The Steric and Electronic Structure of Onium Ions

Onium ions, as has been own in previous sections, are either the exclusive or dominating structures of the wing species in the cationic polymerization of heterocyclic monomers. literature concerning their steric and electronic structure is, however, rather scarce. 

The formation of onium ions requires that one of the lone electron pairs of N, O, P and S atoms in compounds containing C—X—C unit (where X is a heteroatom) is used in the formation of a new bond. In the simplest case, the lone electron pair of the oxygen atom in a water molecule accepts a proton to form a hydronium ion. 

The heat of formation of HjO from the components is 180 kcal mole (the solvation of hydronium ions is discussed in Sect. 3.1). 

The electronic change in the formation of the oxonium ions can be visualized in the following way  

Ammonium ions are also formally formed by removal of one electron firom the nitrogen 

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