Radical Chemistry Rate constants

A review of chemiluminescent and bioluminescent methods in analytical chemistry has been given by Kricka and Thorpe. A two-phase flow cell for chemiluminescence and bioluminescencc has been designed by Mullin and Seitz. The chemiluminescence mechanisms of cyclic hydrazides, such as luminol, have been extensively analysed. " Fluorescence quantum yields of some phenyl and phenylethynyl aromatic compounds in peroxylate systems have been determined in benzene. Excited triplet states from dismutation of geminate alkoxyl radical pairs are involved in chemiluminescence from hyponitrite esters. Ruorophor-labelled compounds can be determined by a method based on peroxyoxalate-induced chemiluminescence. Fluorescence and phosphorescence spectra of firefly have been used to identify the multiplicity of the emitting species. " The chemiluminescence and e.s.r. of plasma-irradiated saccharides and the relationship between lyoluminescence and radical reaction rate constants have also been investigated. Electroluminescence from poly(vinylcarbazole) films has been reported in a series of four... 

Chiorboli, C, Piazza, R., Tosato, M.L. and Carassiti, V. (1993c). Atmospheric Chemistry Rate Constants of the Gas-Phase Reactions Between Haloalkanes of Environmental Interest and Hydroxyl Radicals. Coordination Chemistry Reviews, 125,241-250. 

Aromatic compounds are of great interest in the chemistry of the urban atmosphere because of their abundance in motor vehicle emissions and because of their reactivity with respect to ozone and organic aerosol formation. The major atmospheric sink for aromatics is reaction with the hydroxyl radical. Whereas rate constants for the OH reaction with aromatics have been well characterized (Calvert et al. 2002), mechanisms of aromatic oxidation following the initial OH attack have been highly uncertain. Aromatic compounds of concern in urban atmospheric chemistry are given in Figure 6.16. 

Very few data are available for self-reactions of secondary alkylperoxy radicals. Our main contribution in this case is the study of cycloalkylperoxy radicals C-C5H9O2 and C-C6H11O2 radicals. The rate constants are 40 times larger than the only linear secondary peroxy radical studied so far, /-C3H7O2 and exhibit a slight positive temperature dependence. More data would be necessary for a better description of such reactions but, due to their relatively low rate constants (< 5 X lO" " cm molecule s ) their contribution to atmospheric chemistry is negligible. 

While OH radical reaction rate constants have been successfully generated to extend structure-activity predictive capability for thiocarbamates and chlorinated aromatics, further e q>erimental work will be inq>ortant to assess OH reaction rates for die more complex chemistries diat con rise die majority of high-use urban and agricultural pesticides. 

Analogy with the known chemistry of small radicals. The rate constants for self-reartion of small radicals approach the diffusion-controlled limit and the rate constants can be predicted using the Smoluchowski equation. 

The mechanism of photooxidation of aromatic species in the atmosphere is perhaps the area of greatest incertainty in atmospheric hydrocarbon chemistry. The principal reaction of aromatics is with the hydroxyl radical. Absolute rate constants have recently been determined at room temperature for the reaction of OH radicals with benzene and toluene (l8,l ) and with a series of aromatic hydrocarbons (19). Recently, absolute rate constants for the reaction of OH radicals with a series of aromatic hydrocarbons have been determined over the temperature range 296-U73K (20). For aromatic-OH reactions, the initial step can be either abstraction or addition to the aromatic ring. For toluene, for example. 

The general chemistry of acyl radicals has been recently reviewed/88 Acyl radicals have nucleophilic character. Absolute rate constants for substituted phenacyl radical addition to BA have been reported to be in the range 1.3-5.5xl05... 

Aliphatic acyloxy radicals undergo facile fragmentation with loss of carbon dioxide (Scheme 3,69) and, with few exceptions,428 do not have sufficient lifetime to enable direct reaction with monomers or other substrates. The rate constants for decarboxylation of aliphatic acyloxy radicals are in the range l 10xl09 M 1 s at 20 °C.429 lister end groups in polymers produced with aliphatic diacyl peroxides as initiators most likely arise by transfer to initiator (see 3.3.2.1,4). The chemistry of the carbon-centered radicals formed by (3-scission of acyloxy radicals is discussed above (see 3.4.1). 

Before any chemistry can take place the radical centers of the propagating species must conic into appropriate proximity and it is now generally accepted that the self-reaction of propagating radicals- is a diffusion-controlled process. For this reason there is no single rate constant for termination in radical polymerization. The average rate constant usually quoted is a composite term that depends on the nature of the medium and the chain lengths of the two propagating species. Diffusion mechanisms and other factors that affect the absolute rate constants for termination are discussed in Section 5.2.1.4. 

Radiolytic ethylene destruction occurs with a yield of ca. 20 molecules consumed/100 e.v. (36, 48). Products containing up to six carbons account for ca. 60% of that amount, and can be ascribed to free radical reactions, molecular detachments, and low order ion-molecule reactions (32). This leaves only eight molecules/100 e.v. which may have formed ethylene polymer, corresponding to a chain length of only 2.1 molecules/ ion. Even if we assumed that ethylene destruction were entirely the result of ionic polymerization, only about five ethylene molecules would be involved per ion pair. The absence of ionic polymerization can also be demonstrated by the results of the gamma ray initiated polymerization of ethylene, whose kinetics can be completely explained on the basis of conventional free radical reactions and known rate constants for these processes (32). An increase above the expected rates occurs only at pressures in excess of ca. 20 atmospheres (10). The virtual absence of ionic polymerization can be regarded as one of the most surprising aspects of the radiation chemistry of ethylene. 

The improvement in the rate of chemical reactions is reversed when temperature is cooler and at temperatures as low as 30 K (a warm comer of TMC-1) the exponential term is of order 10-279 and nearly all reactions between neutral species are frozen out at 50 K. Two important classes of reactions survive radical-radical chemistry and ion-molecule chemistry. The importance of these different reaction types will become apparent later with the construction of the models of molecular clouds. For the moment, however, laboratory measurements of reactions in radicals such as C2H have shown that even with temperatures as low as 15 K the rate constant for reactions of the type ... 

Extensive research has been conducted into the atmospheric chemistry of organic chemicals because of air quality concerns. Recently, Atkinson and coworkers (1984, 1985, 1987, 1988, 1989, 1990, 1991), Altshuller (1980, 1991) and Sabljic and Glisten (1990) have reviewed the photochemistry of many organic chemicals of environmental interest for their gas phase reactions with hydroxyl radicals (OH), ozone (03) and nitrate radicals (N03) and have provided detailed information on reaction rate constants and experimental conditions, which allowed the estimation of atmospheric lifetimes. Klopffer (1991) has estimated the atmospheric lifetimes for the reaction with OH radicals to range from 1 hour to 130 years, based on these reaction rate constants and an assumed constant concentration of OH... 

The biological chemistry of NO is ultimately defined by its activity at the molecular level. For example, NO readily reacts with other free radicals such as HO to give nitrite or with O2 to give peroxynitrite (Eq. (44)) at near diffusion-limited second-order rate constants (e.g. k2 1010 1VT1 s 1 for 02 ) (97). 

In the stepwise case, the intermediate ion radical cleaves in a second step. Adaptation of the Morse curve model to the dynamics of ion radical cleavages, viewed as intramolecular dissociative electron transfers. Besides the prediction of the cleavage rate constants, this adaptation opens the possibility of predicting the rate constants for the reverse reaction (i.e., the reaction of radicals with nucleophiles). The latter is the key step of SrnI chemistry, in which electrons (e.g., electrons from an electrode) may be used as catalysts of a chemical reaction. A final section of the chapter deals... 

Although cycloaddition reactions have yet to be observed for alkene radical cations generated by the fragmentation method, there is a very substantial literature covering this aspect of alkene radical cation chemistry when obtained by one-electron oxidation of alkenes [2-16,18-26,28-31]. Rate constants have been measured for cycloadditions of alkene and diene radical cations, generated oxidatively, in both the intra- and intermolecular modes and some examples are given in Table 4 [91,92]. 

Shortly after the discovery of the hydrated electron. Hart and Boag [7] developed the method of pulse radiolysis, which enabled them to make the first direct observation of this species by optical spectroscopy. In the 1960s, pulse radiolysis facilities became quite widely available and attention was focussed on the measurement of the rate constants of reactions that were expected to take place in the spurs. Armed with this information, Schwarz [8] reported in 1969 the first detailed spur-diffusion model for water to make the link between the yields of the products in reaction (7) at ca. 10 sec and those present initially in the spurs at ca. 10 sec. This time scale was then only partially accessible experimentally, down to ca. 10 ° sec, by using high concentrations of scavengers (up to ca. 1 mol dm ) to capture the radicals in the spurs. From then on, advancements were made in the time resolution of pulse radiolysis equipment from microseconds (10 sec) to picoseconds (10 sec), which permitted spur processes to be measured by direct observation. Simultaneously, the increase in computational power has enabled more sophisticated models of the radiation chemistry of water to be developed and tested against the experimental data. 

Alfassi, Z. B S. Padmaja, P. Neta, and R. E. Huie, Rate Constants for Reactions of NO, Radicals with Organic Compounds in Water and Acetonitrile, J. Phys. Chem., 97, 3780-3782 (1993). Allen, H. C., J. M. Laux, R. Vogt, B. J. Finlayson-Pitts, and J. C. Hemminger, Water-Induced Reorganization of Ultrathin Nitrate Films on NaCI—Implications for the Tropospheric Chemistry of Sea Salt Particles, J. Phys. Chem., 100, 6371-6375 (1996). Allen, H. C., D. E. Gragson, and G. L. Richmond, Molecular Structure and Adsorption of Dimethyl Sulfoxide at the Surface of Aqueous Solutions, J. Phys. Chem. B, 103, 660-666 (1999). Anthony, S. E R. T. Tisdale, R. S. Disselkamp, and M. A. Tolbert, FTIR Studies of Low Temperature Sulfuric Acid Aerosols, Geophys. Res. Lett., 22, 1105-1108 (1995). 

I. As discussed in this chapter, a relatively new area in indoor air pollution is that of hydroxyl radical chemistry. However, the importance of indoor OH chemistry (as well as 03 and N03 chemistry) is determined by the rates of the reactions compared to the rate of air exchange. An OH concentration of 7 X 105 cm-3 has been reported in an indoor air environment by Weschler and Shields (1997a). Assess the importance of the OH reaction for the removal of limonene indoors compared to its removal by reaction with 03. The 03-limonene rate constant is 2 X 1CT16 and the OH-limonene rate constant is 1.7 X 10-l() cm3 molecule-1 s-1. Take the 03 concentration to be 20 ppb. How do these compare with a typical air exchange rate of 0.75 h-1 used in these experiments ... 

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