On the Trapping of Free Radicals of Oxygen and Chlorine

Gas mixtures of Cl₂ (∼2 mm Hg) and O₂ (∼50 mm Hg), when exposed to light in the near ultraviolet and then trapped at liquid N₂ temperatures, yield a solid Cl₂ matrix, which on warming to room temperature liberates small quantities of O₂ in excess of that trapped mechanically in the absence of light. It is shown that this excess O₂ can be reasonably accounted for in terms of the formation of ClO, Cl₂O₂, or ClOO. Under the experimental conditions the gas‐phase concentration of ClO and Cl are about equal, while Cl·OO/Cl is small. If the source of O₂ is Cl·OO it must arise from reaction between Cl and O₂ at the cold walls, the gas phase concentration of Cl·OO being negligible. Very small amounts of Cl₂O and OClO may be formed in the present system, but these would not produce O₂ on warming. Analysis of the flash photolysis experiments on Cl₂–O₂ mixtures indicates that measurable amounts of OClO would be formed unless there was a very rapid isomerization reaction, $$\mathrm{ClO}+\mathrm{OClO}\to \mathrm{Cl}·\mathrm{OO}+\mathrm{ClO}.$$ The absence of Cl₂O in the flash experiments can be shown to imply a very rapid reaction, $$\mathrm{Cl}+{\mathrm{Cl}}_2\mathrm{O}\to {\mathrm{Cl}}_2+\mathrm{ClO},$$ with an activation energy of less than 4 kcal.