Basic Mechanisms of Ion Fragmentation

Such unimolecular ion-decomposition reactions can be viewed as another field of chemistry, but fortunately for most chemists studying this book, there are many close similarities to pyrolytic, photolytic, radiolytic, and other energetic reactions, and there are even many general similarities to condensed-phase (solution) organic reactions. The largest points of difference are that ionic and often radical species are involved in each reaction in the mass spectrometer, and their combined effects sometimes appear unusual to the organic chemist. Chemists may also question the reliability of structural relationships based on rearrangement reactions. However, many of these are based on well-established chemistry and can provide key molecular information 

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The basic mechanism of fragmentation (equation 1) is a concerted one, with a contribution from each of the five centers to the transition state. Two-step carbonium ion or, very rarely, carbanion mechanisms mean in all cases that substitution, elimination or ring closure can take place rather than fragmentation. However, the one-step synchronous fragmentation is allowed only when the compound can adopt a conformation where the leaving group X as well as the electron pair of the electrofuge Y are antiperiplanar to the C(2)—C(3) bond (Scheme 5). ... 

Chapter 1 provides a historical viewpoint (perspective) on the study of ion/mol-ecule association (cationization) MS as well as explanation on the evolution of developments of the instmmental methods. In addition to serving as an introduction for the subject of cationization MS as it pertains to ion chemistry, this chapter briefs thermochemistry and chemical dynamics (and analytical application) of metal ion association reaction. The fundamentals for iorr/molecule association reaction are described in Chapter 2, providing a basic introduction to the mechanism and dynamics of termolecrrlar association reaction, dissociation and fragmentation reaction of associated ion and ion/molecule association mechanism in the corrderrsed-phase. 

The starting point for a fragmentation is the molecular ion (El) or the quasimolec-ular ion (Cl). A large number of reactions which follow the primary ionization need to be described here. All the reactions follow the thermodynamic aim of achieving the most favourable energy balance possible. The basic mechanisms which are involved in the generation of spectra of organic compounds will be... 

A wide range of thermochemical properties can be measured, including not only proton affinity or gas-phase basicity, but also electron affinity, ionization energy, gas-phase acidity and cation affinity Entropy changes upon attachment of an ion to a molecule are also accessible and provide information on both the nature of the bonding and fragmentation mechanisms in cluster ions, especially in biological compounds. Thermochemical determinations by the kinetic method also provide very useful structural information e.g., two-electron three-center bond has been observed in the gas phase by means of the kinetic method. " In the last years, the kinetic method has been also applied to characterize chiral ions in the gas phase. 

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