Any constitutionally or isotopically distinct atom, molecule,
ion,
ion pair, radical,
radical ion, complex,
conformer etc., identifiable as a separately distinguishable entity. Molecular entity is used
in this Compendium as a general term for singular entities, irrespective of their
nature, while
chemical species stands for sets or ensembles of molecular entities. Note that the name of a compound
may refer to the respective molecular entity or to the chemical species, e.g. methane,
may mean a single molecule of
CH4
(molecular entity) or a molar amount, specified or not (chemical species), participating
in a reaction. The degree of
precision necessary to describe a molecular entity depends on the context. For example '
hydrogen molecule' is an adequate definition of a certain molecular entity for some purposes, whereas
for others it is necessary to distinguish the
electronic state and/or vibrational state and/or nuclear spin, etc. of the hydrogen molecule.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1142
