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 be en 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 p rotonated species 6-31+G* and 6-311+G** basis sets have been used. Al l the complexes except H2F+, H2Cl+, LiHCl+, and NaHCl+ are predicted t o have linear structure. The calculated structural parameters and cati on affinities are in very good agreement with available experimental d ata. The nature of bonding has been studied on the basis of NPA (natur al 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* ba sis sets. In all cases the bond index of the donor-acceptor bond varie s linearly as the amount of charge transfer. The bonding in M1XM2(+)(M 1, M2 = Li, Na) complexes has been found to be essentially electrostat ic in nature. Making use of this observation K+, Rb+, and Cs+ affiniti es of hydrogen and alkali halides have been predicted. An appreciable degree of valence interaction is found to be operative in the protonat ed 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.