Photochemistry quantum yields

N. Y. C. Chu, in Proceedings of the 10th IUPAC Symposium on Photochemistry, Quantum yield for photodegradation, Interlaken, Switzerland (1984). 

Nitrogen dioxide Nitrate Nitrite Ozone Photochemistry Quantum yield Rate law... 

The thermodynamic force for light reactions is The dependence of X/ on photochemistry quantum yield (j> = JH may be described by... 

Seller, R., Nicovich, J.M., Wine, P.H. Bromine nitrate photochemistry quantum yields for O, Br, and BrO over the wavelength range 248-355 nm. J. Phys. Chem. A 106, 8378-8385 (2002) Stevens, C.G., Swagel, M.W., Wallace, R., Zare, R.N. Analysis of polyatomic spectra using tunable laser-induced fluorescence applications to the NO2 visible band system. Chem. Phys. Lett. 18,465 9 (1973)... 

Studies of actinide photochemistry are always dominated by the reactions that photochemically reduce the uranyl, U(VI), species. Almost any UV-visible light will excite the uranyl species such that the long-lived, 10-lt seconds, excited-state species will react with most reductants, and the quantum yield for this reduction of UQ22+ to U02+ is very near unity (8). Because of the continued high level of interest in uranyl photochemistry and the similarities in the actinyl species, one wonders why aqueous plutonium photochemistry was not investigated earlier. 

Finally a few sentences are deserved for the vast area of DNA photochemistry. Thymine dimerization is the most common photochemical reaction with the quantum yield of formation in isolated DNA of all-thymine oligodeoxynucleotides 2-3% [3], Furthermore, a recent study based on femtosecond time-resolved transient absorption spectroscopy showed that thymine dimers are formed in less than 1 ps when the strand has an appropriate conformation [258], The low quantum yield of the reaction in regular DNA is suggested to be due to the infrequency of these appropriate reactive conformations. 

A study of the photochemistry of 2-trimethylstanny 1-1,3-butadiene (10 in Scheme 1) shows that only isomerization is observed, with no Sn—C cleavage or polymerization26. Three different isomers are formed in approximately equal proportions, as shown in Scheme 1. In the case of the formation of 3-trimethylstanny 1-1,2-butadiene (11), an intermediate is observed, but not characterized. The intermediate subsequently either reforms 10, or forms 11. The quantum yield of the reaction shows a strong dependence on the concentration of 10. At [10] = 10 2 moll-1,... 

Conversely, a quantum yield

greater than unity cannot be achieved during a straightforward photochemical reaction, since the second law of photochemistry clearly says that one photon is consumed per species excited. In fact, values of > 1 indicate that a secondary reaction(s) has occurred. A value of > 2 implies that the product of the photochemical reaction is consumed by another molecule of reactant, e.g. during a chain reaction, with one photon generating a simple molecule of, say, excited chlorine, which cleaves in the excited state to generate two radicals. Each radical then reacts in propagation reactions until the reaction mixture is exhausted of reactant. 

To help clarify the situation, we generally define two types of quantum yield primary and secondary. The magnitude of the primary quantum yield refers solely to the photochemical formation of a product so, from the second law of Photochemistry, the value of 0(primary) cannot be greater than unity. 

As a natural consequence of the second law of photochemistry, the sum of the primary quantum yields cannot be greater than unity. 

The amide functionality plays an important role in the physical and chemical properties of proteins and peptides, especially in their ability to be involved in the photoinduced electron transfer process. Polyamides and proteins are known to take part in the biological electron transport mechanism for oxidation-reduction and photosynthesis processes. Therefore studies of the photochemistry of proteins or peptides are very important. Irradiation (at 254 nm) of the simplest dipeptide, glycylglycine, in aqueous solution affords carbon dioxide, ammonia and acetamide in relatively high yields and quantum yield (0.44)202 (equation 147). The reaction mechanism is thought to involve an electron transfer process. The isolation of intermediates such as IV-hydroxymethylacetamide and 7V-glycylglycyl-methyl acetamide confirmed the electron-transfer initiated free radical processes203 (equation 148). 

Nitrosobenzene was studied by NMR and UV absorption spectra at low temperature146. Nitrosobenzene crystallizes as its dimer in the cis- and fraws-azodioxy forms, but in dilute solution at room temperature it exists only in the monomeric form. At low temperature (—60 °C), the dilute solutions of the dimers could be obtained because the thermal equilibrium favours the dimer. The only photochemistry observed at < — 60 °C is a very efficient photodissociation of dimer to monomer, that takes place with a quantum yield close to unity even at —170 °C. The rotational state distribution of NO produced by dissociation of nitrosobenzene at 225-nm excitation was studied by resonance-enhanced multiphoton ionization. The possible coupling between the parent bending vibration and the fragment rotation was explored. 

The photochemistry of di-terf-butyl nitroxide was studied149. When di-tert-butylnitroxide (DTBN) is excited at 254 nm to the rnt state in pentane solution, it is cleaved to terf-butyl radical and 2-methyl-2-nitrosopropane (with quantum yield of 0.21). The tert-butyl radical is scavenged by DTBN to give di-ferf-butyl-terf-butoxyamine150 (equation 129). 

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