Which of the following is NOT a type of isomer?

Isomers are molecules that have the same molecular formula but different structures or spatial arrangements. The types of isomers include structural isomers, geometric isomers, and enantiomers, each representing distinct ways that atoms can be arranged or oriented within a molecule.

Structural isomers differ in the covalent arrangement of their atoms. For example, butanol and isobutanol have the same chemical formula (C4H10O) but different connectivity between atoms, resulting in different chemical properties.

Geometric isomers, also known as cis-trans isomers, arise from differing spatial arrangements around a double bond or a ring structure, affecting the physical and chemical properties of the molecules despite having the same connectivity.

Enantiomers are a specific type of stereoisomer; they are non-superimposable mirror images of each other. This phenomenon is significant in many biological contexts, especially in the behavior of chiral molecules in biological systems.

Anionic isomers, however, do not exist as a recognized category of isomers in the study of organic chemistry or structural biology. Anions are negatively charged ions, and the term does not apply to isomerization, highlighting that this choice does not fit into the established categories of isomers. Thus, it is the