18O16O

18O16O, often written as 18O16O or 16O18O, is the isotopologue of diatomic oxygen (O2) in which one oxygen atom is the isotope 18O and the other is 16O. Because the two nuclei have the same chemical element but different masses, 18O16O is a distinct isotopologue from 16O16O and 18O18O, though it occupies the same chemical formula in a basic sense.

Natural occurrence and abundance: In Earth's atmosphere, the major isotope is 16O, with 18O making up a small fraction (about 0.2% to 0.2%). Consequently, the mixed isotopologue 18O16O occurs as a small fraction of O2 molecules, on the order of a few tenths of a percent of total O2, typically described as around 0.4% of O2 molecules.

Spectroscopic and physical properties: The presence of a heavier oxygen atom changes the reduced mass of the diatomic molecule, which in turn shifts its vibrational and rotational energies relative to 16O16O. For 18O16O, these shifts lead to slightly different spectral line positions, making the isotopologue detectable with high-resolution spectroscopy or mass spectrometry. In practice, O2 isotopologues are studied primarily via high-resolution techniques and specific electronic transitions, as pure rotational–vibrational transitions in the infrared are weak for O2-like species.

Applications: 18O16O is used in isotope ratio measurements to study atmospheric chemistry, biosphere–atmosphere exchange, and global biogeochemical cycles. By comparing the abundance of this isotopologue with other oxygen isotopologues, researchers can infer processes such as photosynthesis, respiration, and atmospheric transport, contributing to paleoclimate and modern-day carbon–oxygen cycling studies.