Isotopologue

In chemistry, isotopologues (also spelled isotopologs) are molecules that differ only in their isotopic composition.^[1] They have the same chemical formula and bonding arrangement of atoms, but at least one atom has a different number of neutrons than the parent.

An example is water, whose 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 atoms (D₂O or ²H₂O); and "super-heavy water" or tritiated water (T₂O or ³H₂O, as well as HTO [¹H³HO] and DTO [²H³HO], where some or all of the hydrogen is the radioactive tritium isotope). 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. Altogether, there are 9 different stable water isotopologues,^[2] and 9 radioactive isotopologues involving tritium,^[3] for a total of 18. However only certain ratios are possible in mixture, due to prevalent hydrogen swapping.

The atom(s) of the different isotope may be anywhere in a molecule, so the difference is in the net chemical formula. If a compound has several atoms of the same element, any one of them could be the altered one, and it would still be the same isotopologue. When considering the different locations of the same isotope, the term isotopomer, first proposed by Seeman and Paine in 1992, is used.^[4][5] Isotopomerism is analogous to constitutional isomerism or stereoisomerism of different elements in a structure. Depending on the formula and the symmetry of the structure, there might be several isotopomers of one isotopologue. For example, ethanol has the molecular formula C₂H₆O. Mono-deuterated ethanol, C₂H₅DO or C₂H₅²HO, is an isotopologue of it. The structural formulas CH₃−CH₂−O−D and CH₂D−CH₂−O−H are two isotopomers of that isotopologue.