AB-INITIO STUDY OF CATION-DIPOLE INTERACTIONS - PROTON, LITHIUM, AND SODIUM AFFINITIES OF HYDROGEN AND ALKALI-HALIDES AND NATURAL ORBITAL STUDY OF BONDING

The H+, Li+, and Na+ affinities of hydrogen and alkali halides have been calculated at 6-31+G*//6-31+G*, MP2(FULL)/6-31+G*//MP2(FULL)/6-31+G*, HF/6-311+G*//MP2(FULL)/6-31+G*, MP2(FC)/6-311+G*//MP2(FULL)/6-31+G*, and MP4SDTQ(FC)16-311+G*//MP2(FULL)/6-31+G* levels. In the case of protonated species 6-31+G** and 6-311+G** basis sets have been used. All the complexes except H2F+, H2Cl+, LiHCl+, and NaHCl+ are predicted to have linear structure. The calculated structural parameters and cation affinities are in very good agreement with available experimental data. The nature of bonding has been studied on the basis of NPA (natural population analysis) atomic charges and bond indices calculated at the HF level at the MP2(FULL) geometries using 6-31+G* and 6-311+G* basis sets. In all cases the bond index of the donor-acceptor bond varies linearly as the amount of charge transfer. The bonding in M1XM2(+)(M1, M2 = Li, Na) complexes has been found to be essentially electrostatic in nature. Making use of this observation K+, Rb+, and Cs+ affinities of hydrogen and alkali halides have been predicted. An appreciable degree of valence interaction is found to be operative in the protonated species which accounts for their very high interaction energies and bent structure in most cases. The trends in the electronic structure and stability of the complexes have been rationalized on the basis of charge transfer, bond indices, and other considerations.