Peroxy radicals spectroscopy

Determination of Rate Constants for the Self-Reactions of Peroxy Radicals by Electron Spin Resonance Spectroscopy... 

Rate constants for the self-reactions of a number of tertiary and secondary peroxy radicals have been determined by electron spin resonance spectroscopy. The pre-exponential factors for these reactions are in the normal range for bi-molecular radical-radical reactions (109 to 1011 M"1 sec 1). Differences in the rate constants for different peroxy radicals arise primarily from differences in the activation energies of their self reactions. These activation energies can be large for some tertiary peroxy radicals (—10 kcal. per mole). The significance of these results as they relate to the mechanism of the self reactions of tertiary and secondary peroxy radicals is discussed. Rate constants for chain termination in oxidizing hydrocarbons are summarized. 

Spectroscopic Methods. HO and the other peroxy radicals have characteristic absorptions due to various molecular processes. In principle, these spectroscopic features could be used to determine atmospheric concentrations of peroxy radicals. The discussion of spectroscopic techniques in the measurement of peroxy radicals is divided into descriptions of specific spectral regions. General issues related to the use of spectroscopy for quantitative analysis are presented next. 

No data have been presented to date on the utility of far-infrared or millimeter-wave spectroscopy for other peroxy radicals or on its possible use in the troposphere when combined with a low-pressure absorption cell. 

The problem of bringing a large magnet into the field for ambient measurements has been overcome in electron paramagnetic resonance (EPR, also called electron spin resonance, ESR) by Mihelcic, Helten, and coworkers (93-99). They combined EPR with a matrix isolation technique to allow the sampling and radical quantification to occur in separate steps. The matrix isolation is also required in this case because EPR is not sensitive enough to measure peroxy radicals directly in the atmosphere. EPR spectroscopy has also been used in laboratory studies of peroxy radical reactions (100, 101). 

Perhydroxyl radical, 75 thermal generation from PNA of, 75 Peroxy radical generation, 75 Peroxide crystal photoinitiated reactions, 310 acetyl benzoyl peroxide (ABP), 311 radical pairs in, 311, 313 stress generated in, 313 diundecanyl peroxide (UP), 313 derivatives of, 317 EPR reaction scheme for, 313 IR reaction scheme for, 316 zero field splitting of, 313 Peorxyacetyl nitrate (PAN), 71, 96 CH3C(0)00 radical from, 96 ethane oxidation formation of, 96 IR spectroscopy detection of, 71, 96 perhydroxyl radical formation of, 96 synthesis of, 97 Peroxyalkyl nitrates, 83 IR absorption spectra of, 83 preparation of, 85 Peroxymethyl reactions, 82 Photochemical mechanisms in crystals, 283 atomic trajectories in, 283 Beer s law and, 294 bimolecular processes in, 291 concepts of, 283... 

Peroxy radicals are intermediates in the atmospheric oxidation of air pollutants and in oxidation reactions at moderate temperatures. They are rapidly formed from free radicals by addition of 02. Free radicals in the atmosphere are quantitatively converted to R02 with a half-time of about 1 fis. The peroxy radicals are then removed by reaction with other trace species. The dominant pathways are reactions with NO and NOz. Only a few peroxy radicals have been detected with a mass spectrometer, and extensive research on these reactions has been done by UV absorption spectroscopy with the well-known and conveniently accessed band in the 200- to 300-nm region. Nevertheless, FPTRMS has been used for some peroxy radical kinetics investigations. These have usually made use of the mass spectrometer to observe more than one species, and have given information on product channels. The FPTRMS work has been exclusively on atmospheric reactions of chlorofluoromethanes and replacements for the chlorofluoromethanes. 

Reference to the possible secondary radicals observed in mechanically degraded polymers, shows that the peroxy radical (RO ) frequently occurs. Since the polymer nuilecule contains no oxygen in many cases, the peroxy radical can only arise by reaction of the primary radical with oxygen either dissolved in the specimen or in the test environment. The detection of oxy-radicals (R0 ) by ESR spectroscopy has been reported in one instance but disclaimed in later papers ... 

Fluorinated peroxy radicals. Kinetic, Nitric Oxides, Organic nitrates, UV Spectroscopy... 

Peroxy radicals are stable with respect to reaction with the major constituents of the atmosphere. Their chemistry is dominated by reactions with peroxy radicals, HO2 and RO2, and with the nitrogen oxides, NO, NO2, and NO3. Two recent articles review the spectroscopy, kinetics, and reaction mechanisms of organic peroxy radicals [11,12], Three of the more important reaction channels are illustrated in Figure 1. The reaction with NO2 proceeds exclusively by addition to form the corresponding alkyl-peroxy nitrate. This reaction is generally not important in determining the end products of the degradation process due to the usually rapid dissociation rate of the peroxy nitrate however, it can be relevant to dynamical issues, because it sequesters N02-... 

Radicals, generated in polypropylene fibers during irradiation as well as the transition frcan these reidicals (R ) to peroxy radicals (ROa ) are monitored by electron spin resonance spectroscopy. Experimental data exhibit an anomaly in the temperature dependence of RO, concentration, around the glassy transition temperature (Tq). The dependence of RO, concentration on temperature, around T, is described by a Hil-liam-Landel-Perry equation rather than by an Arrhenius one. Consequently, the transition consists of two steps the first is associated with diffusion processes and the second with the proper chemical reaction. 

Tkdc followed the free radical distribution in the molten phase as well as in the pre-flame and flame zones of a burning specimen of polypropylene rod by ESR spectroscopy. Using several entrapping techniques for the radicals, he detected free radicals, bi-radicals, and non-radical fragments leaving the molten phase for the gas phase where they may be combined with traces of oxygen on the surface into peroxy radicals or recombined by cyclization as well as by intermolecular reactions. 

As part of this study, two potential RO2 measurement techniques have been investigated the chemical amplification and tuneable diode laser absorption spectroscopy. The first approach has proven to be successful for the measurement of ambient mixing ratios of the sum of all peroxy radicals which react with NO to form NO2. 

P.D. Lightfoot, R.A. Cox, J.N. Crowley, M. Destriau, G.D. Hayman, M.E. Jenkin, G.K. Moortgat, F. Zabel Organic peroxy radicals kinetics, spectroscopy and tropospheric chemistry, Atmos. Environ. 26A (1992) 1805. 

The goals set with respect to the spectroscopy and kinetics of the simple peroxy radicals was mainly achieved. A detailed review on the chemistry and role of the peroxy radicals in the photo-oxidation of VOC was written within the LACTOZ project [25]. Further studies on quantum yields determination of photolabile carbonyl compounds and on the mechanism of the ozonolysis of alkenes are required. 

The rate constants for the reactions of OH with a series of organic nitroalkanes, nitrites and nitrates and of NO with a series of peroxy radicals were measured at 298 K and a total pressure of 1 atm. The rate constants were obtained using the absolute technique of pulse radiolysis combined with time-resolved UV-VIS spectroscopy. The results are discussed in terms of reactivity trends and the atmospheric chemistry. 

Because of the rapidity of the self-reaction of RCO.O2 radicals and because RCO.O radicals are generally unstable, it is not possible to use a similar approach for the study of acyl peroxy radicals. Instead, we have used the thermal decomposition of PAN, CH3CO.O2NO2, as an in situ source of CH3CO.O2. In our experiments, concentrations of PAN were measured by Fl lR spectroscopy, and the decay of NO3 in a (slow) flow system provides the kinetic information. It is clear that radical reactions, and not direct reaction with PAN, are responsible for the losses of NO3. The current best fits to the experimental data were obtained with the rate coefficient given in Table 1 for the reaction... 

Organic peroxy radicals kinetics, spectroscopy and tropospheric chemistry,... 

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