Bond Cleavage in Small Nonfunctionalized Molecules

The most intense peaks in the El mass spectrum of methane are the molecular ion at m/z 16 and the fragment ion at m/z 15 (Fig. 6.1). Explicitly writing the electrons helps to understand the subsequent dissociations of to yield CHs , m/z 15, by H loss (oi) or H by CH3 loss (02), respectively. In general, it is more convenient to write the molecular ion in one of the equivalent forms. The charge and radical state are then attached to the brackets (often abbreviated as 1) enclosing the molecule. (It is advised to review the rules of Chap. 1 for terminology in the description of mass spectra). 

The CHs fragment ion can also be seen in its relation to the molecular ion, i.e., it may be described as an [M-H] ion. Accordingly, the proton could be written as [M-CHb]. The [M-H2] ion at m/z 14, i.e., carbene molecular ion in this case, results from a rearrangement fragmentation (rd). Rearrangements are discussed later in this chapter. 

The a-bond cleavage represents a simple but very common type of fragmentation. Its occurrence does neither require particular functional groups nor heteroa- 

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