Can water be a catalyst on the HO2Â +Â H2OÂ +Â O3 reactive cluster?

►The reaction of hydroperoxyl radical with ozone is examined in the presence of a water molecule. ► The relative position of the saddle points for hydrogen- and oxygen- exchange are examined. ► It is found that oxygen-abstraction can be facilitated by the presence of the water molecule. ► This study reinforces a previous suggestion that H-exchange cannot explain per se the kinetics of the title reaction. ► Since bonds in the water molecule are not disrupted, it can be loosely considered as a catalyst.