The primary radicals

Despite great efforts put forth by experimentalists, the exact identity of most radical products generated in irradiated DNA is still unknown. The first ESR studies on fuU DNA only provided evidence for the formation of a thjonine centered radical [54,55]. Work performed on DNA irradiated by ultraviolet light [56] and on oriented fibers [57] confirmed this radical to be formed through net hydrogen atom addition to C6 in thymine [T(C6H)]. The definitive 

The debate over the site of electron loss in DNA is much less pronounced since it has been estimated that over 90% of the cations generated in DNA are centered on guanine [67] and guanine end products account for 90% of the electron loss products in DNA [70]. However, the spectra of G recorded in solid-state studies of nucleotides and nucleosides do not correspond to the spectrum recorded in fiiU DNA [71] and investigations of the strand-break specificity determined that some adenine cations could be generated [64]. Thus, it is also possible that other cations are formed, primarily A.  

Limited experimental data is available for the EAs of the DNA bases. The trend in the estimated EAs (obtained by correcting the HF Koopmanns EA by the calculated nuclear relaxation energy) is T C A G, which is in agreement with early studies on DNA predicting that the thymine anion is the major reduction product upon irradiation [72]. Alternatively, the trend predicted through examination of the adiabatic EAs calculated with DFT (C T G A) supports experimental data predicting cytosine to be the major reduction site in 

---

Because the starting material (propane) and one of the products (H ) are the same m both processes the difference m bond dissociation energies is equal to the energy dif ference between an n propyl radical (primary) and an isopropyl radical (secondary) As depicted m Figure 4 20 the secondary radical is 13 kJ/mol (3 kcal/mol) more stable than the primary radical... 

The following conditions are stipulated the catalyst decomposition rate constant must be one hour or greater the residence time of the continuous reactor must be sufficient to decompose the catalyst to at least 50% of the feed level the catalyst concentration must be greater than or equal to 0.002 x Q, where the residence time, is expressed in hours. An upper limit on the rate of radical formation was also noted that is, when the rate of radical formation is greater than the addition rate of the primary radicals to the monomers, initiation efficiency is reduced by the recombination of primary radicals. 

No clear picture of the primary radical intermediate(s) in the HO2 photooxidation of water has appeared. The nature of the observed radical species depends on the origin and pretreatment of the HO2 sample, on the conditions and extent of its reduction, on the extent of surface hydroxylation, and on the presence of adventitious electron acceptors such as molecular oxygen (41). The hole is trapped on the terminal OH group (54). 

The departure of dependence of Rp on the concentration of CHP from 0.5 order might be ascribed to induction decomposition of ROOH type to form ROO- radical, which has very low activity to initiate monomer polymerization [40], but can combine with the propagation chain radical to form the primary radical termination. For the same reason, the order of concentration of TBH was also lower than 0.5 when the TBH-DMT system was used as the initiator in MMA bulk polymerization. But in the BPO-DMT initiation system as shown in Table... 

CH2N(CH3)R rather than (CH3)2N ( CH R) showed that the methyl group is the preferable group for substitution. Meanwhile, a secondary product was also formed and verified through ESR as -CH2CH2N(CH3)2 (N,N-dimethylaminoethylene radical) from TMEDA, which was considered to form from the scission of the primary radical as follows ... 

Bamford and Mullik [23] have also investigated a new photoinitiating system composed of Mn2(CO)io or Re2(CO)io with acetylene, acetylene dicarboxylic acid, diethyl fumarate, diethyl maleate, or maleic anhydride. It was concluded that the primary radical responsible... 

Neutron activation analysis of a polymer suggests that when Py is used as the electron doner (D), the initiation proceeds through the Cl atom, but when D = DMSO, both Cr and DMSO residues are the primary radicals produced from the photoexcited ion-pair complex. The following reaction scheme is proposed ... 

In the mass spectrum (Figure 8) of the corresponding ketal of 5-deoxy-D-xt/Zo-hexose, 5-deoxy-l,2-0-isopropylidene-D- rt/Zo-hexofuranose (11), the peak from C-4-C-5 cleavage, m/e 159, is of minor relative intensity. Since the ions at m/e 159 are the same from both isomers, 10 and 11, the intensity difference must be attributable to the lower stability of the primary radical formed from C-5 of 11 compared with the secondary radical from 10 ... 

The simple initiation process depicted in many standard texts is the exception rather than the rule. The yield of primary radicals produced on thermolysis or photolysis of the initiator is usually not 100%. The conversion of primary radicals to initiating radicals is dependent on many factors and typically is not quantitative. The primary radicals may undergo rearrangement or fragmentation to afford new radical species (secondary radicals) or they may interact with solvent or other species rather than monomer. 

The radicals formed by imimolecular rearrangement or fragmentation of the primary radicals arc often termed secondary radicals. Often the absolute rate constants for secondary radical formation are known or can be accurately determined. These reactions may then be used as radical clocks",R2° lo calibrate the absolute rate constants for the bimolecular reactions of the primary radicals (e.g. addition to monomers - see 3.4). However, care must be taken since the rate constants of some clock reactions (e.g. f-butoxy [3-scission21) are medium dependent (see 3.4.2.1.1). 

The polarization of biphenyl, deserves special comment. If, as indicated in Scheme 2, its immediate precursor is a radical pair consisting of two phenyl radicals, then it should be formed without detectable net polarization since if Ag = 0. Analogous results have been reported in the decomposition of other peroxides for example, ethane formed from acetyl peroxide shows net emission. To account for this, it has been suggested (Kaptein, 1971b, 1972b Kaptein et al., 1972) that nuclear spm selection which occurs in the primary radical pair—in... 

Production of considerable amounts of cyclohexanol and cyclohexanone as well as benzaldehyde and benzoic acid in the oxidation of benzyl cyclohexyl ether shows the primary radical to be CgHjCHOCeHjj. Abstraction from aliphatic C-H bonds cannot occur in the case of diphenyl ether which is oxidised rapidly, and removal of a 7t-electron is likely. 

The inhibition method has found wide usage as a means for determining the rate at which chain radicals are introduced into the system either by an initiator or by illumination. It is, however, open to criticism on the ground that some of the inhibitor may be consumed by primary radicals and, hence, that actual chain radicals will not be differentiated from primary radicals some of which would not initiate chains in the absence of the inhibitor. This possibility is rendered unlikely by the very low concentration of inhibitor (10 to 10 molar). The concentration of monomer is at least 10 times that of the inhibitor, yet the reaction rate constant for addition of the primary radical to monomer may be less than that for combination with inhibitor by only a factor of 10 to 10 Hence most of the primary radicals may be expected to react with monomer even in the presence of inhibitor, the action of the latter being confined principally to the termination of chain radicals of very short length. ... 

The termination constants kt found previously (see Table XVII, p. 158) are of the order of 3 X10 1. mole sec. Conversion to the specific reaction rate constant expressed in units of cc. molecule" sec. yields A f=5X10". At the radical concentration calculated above, 10 per cc., the rate of termination should therefore be only 10 radicals cc. sec., which is many orders of magnitude less than the rate of generation of radicals. Hence termination in the aqueous phase is utterly negligible, and it may be assumed with confidence that virtually every primary radical enters a polymer particle (or micelle). Moreover the average lifetime of a chain radical in the aqueous phase (i.e., 10 sec.) is too short for an appreciable expectation of addition of a dissolved monomer molecule by the primary radical prior to its entrance into a polymer particle. 

It has already been shown above that the radicals are most probably produced at the TMA - TiCl4/MgCl2 interface, where an alkylation of the TiCU by ligand exchange is supposed to happen (see reaction scheme above). Assuming this ligand exchange occurs, the primary radical would be a methyl... 

Heterolytic (two-electron, ionic) oxidation of 1, or alternatively further one-electron loss from the primary radical 2, affords chromanoxylium cation 4 with its positive charge mainly localized at C-8a. Cation 4 is stabilized by resonance so that a positive partial charge results also at C-5 and C-7, where nucleophilic attack is... 

A spin trap is a diamagnetic compound that reacts with a radical by addition of the radical functionality typically to a double bond in the trap, thus forming a new radical that is more stable (better, less unstable) than the original radical. By far the most common class of spin traps are nitrone compounds that, upon addition of the primary radical, produce a stable aminoxyl radical (Figure 10.1). The compound DMPO is the paradigmatic spin trap it is readily available, widely used, and its EPR spectra are relatively easy to interpret. Some of its radical adducts have unpractically short lifetimes. 

The primary radical yields are often 3. A much higher value (>10) indicates chain reaction. In fact, the chain reaction mechanism for the formation of HC1 from a gaseous mixture of hydrogen and chlorine exposed to radium irradiation is one of the earliest example of this kind, although the detailed chemistry was later shown to involve dissociated atoms rather than electrons and ions, as was originally proposed (see Bansal and Freeman, 1971). 

In the stepwise process, the cleavage of the intermediate primary radical is a fast reaction, endowed with a large driving force. Cases where the cleavage of the primary radical is an uphill reaction will be discussed in Sections 4 and 5. 

Ion radicals or, more generally, the primary radicals resulting from single electron transfer to or from a parent molecule are often frangible species that decompose more readily than their parents. Starting from an ion radical (Scheme 9), it is useful to distinguish two cases according to the location of the... 

The kinetics of the electron transfer reaction leading to the homolytically dissociating primary radical is also a question of interest. It may be modeled using the Morse curve for the reactant and the Morse curve shown in Fig. 10 representing the homolytic dissociation of the primary radical. This point will be discussed in detail in Section 5. 

---