PROTON-TRANSFER IN H5O2- AN EXTERNAL RESTRAINING FORCE( AND H3O2)

A comparative study between proton transfer in H5O2+ and H3O2- has bee n carried out using the 6-31+ G* basis set at the MP level. An extern al harmonic force is imposed between the two terminal hydrogens (one o n either end of the complex) to restrain the H bond lengths to a range where two minima exist in the potential energy surface, while providi ng the OH2 and OH- groups appropriate flexibility to approach one anot her during the course of transfer. The H bond length in the anion is f ound 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 ar e 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 valu es of R(OO). The energy barrier for the anion is higher than that of t he 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, t he hydrogen-bonded hydrogen moves upward, perpendicular to the line jo ining the O atoms, during the first step. The first steps account for about 30% of the energy required for proton transfer in the cation,whe reas for the anion it is about 45%. (C) 1995 John Wiley & Sons, Inc.