An ensemble of chemically identical
molecular entities that can
explore the same set of molecular energy levels on the time scale
of the experiment. The term is applied equally to a set of
chemically identical atomic or molecular structural units in a solid
array. For example, two conformational
isomers may be
interconverted sufficiently slowly to be detectable by separate
NMR spectra and hence to be considered to be separate chemical
species on a time scale governed by the radiofrequency of the
spectrometer used. On the other hand, in a slow chemical reaction
the same mixture of conformers may behave as a single chemical
species, i.e. there is virtually complete equilibrium population of
the total set of molecular energy levels belonging to the two
conformers. Except where the context requires otherwise, the term
is taken to refer to a set of molecular entities containing
isotopes in
their natural abundance. The wording of the definition given in the
first paragraph is intended to embrace both cases such as
graphite,
sodium chloride or a surface oxide, where the basic structural units
may not be capable of isolated existence, as well as those cases
where they are. In common chemical usage generic and specific
chemical names (such as
radical or hydroxide ion) or chemical
formulae refer either to a chemical species or to a
molecular entity.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1096
PAC, 1996, 68, 2193
(Basic terminology of stereochemistry (IUPAC Recommendations 1996))
on page 2202
