PROTON AND LITHIUM ION TRANSFER BETWEEN 2 WATER-MOLECULES WITH AN EXTERNAL RESTRAINING FORCE

Transfer of the central ion in (H2O..H..OH2)(+) and H2O..Li..OH2)(+) is studied by ab initio calculations using the 6-31+G** basis set at the SCF and MP2 levels. An external harmonic force is imposed to restrain the H/Li bond length to the range where two minima exist in the potential energy surface, while providing the two water molecules appropriate flexibility to approach one another during the course of the transfer. The proton transfer barrier is low for a weak external force and climbs as the spring is stiffened. Similar trends are noted as the spring is lengthened with a uniform force constant. 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 Li+ transfer is somewhat higher than that for proton transfer. The two oxygen atoms more closely approach one another at the midpoint of transfer in either case, and nonlinearity is introduced into the bond as each water molecule pivots around its anchor. The half transfer of the proton involves a displacement of 0.3 Angstrom, as compared to the 1 Angstrom motion of the Li+. 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 central ion. The second step accounts for 70% of the energy required for proton transfer and about 90% in the Li+ case. Most of the electron density rearrangement takes place in the second step of either transfer.