Photodissociation of dimeric

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. 

Photodissociation of dimer coupled to current measurement of electrochemical oxidation of the pyridinyl radical to the pyridinium ion has been described in Section 3.1.3. Oxidation of the l-methyl-3-carbamidopyridinyl and NAD (nicotinamide adenine dinucleotide radical) after dissociation of the dimers has been reported the agents being either oxygen or OH radical. Reasonable mechanisms for the latter are either electron transfer or radical combination, followed by dissociation to Py+ and OH. 

The dissociation energy of Na2 to two S sodium atoms is 6022 cm (AA). Therefore the photodissociation of dimeric sodium to produce the onset of D-llne fluorescence would require that Na2 possess 5500 cm of internal excitation (X Zg", v" A5). 

In contrast to a straightforward and predictable decomposition pattern of photolysis with >400 nm light, irradiation of nitrosamides under nitrogen or helium with a Pyrex filter (>280 nm) is complicated by the formation of oxidized products derived from substrate and solvent, as shown in Table I, such as nitrates XXXIII-XXXV and nitro compound XXXVI, at the expense of the yields of C-nitroso compounds (19,20). Subsequently, it is established that secondary photoreactions occur in which the C-nitroso dimer XIX ( max 280-300 nm) is photolysed to give nitrate XXXIII and N-hexylacetamide in a 1 3 ratio (21). The stoichiometry indicates the disproportionation of C-nitroso monomer XVIII to the redox products. The reaction is believed to occur by a primary photodissociation of XVIII to the C-radical and nitric oxide followed by addition of two nitric oxides on XVIII and rearrangement-decomposition as shown below in analogy... 

The production of NOz, with NO as a possible precursor to NOz, has been observed when synthetic air or 02/N2 mixtures are photolyzed using a deuterium lamp, an argon flash lamp, or a 185-nm mercury line (Zipf and Prasad, 1998a). They proposed that this occurs from the reaction of electronically excited 02(B%) with N2, or photodissociation of 02 N2 dimer, and that the rate of NOx production from this process could be comparable to that from reaction (13b) (Zipf and Prasad, 1998a Prasad, 1998). If this proves to be the case, there must be some unidentified NOx sinks to be consistent with the measured NOx concentrations in the upper atmosphere. 

Perhaps the best-characterized lesion in DNA associated with uv inactivation and mutagenesis is that involving the intrastrand photodimerization of adjacent thymine residues this lesion is almost wholly repaired by photodissociation of the dimers at shorter wavelengths in the photoreactivation process. Production of the chh dimer in this case, promoted by the configuration of adjacent molecules on the same sugar-phosphate strand, must however involve a rotational displacement of 36°, following the reduction of 0.6 A in molecular separation. 

Iron pentacarbonyl exhibits efficient photodecomposition (in the absence of ligands) because the bimolecular reaction between Fe(CO)3 and photogenerated Fe(C0)4 yields insoluble Fe2(C0)g. Sterically unhindered Fe(CO)3[l,4-Me2N4] mimics the behavior of Fe(CO)3 in forming a cluster on irradiation in the absence of ligands. Furthermore selective photodissociation of CO from the tetraazabutadiene complex produces a coordinatively unsaturated species that reacts (like photogenerated Fe(C0)4) with Fe(C0)3 to form a dimer, Equation 10. 

The foregoing is of obvious significance in relation to the photochemical behaviour of cyclobutane photodimers of natural pyrimidines such as uracil (III, Scheme 1), thymine, cytosine12,76). Photodissociation of such photodimers to the parent monomers, which proceed with quantum yields ranging from 0.5 to nearly unity, has, indeed, been employed as one of the criteria for identification of such dimers. The validity of this criterion is now, in the light of the behaviour of the dimer electroreduction product of pyrimidone-2, at best somewhat restricted, notwithstanding that the quantum yields for photodissociation of the latter are lower. 

The photodissociation of the 2-thiopyrimidine dihydrodimers, Dt 3 (Scheme 21) is really a photooxidation process since, formally, the observed products, the parent 2-thiopyrimidines, are regenerated by removal. of two electrons and two protons. The same applies to photodissociation of the dimer of 2-oxopyrimidine (D4). 

In neutral aqueous medium the dimer reduction product, but not the product of reduction on wave II, underwent photodissociation to the parent 2-oxopurine, with a quantum yield at 254 nm of 0.03, as compared to 0.1 for photodissociation of pyrimidine-2. 

The consequent similarities in polarographic reduction of 2-thiopurine and 2-oxo-purine are analogous to the similarities in photochemical reduction of pyrimidone-2 and pyrimidone-2-thione, and of photodissociation of their dimer reduction products. The dimer reduction product of 2-thiopurine, obtained by electrolysis at pH 5, undergoes photodissociation at 254 nm to the parent monomer with a quantum yield comparable to that for the dimer reduction product of 4,6-dimethylpyrimidine-2-thione (Scheme 26). 

Meanwhile we can show (50) that the deazaflavin radical generated by photodissociation of the dimer as shown in Figure 7 can be utilized for selective reduction of flavodoxin radical. With the aid of this le -transfer method Scherings, Haaker, and Veeger (51) were able to show that flavodoxin is the actual electron donor for the Azotobacter nitrogen-ase system and that N2 reduction can be maintained in the light by an artificial chain system of the reactant sequence EDTA-deazaflavin—flavodoxin—nitrogenase. Hence, Azotobacter flavodoxin is actually a carrier... 

Cage effect dynamics or kinetics of geminate recombination was observed for the first time under photodissociation of aC C dimer of aromatic radicals in a viscous media. A suggestion has been made that at least in a number of smdied cases the mumal diffusion coefficient of radicals in the pair is approximately 10 times lower than the sum of macroscopic diffusion coefficients of the individual species. In other words, a geminate recombination proceeds considerably longer than expected. 

Photodissociation of the dimer [2-2] to the pyridinyl radical (2 ) occurs readily in thin films at low temperatures or in acetonitrile solutions. Although excitation spectra could not be obtained from these experiments a new technique was used 1) the wavelength dependence of radical formation from dimer generated electro-chemically in small amounts, 2) the measurement of the radical produced by a rapid jump to a potential at which reoxidation of the radical takes place. Since the photodissociation spectra of dimers may well determine their practical use, a convenient procedure is useful. 

The photodissociation of the dimers to radicals (see 4.1) is an important photochemical consequence of the through-bond interaction. A photodissociation spectrum for 3-3 is similar to the dimer absorption band as determined by the... 

Fig. 23. Photodissociation of l-methyl-2-carbomethoxypyridinyl dimer (2-2) to the monomeric radical, 2 . Irradiation at 350 nm for 30, 180, 360 and 750 s rapidly produces radical 2 and some pimer...

Homolytic photodissociation of a benzyl-anilino C—N bond has also been observed in a series of iV-(arylmethyl)anilines. The main products identified for 27 are aniline, triphenylmethane and 9-phenylfluorene (28) (equation 4)126. The quantum yields for the formation of Pt C are high (0.6-0.8, 248-nm excitation) and independent of solvent. On the basis of the results of laser flash photolysis and ESR studies, the formation of 28 occurs via the intermediate 29 as a result of electrocyclization of Pt C (Scheme 5). In contrast, the dimerization of the benzyl and diphenylmethyl radicals, leading to the formation of 1,2-diphenylethane and 1,1,2,2-tetraphenylethane, respectively, are efficient in the cases of 30 and 31 (equations 5 and 6)127. In addition, products resulting from the coupling of the photodissociated benzyl and aniline radicals are also observed for 31, presumably due to the less sterically hindered PhCH2 radical when compared with Ph2CH and Ph3C radicals. 

C-H bond fission and the production of ethynyl radicals. Butadiyne and vinyl acetate are formed when the T -shaped ethyne dimer is irradiated at 193 nm in argon or xenon. The dynamics of the photodissociation of propyne and allene have been studied. The H2 elimination from propyne is a minor route for propyne dissociation and the major path identified in this study is loss of the alkyne hydrogen. A study of the photodissociation dynamics of allene and propyne has been reported and this work has demonstrated that allene gives rise to a propargyl radical while propyne yields the propynyl radical. Other research has examined the photodissociation of propyne and allene by irradiation at 193 nm. ... 

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