A structural isomer or constitutional isomerism - is one of a set of isomeric molecules with the same molecular formula but with different bonding patterns and atomic organization (i.e. line formulae - see https://goldbook.iupac.org/html/C/C01285.html ).
A stereoisomer is one of a set of isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution), but differ in the configuration of their atoms in three-dimensional space (see https://goldbook.iupac.org/html/S/S05984.html ).
(Extended) Isotopomer - also known as isotopic isomers, these are isomers https://goldbook.iupac.org/html/I/I03289.html with the same number of each isotope of each element but differing in their positions within the chemical structure. For example, CH3CHDCH3 ( CC([2H])C ) and CH3CH2CH2D ( CCC([2H]) ) are a pair of constitutional isotopomers of propane . Isotopomers can also be specific to stereochemistry, where they are known as isotopic stereoisomers (e.g. (R)- and (S)-CH3CHDOH - see https://goldbook.iupac.org/html/I/I03352.html ). By definition, isotopomers have the same mass, since they have the same isotopic composition. However, the set of all isotopomers which span all possible isotopic compositions are often collectively referred to as isotopomers. While technically inaccurate, it is a pragmatic use of the term to refer to all possible isotopomers spanning all isotopic compositions for a given chemical entity. Therefore, we will refer to isotopomer as any isotope-position-distinct isomer of a given chemical entity. The term “mass-equivalent isotopomer” will be used to refer to the technical definition of isotopomer.
We will use this term to refer to the technical definition of an isotopomer (also see above), as one in a collection of isomers with the same number of each isotope of each element but differing in their positions within the chemical structure.
In nuclear magnetic resonance spectroscopy (NMR) and in rare circumstances in tandem mass spectrometry, individual spectral features may directly indicate that certain atoms of a particular molecule have certain isotopes. As an example from NMR, certain analytical features may indicate that carbon 1 of alpha-D-glucopyranose is 13 C labeled without knowing the isotopic status of the other carbon atoms.
Isotopologues - representing molecules of the same chemical entity and same isotopic composition. Related isotopologues represent the same chemical entity but differ in the isotopic composition (number of isotopic substitutions - see https://goldbook.iupac.org/html/I/I03351.html ). The isotopologues of water that span the stable isotope compositions of hydrogen and oxygen as as follows:
1H216O, 1H2H16O, 2H216O
1H217O, 1H2H17O, 2H217O
1H218O, 1H2H18O, 2H218O
** A single isotopologue represents a set of mass-equivalent isotopomers ** (e.g. 12C2 1H6 has one isotopomer: 12C21H52H has two isotopomers).
Nominal-Mass Isotopologue - a set of isotopologues with the same elemental and nucleon compositions. In many mass spectra, the detected spectral features are not isotopically-resolved, but are elementally and nucleon resolved, thus representing what we will term a nominal-mass isotopologue.
Isotopologue Fragment - a refined set of isotopomers where the ambiguity of isotope location is limited to a subset of the atoms. This is a subset of the mass-equivalent isotopomers representing the full isotopologue. Tandem mass spectrometry may provide spectral features that indicate where (possibly mixed) isotopes are localized within a chemical structure. For example, 13C1 may be limited to the glycerol part of a glycerophospholipid and not the fatty acid chains.
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