16O18O

16O18O is an isotopologue of molecular oxygen (O2) consisting of one oxygen-16 atom and one oxygen-18 atom. It is written as 16O18O or O16O18 and is part of the set of oxygen isotopologues used to study atmospheric and planetary processes, as well as isotopic chemistry.

Natural occurrence and abundance: In Earth's atmosphere, 18O accounts for about 0.2% of oxygen nuclei, while 16O accounts for about 99.8%. Because of this, a small fraction of O2 molecules are 16O18O. Roughly, about 0.4% of atmospheric O2 molecules are the 16O18O isotopologue, with the exact share influenced by atmospheric and biological fractionation over time.

Structure and physical properties: Like all O2 molecules, 16O18O has a double bond between two oxygen nuclei and is a linear diatomic nonpolar molecule. The electronic structure is the same as for 16O16O, so it has no permanent electric dipole moment. Because the two atoms are identical in chemical charge, isotopic substitution does not create a large dipole; consequently, purely electric-dipole rotational or vibrational transitions are very weak or forbidden, and spectroscopic signatures arise mainly from electronic transitions and, to a lesser extent, magnetic-dipole or quadrupole effects. The bond length is similar to 16O16O, while the vibrational frequency shifts to lower energy because the increased reduced mass (approximately μ ≈ (16×18)/(16+18) ≈ 8.47 amu) lowers the vibrational wavenumber. The fundamental vibration of 16O18O is shifted downward by a few percent relative to 16O16O, on the order of a couple thousandths of a centimeter.

Applications: The 16O18O isotopologue is used to study isotopic fractionation in the atmosphere, oceans, and biosphere, and to constrain models of atmospheric chemistry and planetary evolution. In spectroscopy, it provides a reference for isotopic shifts and helps interpret measurements of O2-related bands in Earth and space environments.

See also: isotopologue, oxygen, isotopic abundance.