6.2.1 Ionization

In general, ionization occurs at the molecular orbitals that have the smallest ionization energies. Often, ionization is not a localized event (in the molecule orbital theory), but many straightforward mechanisms of MI decomposition begin with a localized charge site.

• Non-bonding electrons (also called 'lone pair' electrons), present in N, O and S atoms for example, often have small ionization energies. Mechanisms often begin with these electronegative atoms having the charge.
• Valence $\pi$ electrons generally have smaller ionization energies than $\sigma$ electrons for most organic compounds. Electron rich regions with relatively small ionization energies, such as aromatic rings, represent a reasonable site to place the initial charge in a mechanism.
• We are concerned virtually entirely with the removal of valence electrons, not core electrons.