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)(+) i s studied by ab initio calculations using the 6-31+G* basis set at th e SCF and MP2 levels. An external harmonic force is imposed to restrai n the H/Li bond length to the range where two minima exist in the pote ntial energy surface, while providing the two water molecules appropri ate flexibility to approach one another during the course of the trans fer. The proton transfer barrier is low for a weak external force and climbs as the spring is stiffened. Similar trends are noted as the spr ing is lengthened with a uniform force constant. The barrier reaches i ts asymptotic maximum for intermolecular force constants larger than a bout 7 mdyn/Angstrom, as do the equilibrium and transition state value s of R(OO). The energy barrier for Li+ transfer is somewhat higher tha n that for proton transfer. The two oxygen atoms more closely approach one another at the midpoint of transfer in either case, and nonlinear ity is introduced into the bond as each water molecule pivots around i ts 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 int rinsic reaction coordinate divides the transfer process into two conse cutive steps: The approach of the two O atoms is followed by the actua l motion of the central ion. The second step accounts for 70% of the e nergy 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.