The Concept of Charge and Unpaired Electron Localization

An extremely useful approach was proposed by Budzikievicz and coworkers [14] and later developed by McLafferty and Turecek [26]. The concept is based on the postulate that reactions of fragmentation of the molecular ions of complex organic molecules are initiated by the charge or unpaired electron, localized at a certain moiety. Despite its limitations this approach is very convenient to remember the different reactions of various particles. 

Dealing with a molecular ion it is necessary to identify its ground state, that is to remove an electron from the highest occupied molecular orbital (HOMO). The most favorable sites for the charge and unpaired electron localization may be established by taking away an electron with minimal ionization energy. The energy requirements in this case are similar to these known in UV-spectroscopy for the electron transitions a tt n. 

The charged and radical centers may coincide (e.g., elimination of n-electron) or may be separated (e.g., ionization of rr-bond). These centers may be quite far away from one another in the fragment ions. For example, SO2 loss from the molecular ion of tetrahydrothiophendioxide initially leads to a linear chain of four methylene groups, while the charge and the unpaired electron are located at the opposite ends of the chain. Certainly such an ion may further isomerize into a more stable structure. 

The tendency for the fragmentation initiation with the radical site is parallel to the donor properties of this site. The most spectacular examples involve the processes triggered by the removal of a nitrogen n-electron. Halogens are the least active in these reactions. 

INTRODUCTION TO MASS SPECTRA INTERPRETATION ORGANIC CHEMISTRY 

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The most accepted among the qualitative theories of mass spectral fragmentation are the conception of charge and unpaired electron localization and the estimation of ions and neutral particles stability. Despite their qualitative character these approaches are quite useful to work with mass spectra. Both theories use the principle of the minimal structural changes at each stage of fragmentation, while the structure of the molecular ion is considered to be the same as that of the initial molecule. Certain isomerization processes of M+ before the fragmentation are usually a matter of special study. 

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