The Structure of Radical Ions

The species initially produced in the mass spectrometer are ions derived from normal molecules by loss of an electron. Such an ion will normally contain an odd number of electrons and is consequently described as a radical ion. The same is true of the negative radical ion formed from a neutral molecule by capture of an electron [equation (7.3)]. The relationships between a neutral molecule M, the derived radical ions and M , and the lowest singlet excited state M are shown in Fig. 7.5. 

FIGURE 7.5. Relations between a neutral molecule M and the corresponding lowest excited singlet state M and radical ions and M . 

FIGURE 7.6. Hypothetical orbital energy diagram for the lowest singlet transition in (a) a radical cation (b) a radical anion. 

In the majority of cases that concern us, radical cations are formed by loss of a 7C electron or a lone-pair electron, and radical anions by capture of an electron into an antibonding n MO. The relative stabilities of the n radical ions can be estimated by considering their formation through union of an AH radical with an AH cation or anion (Fig. 7.7). 

FIGURE 7.7. First-order interactions between an AH radical S and an AH anion 

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