Fragmentation reactions, loss small molecules

There are two general types of fragmentation reactions. In one type the radical cation fragments to a neutral molecule and a new radical cation. This process is especially favorable when the neutral product is a small, stable molecule. For example, the loss of water from the molecular ion of alcohols is very facile. For this reason the M+ peak is very small for primary and secondary alcohols, and it is usually undetectable for... 

A coupling reaction, broadly defined, joins two fragments with the accompanying loss of a small molecule (or ion). In one well-known coupling reaction, the nucleophilic amino group of one amino acid reacts with the electrophilic carboxyl carbon of a second amino acid to form an amide bond with the loss... 

When an electron neutralizes a positive ion, the energy released can be dissipated either in photon emission (radiative recombination), or by a third body encounter with the transient excited atom or molecule (three-body recombination) or by the fragmentation of the transient excited molecule (dissociative recombination). Radiative recombination only occurs with a very small probability and three-body recombination only occurs at high pressures or high charge densities, neither of these being appropriate to the atmospheric plasma. It is the dissociative process, exemplified by reactions (5a) and (5b), which is dominant in the ionosphere. In fact, reactions (5a) and (5b) are almost entirely responsible for the loss of ionization in the ionosphere above 85 km altitude (with N2 recombination contributing somewhat) as is readily shown by simple calculations based on laboratory determinations of dissociative recombination coefficients, are, for the dominant molecular ions 02 and NO+. 

Many radical processes involve the loss of small, stable molecules, such as carbon dioxide, nitrogen, or carbon monoxide. These kinds of reactions are called fragmentations. 

We ve seen loss of CO from ketones under certain circumstances (Eq. 16.61), and expulsion of a small stable molecule is a common photochemical reaction. Dinitrogen, N2, is an extremely stable fragment, and photochemical expulsion of dinitrogen can occur from several different types of structures. Photochemical elimination of N, has been especially useful in a wide array of studies of reactive intermediates. Whether under conventional conditions or cryogenic, matrix isolation conditions, photolysis with loss of N2 has been used to generate many types of reactive intermediates. We saw examples of this in Chapter 10 when radical additions were discussed and in Chapter 13 in the context of radical polymerizations. 

A proof-of-principle study, which sampled the headspace at room temperature above small samples of the explosives RDX, TNT, PETN and HMX, demonstrated that even with explosive samples at room temperature sufficient quantities of explosive molecules could reach the reaction region of a PTR-MS to provide a detectable signal of the protonated parent molecule provided a heated inlet line system is used [5], Although the protonated parent molecules for RDX, TNT, PETN and HMX were observed in the pilot study, in the case of RDX it has been shown that the non-dissociative proton transfer channel has a low branching ratio [6], The dominant reaction channel is in fact loss of HONO from protonated RDX resulting in an ion fragment being observed at miz 176. 

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