Even-electron ions

Even-electron ion. An ion containing no unpaired electrons, e.g., CHj in its ground state. 

Processes occurring upon ionization and notations used for odd-electron ions (4) and even-electron ions ( + ) are illustrated in Equations 1, 2, and 3. 

DI methods tend to produce even-electron ions such as protonated molecules [M + H]+, or cationised molecules such as [M + Na]+ these stable ions undergo only a minimum amount of fragmentation. Desorption/ionisation mass-spectral methods (in particular FD, EH and LD) may be used for determining molecular... 

Poor understanding of fragmentation of even-electron ions... 

Whilst for a molecule it is enough to specify its molecular weight (MW), this is not true for ions. In fact, an ion is defined by its mass (m) and by its charge (z). Both even-electron ions, like H30+, and radical ions, i.e. radicals that carry an electric charge, like CH4+", can be formed in a mass spectrometer. With MS it is possible to study positive or negative ions molecules with basic properties are easily protonated, while those having acidic character are more easily deprotonated. 

In soft ionization methods the excess energy deposited onto the ionized molecule is very small and stable even-electron ions are formed. This leads to easy determination of the molecular weight of the analyte, but as fragmentation is absent or it occurs to a very low extent, structural information is missing in the mass spectrum. However, one can obtain structural information by causing ion fragmentation out of the source by means of tandem mass spectrometry experiments (see below). 

An analog of Stevenson s mle for even electron ions is proposed by Field [12] and Bowen et al. [13]. Let us discuss a reaction of fragmentation of an even electron ion ... 

If an alkyl group is bonded to a substitute of high ionization energy one can observe a homologous series (sometimes quite abundant) of even-electron ions with general formula CnH2n+. 

While the doubly charged ion, is an even-electron ion, the triply charged ion, again is an odd-electron ion. In addition, there are several other events possible from the electron-neutral interaction, e.g., a less effective interaction will bring the neutral into an electronically excited state without ionizing it. 

Note As one electron charge is neutralized upon EC, the precursor ion for ECD must at least be a doubly charged positive even-electron ion to yield a singly charged radical ion for subsequent dissociation. 

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. [12]... 

The r + d algorithm produces integers for odd-electron ions and molecules, but non-integers for even-electron ions that have to be rounded to the next lower integer, thereby allowing to distinguish even- from odd-electron species. 

Any molecule that can at least formally be synthesized by a Diels-Alder reaction is a potential candidate for the mass spectral RDA reaction. In addition, the RDA reaction is not restricted to positive radical ions, but it may also occur from even-electron ions as well as from negative radical ions, NT. These findings remind us of the fact that ionic reactions are determined by the intrinsic properties and the internal energy of the ions, and thus only indirectly by the ionization technique used for their creation. 

Zollinger, M. Seihl, J. McLafferty Reactions in Even-Electron Ions Org. Mass Spectrom. 1985, 77,649-661. 

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