37Cl35Cl

37Cl35Cl is the diatomic chlorine molecule in which one chlorine nucleus is the isotope 35Cl and the other is 37Cl. It is one of the isotopologues of chlorine gas (Cl2), alongside the symmetric 35Cl-35Cl and 37Cl-37Cl species.

In naturally occurring chlorine, about 75.8% is 35Cl and about 24.2% is 37Cl. Consequently, a substantial fraction of Cl2 molecules in a natural sample are the mixed isotopologue 35Cl-37Cl, typically around one-third of all molecules. The exact proportion follows statistical combinatorics from the isotopic abundances.

Because the two atoms have different masses, 37Cl-35Cl is asymmetric. This mass difference alters the molecule’s reduced mass (μ = m1 m2/(m1+m2)), which affects vibrational and rotational energy levels. For 35Cl-37Cl, μ is about 18 atomic mass units, placing its spectroscopic constants between those of 35Cl-35Cl and 37Cl-37Cl. As a result, infrared and Raman spectra, as well as rotational spectra, show slightly shifted lines compared with the symmetric isotopologues. In mass spectrometry, the presence of multiple chlorine isotopologues contributes to the characteristic isotopic pattern observed for chlorine-containing compounds.

The Cl–Cl bond length and bond dissociation energy are largely insensitive to this isotopic substitution; the observable effects are primarily spectroscopic. 37Cl35Cl serves as a natural example of an asymmetric diatomic isotopologue and is relevant in isotopic labeling and high-resolution spectroscopy studies.