Fundamental Aspects of Lithium Ion Transfer

The transfer of the Li+ ion from one water molecule to another is studied in (H20-Li-OH2)+ by ab initio calculations, and the results are compared with analogous proton transfer in (H20-H-OH2)+. In both cases, the equilibrium geometry contains a centrally located Li or H ion, although R(0-0) is considerably longer in (H20-Li-OH2)+. Only a small stretch of the H bond is needed to yield a double-well potential, whereas the barrier does not appear in the Li potential until R(O-0) has been stretched by 1 A. The Li-transfer barrier rises much more gradually with further intermolecular stretch than in the case of the proton. Similarly, the Li barrier is less sensitive to angular deformations. Whereas the proton-transfer barrier is enlarged monotonically with each addition to the basis set, the behavior of the Li analogue relates instead to the dipole moment calculated for the water monomer. Many of these discrepancies can be understood on the basis of the highly ionic nature of the Li bond