Sigma electron ionization mechanism

If the radical is formed by the loss of an electron from a given a bond, such as a bond to a highly substituted carbon, the cleavage at this location will be favored sigma electron ionization mechanism). 

The rules indicated above seem to not have much connection to the fragmentation results obtained in pyrolysis. For example, Stevenson s rule, the charge site ionization mechanism, and the sigma electron ionization mechanism are not applicable to pyrolysis products, as the molecules in pyrolysis are not ionized. On the other hand, the a cleavage and certain rearrangements may be similar for the two processes. Also the fact that small molecule elimination is favored in mass spectrometry makes possible that, with a certain frequency, pyrolysis products are similar to mass fragments obtained in mass spectrometry. 

The presence of heteroatoms with non-bonding n electrons favors the localization of charges. When the radical and/or the charge are localized, this influences the fragmentation and the reaction mechanisms can be classified as sigma electron ionizations, localized charge initiations, radical site initiations, and rearrangements. 

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