F + (H2O)2 Reaction: The Second Water Removes the Barrier

In light of controversy concerning the classical barrier for the F + H2O -> HF + OH reaction, higher level theoretical methods are applied. Both aug-cc-pV5Z CCSD(T) and cc-pVQZ full CCSDT methods predict a low classical barrier of about 2 kcal/mol. For the analogous water dimer reaction, the presence of the second water molecule erases this barrier entirely. An entrance complex F center dot center dot center dot(H2O)(2) is found lying 7.3 kcal/mol below separated F + (H2O)(2). A barrier follows this complex on the reaction coordinate, but lying 3.1 kcal/mol below the reactants. There is also an exit complex HF center dot center dot center dot H2O3 lying 11.0 kcal/mol below the separated products HF + H2O3. Thus the reactions of atomic fluorine with the water monomer and dimer are qualitatively different.