Peroxy radical absorption spectra

Novel method for the measurement of gas phase peroxy radical absorption spectra, J. Phys. Chem. 96 (1992) 982. 

The analysis of the spectrum is a multiple-step process. The first step is the removal of the contribution to the spectrum due to the sampling apparatus. The next step is subtraction of the relatively large contribution due to N02. The remaining absorptions are due to H02, other organic peroxy radicals, and possibly other unknown radicals. A numerical fitting procedure... 

Fig. 10. Absorption spectrum of the cyclohexyl peroxy radical (from ref. 74).

The optical absorption of the unirradiated and irradiated samples was measured. The spectrum of unirradiated sample has two peaks at 200 and 275 mn. The intensity absorption of ultraviolet region increased with increasing the dose and new absorption band was observed at 215 nm after gamma-ray irradiation. The optical densities normalized by that of unirradiated sample as shown in Fig. 4. Several absorption peaks at 200, 215 and 275 nm were associated with fluorocarbon and peroxy radicals and C=0 groups respectively 131. The cross-linkage and chemical bond breaking of the polymer chain were caused by gamma-ray irradiation. 

The fits of this line shape to the peroxy radical UV spectra are shown by the dashed lines in Figs. 4 to 11. Besides providing an excellent representation of the absorption profile, Eq. (28) conveniently summarizes the spectrum by three physically meaningful parameters, and give, respectively, the intensity and position of the absorption maximum, whereas a provides a measure of the width of the absorption profile. Values of these parameters for the peroxy radical spectra relevant to HCFCs are collected in Table 5. 

Direct spectroscopic measurements of HCFC-based alkoxy radicals and real-time determinations of their reaction rates are scarce. The laser-induced fluorescence (LIF) spectrum [124,12S] of CF3O has been measured and a UV absorption feature has tentatively been assigned [92] to CF3CFHO. Three-center elimination of HCl from CH3CHCIO and CHjClO has been observed by real-time measurement of the HCl production [10,87], but the alkoxy radicals have not been directly detected. In other cases, our understanding of alkoxy radical chemistry comes from product studies and theoretical considerations. Below we summarize the fate of the alkoxy radicals derived from the peroxy radicals listed in Tables 6 and 7. 

The acetonyl peroxy radical exhibits a distinctive spectrum having a second absorption band at longer wavelengths ( 3(K) nm). This band apparently corresponds to the n-7i transition of the carbonyl group, which has been enhanced. However, the reason for this enhancement is not presently understood and has not been explained by semi-empirical calculations. 

A vast majority of the kinetic measurements involving RO2 radicals has made use of the relatively intense UV absorption spectrum of these species in order to obtain time-concentration profiles. Moreover, for the study of peroxy radical selfreactions, absolute concentrations must be known in order to analyse second order decay profiles, indicating the importance of accurately measured absolute absorption cross sections. The cited peroxy radicals were generated by photolysis... 

Guajardo and Mascharak have found that the iron complexes [Fe(PMA)] (n = 1, 2) (84, 85) shown in Fig. 23, which are synthesized as iron bleomycin analogues, promote facile lipid peroxidation in the presence of O2 or H2O2 [140]. Reaction of linoleic acid (59) with O2 catalyzed by 84 and 85 gives the 13-OOH product in the selectivity of 80 and 75%, respectively. In the reaction of arachidonic acid, (86), the 15-OOH product is also selectively formed (80%) by the two complexes. The peroxidation is also promoted by H2O2. As a possible intermediate, a low-spin (hydroperoxo)-iron(III) species, [(PMA)Fe -OOH], has been detected by X-band EPR. The EPR spectrum is identical to that of the activated bleomycin. The reaction has been explained in terms of the radical mechanism, which involves H atom abstraction from lipid (LH) by [(PMA)Fe -OOH]. Peroxidized linoleic acid (L-00 ) has been detected by UV absorption at 234 nm, and a chain propagation reaction by the peroxy radical to produce lipid hydroperoxide (L-OOH) has been proposed. 

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