Isotopologue

In chemistry, isotopologues 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 isotopes of hydrogen per molecule (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 atoms are replaced with 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. All taken together, 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 isotopically modified element, the term isotopomer, first proposed by Seeman and Paine in 1992, is used.^[4]^[5] Isotopomerism is analogous to constitutional isomerism 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, is an isotopologue of it. The structural formulas CH₃−CH₂−O−D and CH₂D−CH₂−O−H are two isotopomers of that isotopologue.

^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (1994) "Isotopologue". doi:10.1351/goldbook.I03351
^ The nine stable isotopologues are H₂¹⁶O, H¹⁶OD, D₂¹⁶O, H₂¹⁷O, H¹⁷OD, D₂¹⁷O, H₂¹⁸O, H¹⁸OD, D₂¹⁸O
^ The nine tritiated isotopologues are H¹⁶OT, D¹⁶OT, T₂¹⁶O, H¹⁷OT, D¹⁷OT, T₂¹⁷O, H¹⁸OT, D¹⁸OT, T₂¹⁸O
^ Seeman, Jeffrey I.; Secor, Henry V.; Disselkamp, R.; Bernstein, E. R. (1992). "Conformational analysis through selective isotopic substitution: supersonic jet spectroscopic determination of the minimum energy conformation of o-xylene". Journal of the Chemical Society, Chemical Communications (9): 713. doi:10.1039/C39920000713.
^ Seeman, Jeffrey I.; Paine, III, John B. (December 7, 1992). "Letter to the Editor: 'Isotopomers, Isotopologs'". Chemical & Engineering News. 70 (2). American Chemical Society. doi:10.1021/cen-v070n049.p002. Retrieved 28 August 2020.

[1] IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (1994) "Isotopologue". doi:10.1351/goldbook.I03351

[2] The nine stable isotopologues are H₂¹⁶O, H¹⁶OD, D₂¹⁶O, H₂¹⁷O, H¹⁷OD, D₂¹⁷O, H₂¹⁸O, H¹⁸OD, D₂¹⁸O

[3] The nine tritiated isotopologues are H¹⁶OT, D¹⁶OT, T₂¹⁶O, H¹⁷OT, D¹⁷OT, T₂¹⁷O, H¹⁸OT, D¹⁸OT, T₂¹⁸O

[Seeman-4] Seeman, Jeffrey I.; Secor, Henry V.; Disselkamp, R.; Bernstein, E. R. (1992). "Conformational analysis through selective isotopic substitution: supersonic jet spectroscopic determination of the minimum energy conformation of o-xylene". Journal of the Chemical Society, Chemical Communications (9): 713. doi:10.1039/C39920000713.

[5] Seeman, Jeffrey I.; Paine, III, John B. (December 7, 1992). "Letter to the Editor: 'Isotopomers, Isotopologs'". Chemical & Engineering News. 70 (2). American Chemical Society. doi:10.1021/cen-v070n049.p002. Retrieved 28 August 2020.

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