Odd- and Even-Electron Ions

As mentioned in Section 1.4, the removal of an electron from a neutral molecule during electron ionization generates an odd-electron (OE) ion, by definition, an ion that contains an unpaired electron. It is denoted by placing - - beside the molecular formula (e.g., CeH ). An OE+ ion may isomerize to a more stable distonic ion, an ion in which the charge and radical sites are located on different atoms [19]  

In contrast, even-electron (EE) neutrals and ions are those species in which the outer-shall electrons are fully paired. When an OE+ ion fragments, in most cases, the charged product is an EE+ ion and the neutral is an OE species (a radical)  

An OE+ ion can also fragment via a rearrangement reaction to expel an EE neutral species and form an OE+ fragment ion [reaction (6.8)]. EE+ ions are more stable than OE+ ions. The fragmentation of EE+ ions follows different rules [20]. Most of the EE+ ions fragment to other EE+ ions and a neutral molecule  

In some cases, they fragment to OE+ ions and an OE radical [reaction (6.9)]. 

---

Both odd- and even-electron ions appear to undergo transitions in which a neutral molecule is eliminated by cleavage of two bonds to the metal. 

H shifts may lead to reorientation of the individual double bonds and open additional paths for C—C bonding between parts of the same or formally isolated jr-electron systems. As a consequence, isomerization by cyclization is prevalent in the odd- and even-electron ions of dienes and polyenes, and negatively charged ions of these compounds also tend to undergo cyclization quite easily. 

Budnik, B.A., Jebanathirajah, J., Ivleva, V.B., Costello, C.E., and O Connor, P.B. (2004) Fragmentation methods of odd and even electron ions in HP MALDI FTMS. Proceedings, 52nd Annual Conference of the American Society for Mass Spectrometry, Nashville, TN. 

Although the earliest suggestion of a complex-mediated dissociation dates back to 1956, it was not until the early 1980s that it became evident that their involvement in the unimolecular dissociations of odd and even electron ions is a widespread phenomenon. 

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

Other monovalent elements (F, Cl, Br, and I) are counted as hydrogens, trivalent elements (P) are counted as nitrogen, and tetravalent elements (Si) are included with carbon. For chemically possible formulae, r+ db> — 1.5. Odd-electron ions (M+ ) will have an integer value and even-electron ions will have 0.5 r + db more than expected, so round up to next lowest integer.32,33 By way of example, Kind and Fiehn139 have described an integrated application of accurate mass data to metabolite identification, constrained by isotope abundance information and valence rules, in addition to the KI (Section 9.10.4.3.2). 

Another convenient way to classify ionization sources, rather than from the perspective of odd- or even-electron ion generation, is in relation to where the ions are created relative to the vacuum system that is, either generated at atmospheric pressure or in a vacuum. The two most common atmospheric pressure ionization sources, electrospray and atmospheric pressure chemical ionization, are arguably the most common ionization techniques applied in quantitative mass spectrometry today. However, discussion of earlier ionization sources is useful, as many of these techniques are still commonplace and their understanding provides a framework for appreciation of atmospheric pressure ionization technology and what it has to offer the pharmaceutical industry. 

The close relations between odd- and even-electron cations is shown in Scheme 9. Viewed in a retrosynthetic manner, the protonated cresols [7 + H]+, representing parent species for the ionic part of the distonic ions 24, can be generated by addition of an H atom to the radical cations 7 + as well as by addition of a proton to the neutral arenes 7. In the same way, the radical cations 22a, representing the ionic fragments of the McLafferty reaction, can be formed both by addition of H to the benzylic cations 9 and by addition of H+ to the benzylic radicals 25. 

There are unusually high-energy conditions inside a mass spectrometer and as a consequence, ions may contain atoms in very variable states, such that the existence criteria (Grl) and (Gr2) from Theorem 1.23 are no longer valid. In particular, positively charged ions that do not satisfy (Grl) can exist. We introduce the terms odd electron ions (OEI) for ions with an unpaired electron and even electron ions (EEI) for ions with no unpaired electrons. OEI satisfy (Grl), EEI do not. We will maintain conditions (Gr2) and (Con) also for ions, although a few possible formulas of fragment ions will be excluded as a result. This is justified, however, by the significant reduction in counts of possible molecular formulas (see Appendix C). 

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. 

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. 

A Mathematica calculation of Franck-Condon factors that determine electronic transition intensities of I2 is presented in Chapter III, and program statements for this are illustrated for I2 in Fig. III-6. In this fignre, note the dramatic differences between the intensity patterns predicted for the harmonic oscillator and Morse cases and compare these patterns with those seen in your absorption spectra. If yon have access to this software, yon might examine the changes in the harmonic-oscillator and Morse-oscillator wavefnnctions for different v, v" choices. A calcnlation of the relative emission intensities from the v = 25, 40, or 43 level conld also be done for comparison with emission spectra obtained with a mercury lamp or with a krypton- or argon-ion laser, hi contrast to the smooth variation in the intensity factors seen in the absorption spectra, wide variations are observed in relative emission to v" odd and even valnes, and this can be contrasted with the calcnlated intensities. Note that, if accnrate relative comparisons are to be made with experimental intensities, the theoretical intensity factor from the Mathematica program for each transition of wavennmber valne v shonld be mnltiphed by v for absorption and for emission. ... 

Introduction of heteroatoms complicates the analysis of charge effects on fragmentation reactions. In addition to the consideration of aromaticity effects discussed above, heteroatoms have profound effects on the stabilities of ions which contain them. There are essentially three cases that merit consideration (1) odd electron ions (2) odd alternant (see footnote p. 98) even electron ions and (3) even alternant (see footnote on p. 98) even electron ions. 

In the thermolysis of cyclohexanone virtually all of the products arise from the (8-cleaved structure 5.23) Jwo effects are important here. In the ion the a-cleavage structure is specifically stabilized (see below) and the jff-cleavage structure 9, would be specifically destabilized by the heteroatom which is at an active site in the odd alternant n system associated with the carbonyl. The primary photolysis products of cyclohexanone 24) are much more closely correlated to its mass spectrum than the thermolysis products are. This is because n- n excitation can be relaxed by an a-cleavage (Norrish type I) analogous to 7. The analogy for the photochemical reactions is, however, far from perfect because of the stabilizing effect of the oxygen atom on the even alternant even electron ions, e.g. 10. The photolysis of... 

The term "molecular ion" by definition refers to a radical cation or anion of an intact molecule. Molecular ions are odd-electron ions, which may thus be generated by El. Unfortimately, the term molecular ion is also frequently used to indicate the even-electron ionic species produced by electrospray and APCl. This obviously is not correct. In the soft ionization techniques, predominantly even-electron protonated molecules are generated in positive-ion mode, and deprotonated molecules in negative ions. In addition, various adduct ions may be generated (Table 2.2). These all are even-electron ions, and should therefore not be referred to as molecular ions. Alternatively, the term protonated molecular ions is used, which again is incorrect one cannot protonate a radical cation ... 

Common notations were used in the previous reactions, such as - e for the subtraction of an electron during the ionization process, / for the inductive cleavage, /-Ha for H rearrangement with the formation of an odd electron ion, rHe for cleavage with the H rearrangement and the formation of an even electron ion.)... 

---