PROTON-TRANSFER IN H5O2+ AND H3O2- AN EXTERNAL RESTRAINING FORCE

A comparative study between proton transfer in H5O2+ and H3O2- has been carried out using the 6-31+ G** basis set at the MP level. An external harmonic force is imposed between the two terminal hydrogens (one on either end of the complex) to restrain the H bond lengths to a range where two minima exist in the potential energy surface, while providing the OH2 and OH- groups appropriate flexibility to approach one another during the course of transfer. The H bond length in the anion is found to be longer and more linear than that in the cation. Geometries and orientations of the subunits play an important role in determining the H bond length and the nonlinearity of the bond. Similar trends are noted for both the ions as the spring stiffens: The barrier reaches its asymptotic maximum for intermolecular force constants larger than about 7 mdyn/Angstrom, as do the equilibrium and transition-state values of R(OO). The energy barrier for the anion is higher than that of the cation. For both systems, the intrinsic reaction coordinate divides the transfer process into two consecutive steps: The approach of the two O atoms is followed by the actual motion of the proton. In fact, the hydrogen-bonded hydrogen moves upward, perpendicular to the line joining the O atoms, during the first step. The first steps account for about 30% of the energy required for proton transfer in the cation,whereas for the anion it is about 45%. (C) 1995 John Wiley & Sons, Inc.