Electron spin resonance resulting from mechanical

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. 

Very primary events in the chemical effect of radiations on matter are excitation and ionization of molecules, which result in the formation of neutral free radicals and radical ions. These reactive species play vital roles in the radiation-induced chemical reactions. As they are paramagnetic with an unpaired electron, electron spin resonance (ESR) spectroscopy has been a useful method for elucidating the mechanism of radiation-induced reactions in solid matter where radical species can be trapped temporarily. Since the early days of the chemical application of ESR, this method has been applied very often to the identification and quantification of free radicals in polymers irradiated by radiation [1]. This is probably because, from the view-point of fundamental research, a variety of free radicals are readily trapped in solid polymers and, from the view-point of applied research, these free radicals have close correlation with radiation-induced crosslinking and degradation of polymers. 

Despite of this inherent limitation, several spectacular results have been obtained. It should be noted that the initiation mechanism of the cationic polymerization of styrene described above was also deduced from the results of pulse radiolysis experiments. The pulse radiolysis combined with other techniques, such as the matrix isolation technique, the electron spin resonance technique and usual polymerization techniques, definitely provides a powerful means for investigating fundamentals of polymerization. 

Butyl rubber is a copolymer of isobutylene and I -2% isoprene. As a result the polymer chains contain internal double bonds which are expected to participate in cross-linking reactions. However, the overall molecular mass is expected to fall on irradiation due to the predominance of main-chain scission through the isobutylene units. Thus the radiation chemistry of the isoprene units within butyl rubber is accessible to study via solution NMR. In a comprehensive study Hill identified the primary free radical species by electron spin resonance spectroscopy at low temperatures, and the products of their subsequent reaction by C solution-state NMR. A number of new cross-link structures were identified and the mechanisms of cross-linking determined. Initial reaction involves addition of radicals either directly to the isoprene double bonds or to allyl radicals. Further addition of hydrogen atoms results in a mixture of fully-saturated and unsaturated cross-link structures. Cross-links of both H- and Y-type were identified and the yields of products agreed closely with the yields determined from measurement of changes in molecular weight on irradiation. 

As already pointed out, terms such as wave function, electron orbit, resonance, etc., with which we describe the formulations and results of wave mechanics, are borrowed from classical mechanics of matter in which concepts occur which, in certain respects at least, show a correspondence to the wave mechanical concepts in question. The same is the case with the electron spin. In Bohr s quantum theory, Uhlenbeck and Goudsmit s hypothesis meant the introduction of a fourth quantum number j, which can only take on the values +1/2 and —1/2- In wave mechanics it means that the total wave function, besides the orbital function, contains another factor, the spin function. This spin function can be represented by a or (3, whereby, for example, a describes the state j = +1/2 and P that with s = —1/2. The correspondence with the mechanical analogy, the top, from which the name spin has been borrowed, is appropriate in so far that the laevo and dextro rotatory character, or the pointing of the top in the + or — direction, can be connected with it. A magnetic moment and a... 

---