Originally, the compound betaine,
(CH3)3N+–CH2C(=O)O−N,N,N-trimethylammonioacetate, and similar zwitterionic compounds derived from other amino
acids. By extension, neutral molecules having charge-separated forms with an
onium atom which bears no hydrogen atoms and that is not adjacent to the anionic atom.
Betaines cannot be represented without formal charges. E.g.
Source:
PAC, 1995, 67, 1307
(Glossary of class names of organic compounds and reactivity intermediates based on
structure (IUPAC Recommendations 1995))
on page 1322
