Quantum yield of photodecomposition

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 photochemical stabihty of the molecules is characterized by the quantum yield of photodecomposition, (P = N/Q [69], where N and Q are the numbers of decomposed molecifles and absorbed photons, respectively. The photochemical properties of the fluorene derivatives were investigated in different organic solvents (hexane, CH2CI2, ACN, and polyTHF) at room temperature by the absorption and fluorescence methods and comprehensively described [70-72]. These methods are based on measurements of the temporal changes in the steady-state absorption and fluorescence spectra during irradiation. For the absorption method, the quantum yield of the photodecomposition under one-photon excitation, c >ipa, can be obtained by the equation [73] ... 

Although the net quantum yield of photodecomposition is found to be very small in the gas phase (typically 0.05), it has been found that in the presence of silver,16 mercury,16 oxygen,17 or nitric oxide,18 the quantum yield increases markedly. The two metals scavenge both atomic and molecular iodine, and the two gases scavenge alkyl radicals, thus suppressing both of the two possible inhibiting secondary steps. 

The photolysis of isopropyl iodide has been investigated by McMillan and Noyes.16 Addition of excess NO increased the quantum yield of iodine formation at 3130 A. to — 0.5, and in the presence of metallic silver the quantum yield of photodecomposition rose to unity. Hence the primary process at 3130 A. is exclusively... 

Upon photolysis, there are two reactive species formed side by side, the vinylic uracilyl radical and Br The latter has oxidizing properties (E7 = 1.66 V Ward-man 1989), and in DNA it can oxidize neighboring G and A moieties (Wojcik et al. 2003). The quantum yield of photodecomposition of 5BrUra is only 1.8 x 10 3, but increases dramatically upon deprotonation (Campbell et al. 1974). In the presence of MeOH as an H-donor, it also increases substantially above 2 mol/1 MeOH. It has been speculated that above this concentration cage reactions come into play. Reported rate constants for the reaction of Br- with MeOH are 5 x 104 dm3 mol-1 s 1 in water and 1 x 106 dm3 mol-1 s 1 in acetonitrile (Neta et al. 1988). 

Since the early 1990s, ruthenium (II) polypyridine complexes have been used as mediators in the interconversion of light and chemical energy. A quite important property for photosensitizers is their stability toward photodecomposition. For [Ru(bpy)3] cation in aqueous solution, the quantum yield of photodecomposition is from 10- to 10-3, depending on the pH and temperature. In the solution containing anions (CF, Br, and NCS ), this quantum yield can be as high as 0.1 for solvents with a low dielectric constant [386]. A way... 

Table II. Quantum Yield of Photodecomposition of cis-1,4-Polyisoprene Hydroperoxide at 313 nm Light Intensity = 7.2 X 10 7 Einstein/min.

Bacher et al. (2001) used acetone and acetaldehyde as reference compounds in pho-tolyses within the reaction chamber to estimate the average quantum yield of process (I) in 1 atm. of air <(>i>0.5 for X = 285 25nm. From measuredy-values in sunlight in the EUPHORE chamber, Magneron et al. (2005) concluded that the effective total quantum yield of photodecomposition was near unity (< = 1.3 0.3). 

Table IX-C-4. Methacrolein absorption cross sections (Gierczak et al., 1997) and estimated total quantum yields of photodecomposition (0totai) at 1 atm. units for a are cm molecule" ...

From our current evaluation of the data, we conclude that the variation in the quantum yield of photodecomposition of methylglyoxal by all processes is described at least qualitatively by the black solid and dashed lines in figure IX-C-20 the (/)totai values that describe these curves are given in table IX-C-8 and are used in deriving the present best estimate of the photolysis frequencies for methylglyoxal that are given in the following section. 

Figure IX-C-30. Approximate photolysis frequencies for CF3CHO versus solar zenith angle for a cloudless day in the lower troposphere. The quantum yield of photodecomposition to form CF3 and HCO radicals is assumed to be 0.17 at all wavelengths of absorbed light (Chiappero et al., 2006). We also used in this calculation the actinic flux available within the lower troposphere (overhead ozone column of 350 DU), and an average of the cross section data of SeUevSg et al. (2004) and Chiappero et al. (2006).

Early studies involved photolysis of pure 3-pentanone using partially filtered, full-arc radiation. Reported quantum yields of photodecomposition and CO formation were... 

Figure IX-E-2. Quantum yields of photodecomposition of cyclopropanone by process (I) cyclo-propanone + hv C2H4 + CO, and those for decomposition by all pathways. Data are from Thomas and Rodriguez (1971) and Rodriguez et al. (1976) figure from Calvert et al. (2008). 

The quantum yields of photodecomposition of ketene versus wavelength were estimated using the data given in figures IX-E-16 and IX-E-17. These are given in figure IX-E-21... 

Figure IX-E-21. Quantum yields of photodecomposition of ketene versus wavelength (solid hlack line) cross sections of ketene versus wavelength, gray curve wavelengths of onset of CH2 and CH2 are indicated by the long-dashed line and short-dashed line, respectively.

Figure IX-F-13. Estimated photolysis frequencies for CF3CF2C(OXiiF(CF3)2 as a function of solar zenith angle. Values are calculated for a cloudless day in the lower troposphere with an overhead ozone column of 350 DU. Cross section data and average quantum yields of photodecomposition of D Anna et al. (2005a) were used in the calculations. The data points shown as filled circles give the range of solar zenith angle during the measurements of D Anna et al. (2005a) that were made in the EUPHORE chamber on July 14, 2003.

IX-F-10.2. Quantum Yields of Photodecomposition of Methyl Vinyl Ketone... 

Figure IX-F-19. Plot of the estimated quantum yields of photodecomposition of methyl vinyl ketone in 760 Torr of air as a function of wavelength. The three data points shown are from the Stern-Volmer plot of figure IX-F-18 using the data of Gierczak et al. (1997). Absorption cross section data shown are from Gierczak et al. (1997).

Maricq and Szente (1995) used time resolved IR and UV spectroscopy to study the photodissociation of CF3C(0)C1 in photolysis at 193 and 248 nm. They concluded that the net reaction shown by process (III) described the results best rapidly dissociating CF3CO and/or COCl radicals presumably were present as intermediates. At 193 nm the CO is formed with extensive internal excitation (TVib = 3, 800 900 K), whereas at 248 nm it is formed predominantly in v = 0. Quantum yields of the photodecomposition were measured by comparing yields of HCl (formed in experiments with added ethane) and RO2 (experiments with added O2) from CF3C(0)C1 to those from the photodisssociation of CH3CI at 193 nm and CI2 at 248 nm they assumed the latter two species have unit photodissociation efficiencies. Values of the total quantum yield of photodecomposition, ((/>i -t- + < ) = 1.10 0.11 at 193 nm and 0.92 ... 

Figure IX-G-3. Comparison of the photolysis frequencies for photodecomposition by all modes versus solar zenith angle for some acyl halides for which absorption of tropospheric actinic flux is measurable within their long-wavelength tails (a) formyl halides and carbonyl halides compared to CH2O (b) haloacetyl chlorides compared to CH3CHO. Calculations were made for a cloudless day within the lower troposphere with a vertical ozone column of 350 DU, cross section data of table IX-G-1, and an assumed quantum yield of photodecomposition of unity figure from Calvert et al. (2008).

There are few estimates of the quantum yield of photodissociation of HONO, but existing evidence suggests that process (I) is the dominant decomposition mode for pho-tolyses at wavelengths of sunlight available within the troposphere. Thus Cox (1974) concluded from his studies of the 330-380 nm photolyses of HONO, NO, NO2 mixtures in air that 0.7 < ( i + < n < 1.9. He stated that when uncertainties are taken into account, it is probable that the total quantum yield of photodecomposition of HONO is close to unity. He also concluded that 10% of the photodecomposition may proceed through process (II). 

There have been several useful studies that have allowed estimates of the quantum yields of photodecomposition of methyl nitrite. Wiebe et al. (1973) provided the first meaningful estimates of primary process (I) in their studies at wavelength 366 nm. They photolyzed mixtures of CH3ONO and and measured the quantum yields of CH30 N0 and formation. From the mechanism outlined in the introduction... 

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