The Symmetric Exchange Reaction OH + H₂O → H₂O + OH: Convergent Quantum Mechanical Predictions

The symmetric hydrogen exchange reaction OH + H₂O → H₂O + OH has been studied using the "gold standard" CCSD(T) method with the correlation-consistent basis sets up to aug-cc-pV5Z. The CCSDT and CCSDT(Q) methods were used for the final energic predictions. Two entrance complexes and two transition states on the H₃O₂ potential surface were located. The vibrational frequencies and the zero-point vibrational energies of these stationary points for the reaction are reported. The entrance complex H₂O···HO is predicted to lie 3.7 kcal mol^-1 below the separated reactants, whereas the second complex HOH···OH lies only 2.1 kcal mol^-1 below the separated reactants. The classical barrier height for the title reaction is predicted to be 8.4 kcal mol^-1, and the transition state between the two complexes is only slightly higher than the second complex. We estimate a reliability of ±0.2 kcal mol^-1 for these predictions. The capabilities of different density functional theory methods is also tested for this reaction.