Odd-electron fragments

Example 1,4-Benzoquinone represents the perfect prototype of this fragmentation pattern. The subsequent eliminations of intact molecules causes a series comprising of odd-electron fragment ions only (Fig. 6.35). 

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. 

Analogously to all similar UHF wave functions describing dissociation of a closed shell molecule Into two odd-electron fragments, the UHF/1 wave fimctions is characterized by a pair of molecular orbitals of opposite spins, which are more and more localized on the boron and on the hydrogen, respectively, as the intemuclear distance R increases. (In the limit R- >, one of these orbitals becomes a pure boron, another a pure hydrogen atomic orbital). 

There should be relatively little effect on the relative stability of odd electron ions that contain heteroatoms, although the relative stability of their even electron fragments may be considerably altered. The absence of a heteroatom effect on odd electron - odd electron fragmentation reactions is clearly illustrated by the McLafferty rearrangement. In the mass spectro-... 

When the molecular ion obtains too high energy during the electron impact, there are fragments that are generated in the ion source. The fragment ions A are commonly even electron ions EE (odd-electron fragments OE are also possible), while the rest of the molecule forms radicals B, which are not detected in the mass spectrometer ... 

The next most important type of fragmentation involves the cleavage of two bonds. In this process, the odd-electron molecular ion yields an odd-electron fragment ion and an even-electron neutral fragment, usually a small molecule of some type. Examples of this type of cleavage include the following. 

When a molecule ionizes it follows the reaction M + e M + e + e, as described earlier. When an odd electron molecular ion fragments it can do so in variety of ways, and resulting odd electron fragment ions can also fragment in similar ways. Simple cleavage results in the expulsion of a neutral fragment that is an odd electron radical (OE ) and formation of an even electron ion (EE ) as shown ... 

Cleavage of three bonds again produces an even-electron ion. It will become evident in further discussions that odd-electron fragment ions may be mistakenly identified as molecular ions. For example, the M-H2O ion of alcohols is an odd-electron fragment ion that corresponds to a molecular ion of the related alkene (Equation 2.10). 

Other rearrangement fragmentations that lead to odd-electron fragment ions are discussed farther in the chapter. 

Unknown 4.14. Ionization from a nonbonding electron orbital can also trigger such ring fragmentations. Predict an abundant odd-electron fragment ion in the mass spectrum of p-dioxane. 

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