Oxygen transient species

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

The flash photolysis of s-trinitrobenzene (TNB) aerated solns in alcohols generated a transient species with absorption maxima at 430 and 51 Onm (Ref 27). The yield of the transient was a function of oxygen concn, and its rate of formation was viscosity dependent. In deaerated solns, instead of the transient, a brown permanent product, identified as a charge transfer... 

Figure 5.11 Variation in the catalytic activity of an Mg(0001) surface when exposed to a propene-rich propene- oxygen mixture at room temperature. The surface chemistry is followed by XPS (a), the gas phase by mass spectrometry (b) and surface structural changes by STM (c, d). Initially the surface is catalytically active producing a mixture of C4 and C6 products, but as the surface concentrations of carbonate and carbonaceous CxHy species increase, the activity decreases. STM images indicate that activity is high during the nucleation of the surface phase when oxygen transients dominate. (Reproduced from Ref. 39).

Because of the inherent reactivity of photochemical redox systems, oxygen and water frequently interfere to produce undesirable side reactions. Elucidation of the mechanism requires the analysis of transient species in extremely complex systems. Therefore, many systems are only partially characterized or completely uncharacterized. 

The hydrolysis Reaction 2 is then realised at 390°C without chlorine formation. The temperature is then raised to 530°C to realise Reaction 3, the dissociation of the melanothallite. During this step, a transient species is observed around 330 nm which disappears at 530°C. The gases formed are collected in three containers and analysed by mass spectrometry. Oxygen formation is measured. DRX spectrum realised on the final product indicates the presence of CuCl and CuO. 

Eisner, J., Oppermann, M Czech, W., Dobos, G., Schopf, E., Norgauer, J., and Kapp, A. (1994) C3a activates reactive oxygen radical species production and intracellular calcium transients in human eosinophils. Eur. J. Immunol. 24, 518-522. 

The transient absorption spectra show that besides the ring-closure product (C), a new transient species was found that could be quenched by oxygen. The authors assigned the new species to an excited triplet state. The time-resolved transient absorption spectra indicated that when the time delay was increased, the excited triplet state was decreased and the ring closure product was increased slightly. The decay of the excited triplet state can be fit to first-order kinetics. The lifetime ( ) of the transient is 0.8 ps in cyclohexane. The authors noted that the photocyclization process proceeds mainly via the excited singlet state, but the excited triplet state is... 

Since hydroxyl radical reacts at very high rates with most organic contaminants (with the exception of some polyhalogenated compounds), it may end in being the main reactive species for the transformation of contaminants that do not appreciably react with the main transient species, i.e. triplet states and singlet oxygen. 

It should be emphasized that the measurement of the singlet oxygen phosphorescence intensity at 1270 nm confirmed the values <1>A for the porphyrins but gave systematically lower values for the bacteriochlorins. PAC measures the amount of heat released in the formation of transient species, whereas phosphorescence at 1270 nm measures the aunoimt of singlet oxygen species. The apparent discrepancy between the two techniques may be related to the natime of the species that are formed from the triplet state of bacteriochlorins in the presence of oxygen and will be discussed in the next section. 

Bicyclo[3.3.0]oct-l(5)-ene 178 (Scheme 4.55) is a stable compound with a flattened alkene fragment and exhibits a regular pattern of reactivity. Computational studies revealed, however, that installation of a short 3,7-bridge should lead to noticeable pyramidalization of the double bond. Compounds like 179-181 were synthesized to check this prediction. Tricyclic hydrocarbon 179, with the smallest possible bridge, was generated as a transient species from diiodide 182. The formation of 179 is implicated by the isolation of its cyclodimer 183 (or respective Diels-Alder adduct if the reaction is carried out in the presence of a 1,3-diene). The next member of this series, 180, is more stable. In fact, the formation of 180 was ascertained not only from the structure of the final products (as was done for 179), but also by its matrix isolation and analysis of spectral data. The selenium derivative 181 was found to be stable at ambient temperature in the absence of oxygen. X-ray data confirmed a noticeable pyramidalization of the double bond in 181 but the distortion was different [Pg.372]

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