

Photodegradation. Casida s group (15, 20) has studied the photodecomposition of R-20458 on silica gel and in water. The major aqueous photoproducts are summarized in Figure 1. The predominant photoproduct in aqueous solution resulted from epoxide hydration to the corresponding diol. The photoproducts on silica were quite similar to aqueous products with an enhanced yield of diepoxide and diminished yield of diol. Photosensitizer dyes had little effect on R-20458 photodegradation. 

When an aqueous solution containing 1,2-dichlorobenzene (190 pM) and a nonionic surfactant micelle (Brij 58, a polyoxyethylene cetyl ether) was illuminated by a photoreactor equipped with 253.7-nm monochromatic UV lamps, photoisomerization took place yielding 1,3- and 1,4-dichlorobenzene as the principal products. The half-life for this reaction, based on the first-order photodecomposition rate of 1.35 x lO Vsec, is 8.6 min (Chu and Jafvert, 1994). 

Photodecomposition of pentachlorophenol was observed when an aqueous solution was exposed to sunlight for 10 d. The violet-colored solution contained 3,4,5-trichloro-6-(2 -hydroxy-3, 4, 5, 6 -tetrachlorophenoxy)-o-benzoquinone as the major product. Minor photo-decomposition products (% yield) included tetrachlororesorcinol (0.10%), 2,5-dichloro-3-hydroxy-6-pentachloro-phenoxy-p-benzoquinone (0.16%), and 3,5-dichloro-2-hydroxy-5-2, 4, 5, 6 -tetrachloro-3-hy-droxyphenoxy-p-benzoquinone (0.08%) (Plimmer, 1970). 

Photolytic. Under smog conditions, indirect photolysis via OH radicals yielded phosgene, di-chloroacetyl chloride, and formyl chloride (Howard, 1990). These compounds are readily hydrolyzed to HCl, carbon monoxide, carbon dioxide, and dichloroacetic acid (Morrison and Boyd, 1971). Dichloroacetic acid and hydrogen chloride were reported to be aqueous photodecomposition products (Dilling et al., 1975). Reported rate constants for the reaction of trichloroethylene and OH radicals in the atmosphere 1.2 x 10 cm /molecule-sec at 300 K... 

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