Rearrangement ionization

Second, rich bimolecular chemistry (attack by nucleophiles, electrophiles, oxidants, or reductants) that can be used to create reactive intermediates in solution is not generally available in the context of matrix isolation (exceptions to this rule will be discussed in the proper context below). Usually, reactive intermediates to be studied by matrix isolation must be accessible by means of unimolecular processes (fragmentations, rearrangements, ionization) induced by external sources of energy (light or other forms of radiation, discharges). 

When neopentyl bromide is boiled in ethanol, it gives only a rearranged substitution product. This product results from a methyl shift (represented by the symbol CH ), the migration of a methyl group together with its pair of electrons. Without rearrangement, ionization of neopentyl bromide would give a very unstable primary carbocation. 

Rearrangement ionization (6) accounts for about 10 and 22% respectively of the thermal reactions of He and Ne with H2" but has been only weakly observed in reactions with other molecules. This diannel has received some attention in the reaction of He(2 iS,2 5) with H2, for which a two-step mechanism has been proposed ... 

Intermediate to Penning ionization, in which the unexcited reaction partner is ionized by the excited one [reactions (4) and (5)], and associative ionization, in which only one molecular entity is formed, is the process (3), which has been called by Berry rearrangement ionization. ... 

Nearly all the mass-spectrometric studies of these rearrangement reactions have yielded relative abundances of the ionic products formed in Penning ionization, associative ionization, and rearrangement ioniza-tion. " For those cases in which the Penning ionization cross sections are known or could be estimated, specific reaction rates have been calculated and these are depicted also in Table VI. Comparison of Tables V and VI shows that when rearrangement ionization and heteronuclear associative ionization are in competition, the former occurs about an order of magnitude faster. 

It is assumed here that the probabilities of associative ionization, rearrangement ionization, and Penning ionization are the same for He(2 S) and He(2... 

For those cases in which heteronuclear associative ionization and/or rearrangement ionization compete with Penning ionization, the relative probabilities of reaction are shown in Table VIII. It is seen here that Penning ionization generally predominates, although recent work with rare gases has shown that the probability of associative ionization relative to Penning ionization increases considerably as the temperature is decreased. On the other hand, cross sections for total ionization are not very sensitive to the temperature. ... 

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