Even-electron rule

Even-electron rule Odd-electron ions (such as molecular ions and fragment ions formed by rearrangements) may eliminate either a radical or an even-electron neutral species, but even-electron ions (such as protonated molecules or fragments formed by a single bond cleavage) will not usually lose a radical to form an odd-electron cation. In other words, the successive loss of radicals is forbidden [9]. 

Example According to the even-electron rule the molecular ion of methane should undergo the following dissociations  

Despite that it might initially be generated in a different electronic state and/or conformation, the fragment ion can be expected to decompose further in a way identical to the molecular ion of carbene  

Loss of a radical from a radical ion creates an even-electron fragment ion, in this case, which preferably may undergo subsequent loss of a molecule  

Loss of H from CHs should not occur. It is noteworthy that even such a single fragmentation scheme offers two independent pathways for the generation of CH  

Note It is important to assign the correct charge and radical state to all species encountered and to carefully track them through a fragmentation scheme. Otherwise, impossible fragmentation pathways may be formulated, thereby misleading the assignment of elemental composition and molecular constitution. 

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A closer look at the spectrum of K-decane also reveals fragment ions at m/z 84, 98, and 112, i.e., rearrangement ions at even mass number. The origin by loss of H from the accompanying carbenium ions at m/z 85, 99, and 113, respectively, can be excluded by application of the even-electron rule. Instead, alkane molecular ions may undergo alkane loss, [76] e.g.,... 

As indicated by metastable ion studies of isopropylbenzoic acids, [202] the formation of the ion at m/z 90 should be described as a two-step process, i.e., the product rather is [M-OH-CHO] than [M-HCOOH]. This would add yet another example to the list of violations of the even-electron rule (Chap. 6.1.3). 

In general, fragmentations obey the even-electron rule (Chap. 6.1.3). Odd-electron fragments from rearrangement fragmentations behave as if they were molecular ions of the respective smaller molecule. 

Kami, M. Mandelbaum, A. The Even-Electron Rule. Org. Mass Spectrom. 1980,... 

Krauss, D. Mainx, H.G. Tauscher, B. Bischof, P. Fragmentation of Trimethyl-silyl Derivatives of 2-Alkoxyphenols a Further Violation of the Even-Electron Rule. Org. Mass Spectrom. 1985, 20, 614-618. 

Numerous exceptions of the even-electron rule have been described [9-11]. Violations tend to occur where the loss of a radical from an even-electron ion leads to the formation of an exceptionally stable ionic species or where highly excited ions are involved. Representatives of highly excited ions are small molecular ions,... 

The decompositions of even-electron ions are strongly influenced by the preference for formation of an EE ion and EE neutral (the even-electron rule, Cummings and Bleakney 1940, Friedman and Long 1953, Karni and Mandelbaum 1980). Production of an OE" ion from an EE" ion must be accompanied by formation of a radical neutral species, involving the energetically unfavorable separation of an electron pair (Equations 4.6). 

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