Isotopologues

Isotopologues are molecules that differ only in the isotopic composition of their atoms. They have the same arrangement of atoms and the same chemical formula, but at least one atom is replaced by one of its isotopes. The mass difference leads to differences in spectra and physical properties, while chemical behavior remains largely similar.

Common examples include water, H2O and D2O; carbon monoxide, 12CO and 13CO; methane, CH4 and CD4; and carbon dioxide, 12C16O2 and 13C16O2. Each pair shares structure but different isotopes, producing distinct vibrational and rotational spectra.

Isotopologues are distinct from isotopomers, which refer to the same isotopic substitutions arranged at different positions within the molecule. Isotopologues focus on isotopic type, whereas isotopomers focus on isotopic location.

Applications include tracing isotopic abundances in chemistry and geoscience, studying reaction mechanisms in spectroscopy and astrochemistry, and labeling for nuclear magnetic resonance and mass spectrometry experiments. Understanding isotopologues helps interpret spectra and quantify isotope ratios in samples.