isotopologen

Isotopologen are chemical species that differ only in the isotopic composition of their atoms, while keeping the same chemical formula and connectivity. In English, the widely used term for this concept is isotopologue; isotopologen is an alternative form found in some language contexts and older texts. Both refer to molecules that arise from substituting one or more atoms with their isotopes without changing the overall structure.

Key idea and examples

- Isotopologues include variants such as 12CO and 13CO, or H2O, HDO, and D2O. These molecules have the same arrangement of atoms and bonds but differ in which atoms are heavy isotopes (for example, carbon-12 versus carbon-13, or hydrogen versus deuterium).

- A related concept is isotopomer, which describes isotopologues that differ only in the positions of isotopic substitution within the molecule.

Physical and analytical consequences

- Isotopic substitution changes molecular mass and incremental mass spectra, producing distinct peaks in mass spectrometry.

- Vibrational and rotational frequencies shift due to the altered reduced mass, leading to changes in infrared and Raman spectra.

- Nuclear magnetic resonance properties may also be affected, depending on the isotope substituted.

Applications

- Stable isotope labeling for tracing biochemical pathways, metabolic flux analysis, and proteomics (for example, labeling with 13C, 15N, or D).

- Environmental and geochemical studies use isotopologues to track sources and processes, such as isotope-ratio analyses in water or carbon-containing compounds.

- Structural elucidation and mechanism studies often leverage isotopic labeling to probe reaction pathways.

Nomenclature note

- While isotopologue is the standard term in English, isotopologen may appear in some languages or older literature. IUPAC and contemporary texts generally prefer isotopologue to describe species that differ in isotopic composition.