Ab initio investigation on the reaction path and rate for the gas-phase reaction of HO + H2O ↔ H2O + OH

This article describes an ab initio investigation on the potential surfaces for one of the simplest hydrogen atom ion reactions, that is, HO + H2O ↔ H2O + OH. In accord with the findings in the previously reported theoretical investigations, two types of the hydrogen‐bonding complexes [HOHOH] and [H2OHO] were located on the potential energy surface. The water molecule acts as a hydrogen donor in the [HOHOH] complex, while the OH radical acts as a hydrogen donor in the [H2OHO] complex. The energy evaluations at the MP2(FC) basis set limit, as well as those through the CBS‐APNO procedure, have provided estimates for enthalpies of association for these complexes at 298 K as −2.1 ≈ −2.3 and −4.1 ≈ −4.3 kcal/mol, respectively. The IRC calculations have suggested that the [H2OHO] complex should be located along the reaction coordinate for the hydrogen ion. Our best estimate for the classical barrier height for the hydrogen ion is 7.8 kcal/mol, which was obtained from the CBS‐APNO energy evaluations. After fitting the CBS‐APNO potential energy curve to a symmetrical Eckart function, the rate constants were calculated by using the transition state theory including the tunneling correction. Our estimates for the Arrhenius parameters in the temperature region from 300 to 420 K show quite reasonable agreement with the experimentally derived values. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1538–1548, 2003