Quantum yield, in photolysis

The yield of unimolecularly formed cyclohexene, and by that the yield of unimolecular H2 elimination (quantum yield in photolysis or G value in radiolysis), XiWj), may be calculated from the yields of alkene and dimer dehydrogenation products by the relation [91,92] ... 

Kirmse and Horner160 have shown that the quantum yield, in photolysis with monochromatic light, of C6H5COCHN2 is lowered considerably by the presence of additional double bonds in conjugation with the carbonyl groups and by para- and mearoyl groups attached to the carbon atom of the diazo group. 

In view of the action of trace impurities, many early observations on product distributions and on quantum yields in photolysis studies should be regarded with caution, particularly in the case of solution or liquid phase in which the reaction behaviour is extremely sensitive to traces of oxygen . ... 

This transformation was demonstrated (14) by evaluating changes in the UV, IR and NMR spectra and comparing these to the spectra of authentic samples of each cluster (15., 16.). Quantum yields for the photoisomerization depicted in Equation 17 were found to be notably dependent both on the CO concentration and on the A rr. Although the resulting optical changes were the same for different A rr, the quantum yields in CO saturated cyclohexane ranged from < 10 at 405 run to 4.9 x 10 at 313 nm. Furthermore, 2 varied linearly from 1.2 x 10" at Pqq " 0.0 to 4.9 x 10 at P q - 1.0 atm for 313 nm photolysis in cyclohexane. 

Region A X > 2100 A. This is a region of very weak, continuous absorption. There have been no attempts to determine quantum yields in this region, but studies139 of the photolysis of mixtures of NzO and hydrocarbons at 2139 A and 2288 A suggest that (b) (Table 9) is an important primary process. Sponer and Bonner137 have attributed a weak maximum in the absorption (at 2900 A) to a transition to a 3II state which dissociates via (a), but it could equally well correspond to an upper state which dissociates via (b). 

Dulin, D., Drossman, H., and Mill, T. Products and quantum yields for photolysis of chloroaromahcs in water. Environ. Sci. Technol, 20(l) 72-77, 1986. 

The early work on the photolysis of water was in the gas phase employing one photon. The branching ratio of the photodissociation into H + OH and H2 + O was reported by McNesby et al. [28] as 3 1 at a photon energy of 10.03 eV. Ever since, that ratio has been consistently revised in favor of the H + OH reaction with the final result of Stief et al. [29] giving 0.99 0.01 for 6.70-8.54 eV photon energy and 0.89 0.11 for the interval 8.54-11.80 eV. In the absence of direct determination these ratios often are assumed valid in the liquid phase. In the early work of Sokolev and Stein [30], mainly the photodissociation quantum yield in liquid water was measured, but a small photoionization yield of -0.05 was attributed to the process... 

Zellner, R., M. Exner, and H. Herrmann, Absolute OH Quantum Yields in the Laser Photolysis of Nitrate, Nitrite, and Dissolved H202 at 308 and 351 nm in the Temperature Range 278-353 K, /. Atmos. Chem., 10, 411-425 (1990). 

Diazomethane. Although CH2N2 photolysis has been widely used as a methylene source, the reaction of CH2 with CH2N2 has received little attention. The quantum yield in the photochemical decomposition of CH2N2 is about four at both 4360 A. and 3650 A.96 The mechanism is believed to involve a short energy chain, in addition to the simple process... 

The high quantum yield of photolysis of C02 suggests the rapid destruction of C02 and the formation of CO and 02 by sunlight of wavelengths below about 2200 A (see Section VI- 5). According to an estimate by McElroy and McConnell (675), the column abundance of C02 in the atmosphere is 2 x 1023 molec cm"2. With a dissociation rate of 2.5 x 1012 cm-2 sec-, the entire C02 may be destroyed in less than 10,000 years. 

Precise determinations of quantum yields in dry ozone photolysis would be of considerable value in clarifying the possible mechanisms. If an energy or photon chain exists, there is no reason why the quantum yield should be limited to about 6. It should be possible to use experimental conditions where it would be much higher. If an energy or photon chain does not exist, the quantum yield should not exceed 4 or, perhaps, 6. It should be emphasized that the most precise work in the ultraviolet yields a maximum quantum yield of 6 and that an error of 2 in this value would be possible. 

Campbell JM, Schulte-Frohlinde D, von Sonntag C (1974) Quantum yields in the UV photolysis of 5-bromo-uracil in the presence of hydrogen donors. Photochem Photobiol 20 465-467 Chapman JD (1984) The cellular basis of radiotherapeutic response. Radiat Phys Chem 24 283-291 Chatterjee M, Mah SC, Tullius TD, Townsend CA (1995) Role of the aryl iodide in the sequence-selective cleavage of DNA by calicheamicin. Importance of thermodynamic binding vs kinetic activation process. J Am Chem Soc 117 8074-8082 Chaudry MA, Weinfeld M (1995) The action of Escherichia coli endonuclease III on multiply damaged sites in DNA. J Mol Biol 249 914-922... 

In both Re(CO)5X and M(CO)s(amine) higher energy photolysis leads to population of LF states which feature population of the dxzy2 bi )° y orbital which labilizes the equatorial CO s and leads to larger CO substitution quantum yields. In all of these C4v complexes the ligand photosubstitution most likely occurs by strictly a dissociative mechanism to yield coordinatively unsaturated intermediates. For the Re(CO)sX, photolysis in the absence of added nucleophiles yields the dimeric species [Re(CO)4X]2 reaction (12), which likely form via coupling of two coordinatively unsaturated Re(CO)4X intermediates.68 ... 

Fig. 34.—Quantum yield in the photolysis of beta-caryophyllene nitrosite.

This work was useful in establishing that the low quantum yield in the photolysis of pure methyl iodide was due to back reactions and that the initial decomposition into a methyl radical and iodine atom occurred with approximately 100% efficiency. In 1942, Blaedel et al.22 showed that the quantum yield for the decomposition of methyl iodide was, in fact, somewhat less than unity in the presence of oxygen because Bates and Spence11 had used an incorrect value for the quantum yield of the chloracetic acid actinometer. They suggested the reactions... 

The little information available (see above) suggests that photolysis of gas-phase PCBs in the troposphere will be negligible for those PCBs with <4 chlorine atoms, and this may also be the case for the more chlorinated PCBs. However, it appears that the PCDDs and PCDFs will absorb radiation in the actinic region in the troposphere, and hence the importance of photolysis will depend on the photodissociation quantum yields in the gas phase, which are, however, not presently known. 

One can see from Table 7.1 that the quantum yield of photolysis of alkyl-substituted anthraquinones only slightly depended on the replacement of the methyl group by the ethyl or isopropyl group.26 However, it decreased drastically after the introduction of electron-donor substituents in 3- or 4-positions of the anthraquinone ring.17 Quantum yields of phototransformations of l-arylcyanomethyl-9,10-anthra-quinones were found to be substantially lower (by two orders of magnitude) than for 1-methylanthraquinone.36... 

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