check where the definition of MolecularEntity comes from

[egonw]

If it comes from the IUPAC Gold Book, it should be cited properly.

From a discussion at an IUPAC meeting just now. cc @stuchalk

[tilfischer]

Dear Egon,

the definition in ChEBI linked on the profile page of molecular entity is the same as the one from IUPAC Goldbook which goes back to Victor Gold and the Glossary of terms used in physical organic chemistry from 1983 see https://doi.org/10.1351/pac198355081281 . The Goldbook online does not reference this IUPAC document, but the one from 1994. Please be aware that this focused on organic chemistry in the 1980s, mainly dealing with molecules and that the definition includes atoms or ion pairs as molecules, while this are no molecules.

Best, Tillmann

[stuchalk]

Just to update you that I am in the process of updating the Gold Book with the most recently defined terms (in general). For this term the 2022 revision of Glossary of Terms in Physical Organic Chemistry will be the new reference, but the definition is now only the first sentence of the 1994 version "Any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer, etc., identifiable as a separately distinguishable entity." as the rest has been moved to notes. This update will come this summer...

[tilfischer]

Dear Stuart,

thank you for pointing on https://doi.org/10.1515/pac-2018-1010 . Nevertheless, it will stay irritating, that this definition defines an ion pair such as NaCl as a molecule, as it defines an atom such as He as a molecule, or an intermetallic compound such as Nitinol or Nb₃Sn as a molecule. This list can be continued, but the types of compounds will not become molecules.

Best, Tillmann

[stuchalk]

Tillmann

I have to respectfully disagree. Although it does seem strange at first as a definition, it is related (from my perspective) to the definition of molecule in the same recommendation which reads "An electrically neutral entity consisting of more than one atom." This does not include a covalent bond as part of the definition. I am going to forward your concern to the Ian Williams, the chief person in the development of the 2022 version, and I will add his explanation of why the term is defined as it is in another comment.

Stuart

[tilfischer]

Dear Stuart,

Thank you for your reply! Also thank you for linking in the definition of molecule. With this definition in hand its clear that a He atom is not a molecule, while it can be described as a molecular entity, as it is an atom. The challenges continue: An isolated ion such as Na⁺ cannot be considered as electrically neutral, while it is defined as a molecular entity. Any further contribution to clearing the fog of confusion is very welcome!

Best, Tillmann