O=Cl=O → O--Cl-O- → (cyclic)O-Cl-O This is an intramolecular reaction which isn't all that common. The double bonds break, causing the O's to each have a negative charge. This forms an ion. The ion bends and eventually the 2 O's bond with each other forming a ring. But would this really happen? I mean yes, sunlight is enough to split Cl2 into 2 Cl radicals but would it be enough to break the O-Cl double bond so that the ion can form?