6.3.1 A Simple Reacting Molecule

Consider a molecular ion represented as ABC $^{\textrm{+.}}$. If this molecule is energized sufficiently to decompose, several 'primary' reactions are possible (if the MI itself is the reactant, the reaction is primary; reactions of products are secondary, etc.). When thinking about how these reactions produce signals in the mass spectromter, remember that only ions are mass analyzed and detected. For ABC $^{\textrm{+.}}$, the expected primary reactions are:

ABC $^{\textrm{+.}}$ $\rightarrow$ AB$^{\textrm{+}}$ + C$^{\textrm{.}}$ (1)

ABC $^{\textrm{+.}}$ $\rightarrow$ AB$^{\textrm{.}}$ + C$^{\textrm{+}}$ (2)

ABC $^{\textrm{+.}}$ $\rightarrow$ A$^{\textrm{+}}$ + BC$^{\textrm{.}}$ (3)

ABC $^{\textrm{+.}}$ $\rightarrow$ A$^{\textrm{.}}$ + BC$^{\textrm{+}}$ (4)

ABC $^{\textrm{+.}}$ $\rightarrow$ AC$^{\textrm{+}}$ + B$^{\textrm{.}}$ (5)

ABC $^{\textrm{+.}}$ $\rightarrow$ AC$^{\textrm{.}}$ + B$^{+}$ (6)

Reactions (1)-(4) are simple cleavages. They result from a simple separation of two atoms in the molecule. A considerable amount of information can be derived about the MI structure by considering only the possible simple cleavage reactions. Further,

• Simple cleavage reactions usually involve breaking a single bond.
• Considering the breaking of a given bond, both ionic products are often observed; for example, the mass spectrum of ABC may have signals corresponding to both reactions (1) and (2). In general, the charge stays with the fragment of smaller ionization energy.