Some Fragmentation Patterns

For most classes of compounds, the mode of fragmentation is somewhat characteristic and hence predictable. This section discusses some of the more important modes of fragmentation. 

It is helpful to begin by describing some general principles that govern fi-agmentation processes. The ionization of the sample molecule forms a molecular ion that not only carries a positive charge but also has an unpaired electron. The molecular ion, then, is actually a radical-cation, and it contains an odd number of electrons. 

When fragment ions form in the mass spectrometer, they almost always do so by means of uni-molecular processes. The pressure of the sample in the ionization chamber is too low to permit a significant number of bimolecular collisions to occur. The unimolecular processes that are energetically most favorable give rise to the most fragment ions. 

The fragment ions thus formed are cations. A great deal of their chemistry can be explained in terms of what is known about carbocations in solution. For example, alkyl substitution stabilizes fragment ions (and promotes their formation) in much the same way that it stabilizes carbocations. Fragmentation processes that lead to the formation of more stable ions are favored over processes that lead to less stable ions. 

Examples of fragmentation via the cleavage of one bond include the following. 

---

Ionization efficiency curves can also be easily obtained in the ion cyclotron resonance spectrometer. As mentioned earlier, the most common modulation techniques now center around some form of electron energy or beam modulation. If the electron beam is amplitude-modulated (switched on and off), it is only necessary to sweep the electron energy to obtain such a curve. With the restriction that the ion residence times are much longer (so that some fragmentation patterns may be different from those obtained in more conventional instruments), the ionization efficiency curves thus obtained should be comparable to those obtained in other mass spectrometers. 

---