ClO

ClO, or chlorine monoxide, is a diatomic radical with the formula ClO. It is a highly reactive intermediate in atmospheric chemistry and is not stable under ordinary conditions. In many contexts, ClO refers specifically to the neutral chlorine monoxide radical, rather than to the related hypochlorite ion (ClO−), which is a different species.

In terms of structure, ClO has a ground electronic state of X2Π and is paramagnetic due to its unpaired electron. The Cl–O bond is relatively short, and its rotational, vibrational, and electronic transitions have been characterized by spectroscopy. The radical is highly reactive with a wide range of atmospheric constituents and generally has a short atmospheric lifetime.

ClO plays a central role in stratospheric ozone chemistry. It is formed by reactions such as Cl + O3 → ClO + O2 and by oxidation of chlorine-bearing precursors. ClO participates in catalytic cycles that destroy ozone, and two ClO molecules can combine to form the ClOOCl dimer (chlorine peroxide). Under sunlight, ClOOCl photolyzes to regenerate ClO, releasing reactive chlorine that can continue to deplete ozone catalytically.

ClO is detected and quantified through atmospheric spectroscopy, including UV-visible absorption and differential optical absorption techniques, in both ground-based and satellite measurements. Its study enhances understanding of ozone layer decline and the broader chemistry of halogen radicals in the atmosphere.