Fragmentation reactions unimolecular decomposition

The attachment of an electron to an organic acceptor generates an umpolung anion radical that undergoes a variety of rapid unimolecular decompositions such as fragmentation, cyclization, rearrangement, etc., as well as bimolecular reactions with acids, electrophiles, electron acceptors, radicals, etc., as demonstrated by the following examples.135"137... 

Semiempirical and ab initio techniques have been applied to the unimolecular decomposition of 2-chloropropanoic acid, which is known to form HC1, CO, and CH3CHO. In keeping with the experimental data, the results suggest that reaction proceeds by a two-step mechanism involving rate-determining formation of intermediate a-propiolactone, which then fragments to form CO and CH3CHO.47... 

We also made molecular-dynamic simulation of thermal decomposition of some individual energetic materials, including RDX, at extremely high temperatures [93,94]. It turned out that the primary fragmentation mechanism at these conditions is entirely different from the low-temperature variant. In the case of the RDX unimolecular decomposition, it can be mentioned that elimination of NO2 group by homolysis of one N-N bond is observed for all reaction conditions whereas perhydrotriazine ring fission (depolymerization to... 

The correlations presented here cannot of course be considered quantitative since the reaction processes are quite different. Reactions in solution depend on such factors as solvation and equilibria, whereas mass spectral reactions are strictly unimolecular decompositions of activated molecular ions. However, as a qualitative approach, these comparisons can be useful in the evaluation of spectra of related compounds. Differences in the intensities of various fragment peaks have been used to distinguish between numerous sets of isomeric compounds. 

The dot will be used to indicate a radical, so that the symbol signifies a radical ion. Species containing such an unpaired electron are termed odd-electron (OE) similarly, an ion with only paired electrons is an even-electron (EE) ion. Note in Equations 1.2 that only one ion can be formed in the unimolecular decomposition of an ion, and that a radical product (neutral or ionic) must be formed from an OE ion. It should be reemphasized that only unimolecular reactions are important in El fragmentations the sample pressure in the ion source is made low enough to avoid reactions between ions and molecules. 

In a unimolecular reaction, a singie moiecule fragments into two pieces or rearranges to a new isomer. A simple example of a unimolecular reaction is the decomposition of N2 O4. An N2 O4 molecule continually vibrates, and these vibrations cause its N—N bond to stretch. If the molecule has sufficient energy, the bond breaks and the moiecuie separates into two moiecuies of NO2, much like a spring that breaks if it is stretched too far. 

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