An example is water, where some of its hydrogen-related isotopologues are: "light water" (HOH or H2O), "semi-heavy water" with the deuterium isotope in equal proportion to protium (HDO or 1H2HO), "heavy water" with two deuterium isotopes of hydrogen per molecule (D2O or 2H2O), and "super-heavy water" or tritiated water (T2O or 3H2O), where the hydrogen atoms are replaced with tritium isotopes. Oxygen-related isotopologues of water include the commonly available form of heavy-oxygen water (H218O) and the more difficult to separate version with the 17O isotope. Both elements may be replaced by isotopes, for example in the doubly labeled water isotopologue D218O.
Isotopologues differ from isotopomers in that the atom(s) of different isotope may be placed anywhere in a molecule, while isotopomers specify where the atom of different isotope is located within a molecule. Logically, it follows that two isotopomers of a compound may be the same isotopologue, but two isotopologues must necessarily be two isotopomers. In the example of mono-deuterated ethanol, CH3-CH2-O-D and CH2D-CH2-O-H are two distinct isotopomers of the same isotopologue with molecular formula C2H5DO. Isotopomerism is analogous to constitutional isomerism (relative positions of atoms in a molecule). Some molecules like water and carbon dioxide only have one isotopomer per isotopologue, as the positions of the non-central atoms are indistinguishable.
- Fractional abundance of atmospheric isotopologues, SpectralCalc.com
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