Reactions, fragmentation rearrangement

Cluster ions are also emitted from organic materials their identity and yield depend on the chemical structure of the materials. Molecular or quasi-molecular ions may be observed as well as other ions that are formed by fragmentation, rearrangement, decomposition, or reaction [52], Several typical ion formation processes are summarized in Table 3 [40]. 

Like fragmentations, unimolecular rearrangements are always uphill at the beginning of the process, because a bond breaks. Unlike fragmentations, rearrangements are downhill at the end as a new bond forms. An example of this kind of energy profile for the Isomerization reaction of ds-2-butene appears in Figure 15-14Z). 

The fragmentation of alkoxyl radicals is especially favorable because the formation of a carbonyl bond makes such reactions exothermic. Rearrangements of radicals frequently occur by a series of addition-fragmentation steps. The following two reactions involve radical rearrangements that proceed through addition-elimination sequences. 

The thermal and photochemical activations of EDA complexes by electron transfer are both enhanced when the radical ions D+- or A--(either paired or free) undergo a facile first-order (unimolecular) transformation such as fragmentation, rearrangement, bond-formation, etc., which pulls the redox equilibrium and thus renders the competition from the energy-wasting back electron transfer less effective (compare Scheme 5). Critical to the quantitative evaluation of the reaction dynamics is the understanding that the typical [D+% A--] intermediates, as described in... 

Unimolecular reactions of organic radical cations are fragmentation, rearrangement and cyclizadon, as illustrated by the following generic examples. The specific details of each of these transfomiations are included in Section 7.4.6. (Note Ar and R represent aryl and alkyl groups, respectively.)... 

If the transient radicals R transform rapidly into other transient radicals R , for example by fragmentation, rearrangement, addition to an unsaturated molecule, atom or group transfer, or by any other reaction, then the cross-reaction products between R - and Y become dominant. The transformation of Y to another persistent Y leads to the selective formation of R—Y. ... 

Keywords Orbital overlap factor Fragmentation [1,5] sigmatropic shift Epimerization Reverse ene reaction Cope rearrangement EIZ-isomerization... 

A chemical reaction is, at the most fundamental level, a rearrangement of the atoms of the reactant molecules. In the simplest case of unimolecular reaction, this rearrangement can be viewed as the limit of very high amplitude vibrational motion. For example, a unimolecular dissociation reaction is simply the extension of a bond stretch vibration to the point of fragmentation. Similarly, a cis-trans isomerization reaction is the result of a very high amplitude torsional vibration. 

Dialkyl [(2-acyloxy)alkyl]phosphonates, neat or in solution, undergo a fragmentation-rearrangement process when pyrolysed at ca 220-240 °C the overall reaction, represented in equation 7, results in moderate to high yields of alkenes and of acetic esters . A similar decomposition occurs under ultraviolet irradiation... 

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