35Cl37Cl

35Cl37Cl is the diatomic chlorine isotopologue composed of one atom of chlorine-35 and one atom of chlorine-37. It is one of three natural isotopologues of molecular chlorine (the others being 35Cl–35Cl and 37Cl–37Cl). In nature, chlorine has approximately 75.8% 35Cl and 24.2% 37Cl, so the mixed isotopologue occurs at a substantial frequency in chlorine-containing samples.

Structurally, 35Cl37Cl is a homonuclear diatomic molecule in terms of its chemical bonding, with a Cl–Cl bond length similar to other chlorine molecules (on the order of about 1.99 Å). Because the two atoms are the same element, the molecule has no permanent electric dipole moment. Consequently, it is infrared inactive and does not exhibit pure rotational transitions in the microwave region. Like other homonuclear diatomics, it is Raman active, and its vibrational transitions can be observed in Raman spectra, with frequencies shifted by the isotopic mass.

The vibrational frequency is determined by the reduced mass, so the fundamental frequency for 35Cl37Cl lies between those of 35Cl–35Cl and 37Cl–37Cl. In mass spectrometry, the mixed isotopologue has a distinct molecular ion corresponding to a mass of about 72 u (approximately m/z 72 when ionized).

In natural samples, the abundance of the 35Cl37Cl isotopologue is about 2 × 0.7578 × 0.2422 ≈ 0.367, or roughly 36–37% of Cl2 molecules. This isotopologue is relevant in isotopic analyses and in spectroscopic studies that rely on the natural distribution of chlorine isotopes.