Two potential paths for OH radical formation on surfaces of pure water microdroplets.

Experimental findings by others suggest that OH radicals are formed in unexpected abundance on or near surfaces of 1-50 µm microdroplets comprised of pure water, but the mechanism by which these radicals are generated is not yet fully resolved. In this work, we examine two possibilities using ab initio electronic structure methods: (1) electron transfer (ET) from a microdroplet surface-bound OH- anion to a nearby H3O+ cation and (2) proton transfer (PT) from such a H3O+ cation to a nearby OH- anion. Our findings suggest that both processes are possible but only if the droplet's underlying water molecules comprising the microdroplet provide little screening of the Coulomb interaction between the anion and cation once they reach ∼10 Å of one another. In the ET event, an OH radical is formed directly; for PT, the OH formation occurs because the new O-H bond formed by the transferred proton is created at a bond length sufficiently elongated to permit homolytic cleavage. Both the ET and PT pathways predict that H atoms will also be formed. Finally, we discuss the roles played by strong local electric fields in mechanisms that have previously been proposed and that occur in our two mechanisms.