What is "isotopologue"

n. (context chemistry English) any of a group of compounds only differing in their isotopic composition; for example water and heavy water

Isotopologues are molecules that differ only in their isotopic composition. Simply, the isotopologue of a chemical species has at least one atom with a different number of neutrons than the parent.

An example is water, where some of its hydrogen-related isotopologues are: "light water" (HOH or HO), "semi-heavy water" with the deuterium isotope in equal proportion to protium (HDO or HHO), " heavy water" with two deuterium isotopes of hydrogen per molecule (DO or HO), and "super-heavy water" or tritiated water (TO or HO), where the hydrogen atoms are replaced with tritium isotopes. Oxygen-related isotopologues of water include the commonly available form of heavy-oxygen water (HO) and the more difficult to separate version with the O isotope. Both elements may be replaced by isotopes, for example in the doubly labeled water isotopologue DO.

Isotopologues may be used for nuclear magnetic resonance experiments, where deuterated solvents do not interfere with the solutes' H signals, and in investigations of the kinetic isotope effect.

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, CH-CH-O-D and CHD-CH-O-H are two distinct isotopomers of the same isotopologue with molecular formula CHDO. 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.