The exothermic HCl + OH.( H2O)reaction: Removal of the HCl plus OH barrier by a single water molecule

The entrance complex, transition state, and exit complex for the title reaction have been investigated using the CCSD(T) method with correlation consistent basis sets up to cc-pVQZ. The stationary point geometries for the reaction are related to but different from those for the water monomer reaction HCl + OH -> Cl + H2O. Our most important conclusion is that the hydrogen-bonded water molecule removes the classical barrier entirely. For the endothermic reverse reaction Cl + (H2O)(2), the second water molecule lowers the relative energies of the entrance complex, transition state, and exit complex by about 4 kcal/mol. The title reaction is exothermic by 17.7 kcal/mol. The entrance complex HCl ... OH.(H2O) is bound by 6.9 kcal/mol relative to the separated reactants. The classical barrier height for the reverse reaction is predicted to be 16.5 kcal/mol. The exit complex Cl ... (H2O)(2) is found to lie 6.8 kcal/mol below the separated products. The potential energy surface for the Cl + (H2O)(2) reaction is radically different from that for the valence isoelectronic F + (H2O)(2) system. (c) 2014 AIP Publishing LLC.