Isotopologue

Isotopologues are molecules that differ only in their isotopic composition. 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. Isotopologues are molecules that differ only in their isotopic composition. 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, where 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, as well as HTO and DTO , where some or all of 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. 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.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. Singly substituted isotopologues may be used for nuclear magnetic resonance experiments, where deuterated solvents such as deuterated chloroform (CDCl3) do not interfere with the solutes' 1H signals, and in investigations of the kinetic isotope effect. In the field of stable isotope geochemistry, isotopologues of simple molecules containing rare heavy isotopes of carbon, oxygen, hydrogen, nitrogen, and sulfur are used to trace equilibrium and kinetic processes in natural environments and in Earth's past. Measurement of the abundance of clumped isotopes (doubly substituted isotopologues) of gases has been used in the field of stable isotope geochemistry to trace equilibrium and kinetic processes in the environment inaccessible by analysis of singly substituted isotopologues alone.