Communication: Some critical features of the potential energy surface for the Cl + H2O → HCl + OH forward and reverse reactions.

The forward and reverse reactions Cl + H2O → HCl + OH are very important in atmospheric chemistry. The entrance complex, transition state, and exit complex for the endothermic reaction Cl + H2O → HCl + OH have been studied using the CCSD(T) method with the correlation consistent basis sets through cc-pVQZ. The vibrational frequencies and the zero-point vibrational energies of the five stationary points for the reaction are reported and compared to the limited available experimental results. Contrary to the valence isoelectronic F + H2O system, the Cl + H2O reaction is endothermic by 18.4 kcal∕mol. The two potential energy surfaces found to be radically different. The Cl + H2O entrance complex is found to lie 3.5 kcal∕mol below the separated reactants. The classical barrier height for the title reaction is predicted to be 20.8 kcal∕mol. The exit complex is bound by 3.7 kcal∕mol relative to separated products. From the atmospherically important Cl + OH side, there is a small classical barrier of 2.4 kcal∕mol.