Odd-electron ions

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

Radical ion (odd electron ion) An ion containing an unpaired electron that is both a radical and an ion. 

Stevenson s Rule (Stevenson-Audier). A simple cleavage of a chemical bond in an odd electron ion may result in two pairs of ions and neutrals ... 

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. 

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. 

Electron impact (El) ionization is one of the most classic ionization techniques used in mass spectrometry. A glowing filament produces electrons, which are then accelerated to an energy of 70 eV. The sample is vaporized into the vacuum where gas phase molecules are bombarded with electrons. One or more electrons are removed from the molecules to form odd electron ions (M+ ) or multiply charged ions. Solids, liquids and gases can be analyzed by El, if they endure vaporization without decomposition. Therefore the range of compounds which can be analyzed by El is somewhat limited to thermally stable and volatile compounds. The coupling with gas chromatography has been well established for... 

APPI is a relatively recent development compared with the other techniques. Here, ionisation is achieved photochemically, either directly or mediated by a dopant such as acetone added to the eluent. Both even- and the less stable odd-electron ions (e.g. M ) may be formed. At the time of writing, the mechanisms involved and scope of the technique are still not fully understood. What is apparent is that it provides a complementary technique to ESI and APCI. 

The cleavage reaction of Equation 23-2 reveals other useful generalizations. Whatever its source, a parent molecular ion, M+, has one unpaired electron and is properly described as an odd-electron ion (a radical cation). When a parent molecular ion fragments, it does so homolytically, as shown in Equation 23-2, and produces a radical and an ion in which the electrons are paired—an even-electron ion. The m/e value of an even-electron ion is an even number for any elemental composition of C, H, O in combination with an odd number of nitrogens. These generalizations are summarized in Table 23-2 and can be useful in the interpretation of mass spectra, as illustrated by Exercises 23-4 and 23-5. 

Identify any odd-electron ions and consider possible rearrangements (see Rearrangements, p. 371). 

Phenols usually give a strong molecular ion. Typical peaks in the spectrum arise from M — 28 (CO), which is a useful odd-electron ion, and M — 29 (CHO). 

McLafferty rearrangements are common for aliphatic aldehydes and ketones, providing that an alkyl group of at least three carbons long is attached to the carbonyl group. Odd-electron ions are formed which are useful in the analysis of the spectrum. 

Thus the spectrum of 4-methylpentan-2-one (Fig. 3.84) shows a strong peak at m/z due to the odd-electron ion (CH3C(OH)=CH2) resulting from the McLafferty rearrangement. 

The peaks at m/z 56 and 61 in the mass spectrum of butyl acetate (Fig. 3.85) can be explained by the above rearrangements. The mass spectrum of ethyl buta-noate, Fig. 3.86, shows two important peaks due to odd-electron ions at m/z 88 and 60, resulting from two successive McLafferty rearrangements. 

Haas and Wilson (117) have studied a number of substituted butadiene-iron tricarbonyl complexes, finding that in most cases loss of three CO groups precedes fragmentation of the ligand. Compared with the ligands the spectra of the complexes show a number of instances where the iron atom tends to stabilize odd-electron ions. In (XV), the presence of the ion... 

---