The Molecular Ion and Fragmentation Patterns

In this text, we shall emphasize compounds with molecular ions that can be identi-hed or deduced with reasonable certainty. If the molecular ion is present, it must have the highest m/z in the spectrum, excluding the effects of isotopes. Examples are shown for methane (Eig. 10.2, Table 10.2), methanol (Fig. 10.3, Table 10.3), and benzene (Eig. 10.4, Table 10.1). In each case, the molecular ion was very abundant and not difficult to identify. This is not always the case, as will be seen in later examples. The student should note that in most of the mass spectra used as examples, the molecular ion m/z value is marked by a black triangle on the x-axis. The x-axis is in units of m/z while the y-axis is relative abundance, even though these units are not marked on the spectra. The most intense peak is set to 100% and the rest of the peaks normalized to that peak. The structure of the compound is also shown on the spectrum, using a shorthand method that does not show the hydrogen atoms or the carbon atoms. The methanol spectrum (Fig. 10.3) demonstrates clearly that the molecular ion is not the base peak in the spectrum the fragment ion at m/z = 31 is the most abundant ion. 

When a molecule is ionized by electron impact, it undergoes the reaction  

a molecular ion is always a radical cation, usually with a single positive charge. It is evident that if an organic molecule loses an electron, it must be left with an unpaired electron (i.e., it is a radical ion), as shown by the dot representing the unpaired electron. This radical ion with an unpaired electron is termed an odd electron ion. The molecular ion is always an odd electron ion. For example, in the reaction CaHe -I- e 2 - - e - - e, the is the molecular ion. 

However, the molecule may fragment in such a way as to leave a pair of electrons behind on one ion. This is an even electron ion. Such an ion arises hy fragmentation and cannot be the molecular ion. Hence, the molecular ion is never an even electron ion. As an example, CH3OH may ionize and fragment as follows  

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Preliminary structural characterization was carried out on the soluble products of treatment with BF3/CH3OH (or LiAlH (8), in order to verify the similarity of our samples to materials studied previously (8-11). Gas chromatography-mass spectrometry (GC-MS) (Finnigan 3300 spectrometer) was used to establish the molecular ion and fragmentation patterns solution-state 13C NMR (IBM Instruments WP-200 spectrometer) was employed for quantitation of CH2, CH2OH, and CHOH moieties. 

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