InChI, the IUPAC International Chemical Identifier

Mesomerism is the concept related to the situation in which the molecular structure cannot be unambiguously represented by a single classical structural formula; rather, two (or more) mesomeric structures must be drawn and considered to contribute to the overall picture.
As the IUPAC Gold Book states, “mesomerism” is “Essentially synonymous with resonance. The term is particularly associated with the picture of π-electrons as less localized in an actual molecule than in a Lewis formula. The term is intended to imply that the correct representation of a structure is intermediate between two or more Lewis formulae” [28]. In other words, mesomers are considered as imaginary objects (or even drawing artifacts) that cannot be distinguished by a simple chemical identifier.
Mesomerism is effectively eliminated in InChI. Mesomers have the same InChIs (this is true for all possible InChI layouts of layers). Actually, this is very natural. Mesomeric structures of a molecular entity have the same basic connectivity but differ in bond orders, and maybe by having atomic charges on different atoms. InChI does not use bond orders and does not place charges on particular atoms; the placement of hydrogen atoms in a mesomeric system, which would be important for InChI, is always the same.

In class yesterday we covered mesomerism with canonical SMILES, tomorrow you need to be prepared to explain how InChI covers this for the molecule you used in class yesterday, and be prepared to lead a discussion with your group on the differences between the two methods.

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