Bond Cleavage in Small Non-Functionalized Molecules

The most intensive peaks in the El mass spectrum of methane are the molecular ion, 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 CH4 to yield CH3, m/z 15, by H loss (oi) or 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. 

Note Ions are detected at a certain m/z value, and therefore only ionic species may be correlated with peaks. Neutral losses are exclusively indicated by the mass difference between peaks which is given in units of u. 

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

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