Self-annihilation

A few free radicals are indefinitely stable. Entries 1, 4, and 6 in Scheme 12.1 are examples. These molecules are just as stable under ordinary conditions of temperature and atmosphere as typical closed-shell molecules. Entry 2 is somewhat less stable to oxygen, although it can exist indefinitely in the absence of oxygen. The structures shown in entries 1, 2, and 4 all permit extensive delocalization of the unpaired electron into aromatic rings. These highly delocalized radicals show no tendency toward dimerization or disproportionation. Radicals that have long lifetimes and are resistant to dimerization or other routes for bimolecular self-annihilation are called stable free radicals. The term inert free radical has been suggested for species such as entry 4, which is unreactive under ordinary conditions and is thermally stable even at 300°C. ... 

An argument against the defect mediated diffusion model is the same one used earlier that is, there are not enough defects as determined by ESR (Brodsky and Title, 1969, 1976) or Deep Level Transient Spectroscopy measurements (Johnson, 1983) to account for the motion of all of the bonded hydrogen in a-Si H. This objection is removed if the floating bonds are 104-106 times more mobile than the hydrogen atoms. However, such highly mobile defects would rapidly self-annihilate via the process. 

The self-annihilation reaction occurs much faster than addition to C2F4. If... 

Positronium can exist in the two spin states, S = 0, 1. The singlet state (5 = 0), in which the electron and positron spins are antiparallel, is termed para-positronium (para-Ps), whereas the triplet state (5 = 1) is termed ortho-positronium (ortho-Ps). The spin state has a significant influence on the energy level structure of the positronium, and also on its lifetime against self-annihilation. 

In organic ECL reactions, the luminescent species are generally derivatives of polyaromatic hydrocarbons where A and B in Eqs. (1) through (4) can be either the same species (leading to self-annihilation) or two different PAHs with either being the analyte (mixed system). Some examples of both self-annihilation and mixed system ECL reactions of organic molecules are listed in Tables 1 and 2. One well-studied example is the self-annihilation reaction between the anion and cation radicals of 9,10-diphenylanthracene (DPA) via an S-route in acetonitrile resulting in blue fluorescence characteristic of DPA [17] ... 

Table 1 Organic Compounds Exhibiting ECL Via Self-Annihilation... 

Figure 1 Proposed mechanism for the generation of the radical anion and radical cation of perylene diimide and self-annihilation of the two to yield the triplet excited state.

Pt2(P205H2)4, also known as Pt2(POP)4, in the presence of tetrabutylam-monium (TBA) salts as electrolyte [19,20], ECL has also been observed from palladium and platinum a, 3,y,5-tetraphenylporphyrin complexes, and also via self-annihilation of the electrogenerated anion and cation radicals [21],... 

In a measurement, lifetimes are observed. They correspond to the inverse of the sum of the self-annihilation the wall bounces dependent annihilation rate (A an). 

It should be noted that the S parameters of both o-Ps pick-off and free-positron annihilation are lower than that of the Si substrate, because positrons predominantly annihilate with electrons of oxygen in the Si02 network. Only p-Ps self-annihilation has a higher S value than that of Si. The S parameter observed in conventional Doppler- broadening-of-annihilation radiation is the average of p-Ps, o-Ps, and free-positron annihilation. Therefore, if the Ps fraction decreases due to the presence of defects, impurities, etc., the intensity of the narrow momentum component due to p-Ps self-annihilation decreases, and as a result the averaged S parameter decreases. 

Positrons emitted for a radioactive source (such as 22Na) into a polymeric matrix become thermalized and may annihilate with electrons or form positronium (Ps) (a bound state of an electron and positron). The detailed mechanism and models for the formation of positronium in molecular media can be found in Chapters 4 and 5 of this book. The para-positronium (p-Ps), where the positron and electron have opposite spin, decays quickly via self-annihilation. The long-lived ortho positronium (o-Ps), where the positron and electron have parallel spin, undergo so called pick-off annihilation during collisions with molecules. The o-Ps formed in the matrix is localized in the free volume holes within the polymer. Evidence for the localization of o-Ps in the free volume holes has been found from temperature, pressure, and crystallinity-dependent properties [12-14]. In a vacuum o-Ps has a lifetime of 142.1 ns. In the polymer matrix this lifetime is reduced to between 2 - 4 ns by the so-called pick-off annihilation with electrons from the surrounding molecule. The observed lifetime of the o-Ps (zj) depends on the reciprocal of the integral of the positron (p+(rj) and electron (p.(r)) densities at the region where the annihilation takes place ... 

The potential for using NCl(a) in a chemically pumped laser was first examined by Benard et al. These investigators used pulsed CO2 laser pyrolysis of ClNs/SFe/Ar mixtures to generate high concentrations (> 10 cm ) of NCI (a). From the rate of NCI (a) decay in this system they estimated an upper limit for the NCl(a) self-annihilation rate constant of > 8 x Energy pooling between NCI (a) and NF(a)... 

To date, the self-annihilation rate constant for NCI (a) has only been determined using photolysis techniques. Henshaw et were able to... 

Since this chapter was compiled, cw chemical lasing of I in an NCl(a)/I flow reactor has been demonstrated by Henshaw, Manke II, Madden, Berman and Hager (work submitted for publication). Komissarov and Heaven have reexamined the rate constant for self-annihilation of NCI (a). They obtained a room temperature value of (7 2) x 10 cm s , which is significantly smaller than previous estimates. 

From the above discussion it should be clear that annihilation of radical ions can result in production of several species. In the very simplest case of self-annihilation of one-electron oxidized and one-electron reduced forms of a particular species, S, three processes compete (Eq. 7). 

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