The structure of lithium and potassium cations coordinated by ammonia molecules in the gas phase as revealed by ab initio SCF-MI calculations

The structure and the energetics of lithium and potassium cations coordinated by ammonia molecules were calculated by an ab initio Hartree Fock study in the absence of basis set superposition error (BSSE). Complexes formed by clusters consisting of up to six ammonia molecules and Li+ or K+ ions were investigated. Gradient geometry optimizations were performed in the framework of the self consistent field for molecular interactions (SCF-MI) theory adopting the recently implemented generalization of the method. The intermolecular interaction energies and PES are a priori corrected for the BSSE. The 6-31+G∗ basis set was used. Ion clusters are stabilized by the metal-N bond and the interaction structure allows the formation of the maximum number of such bonds in the first solvation shell. All the properties studied change in a systematic manner upon addition of a further ammonia molecule. Li+ and K+ complexes exhibit no sign of hydrogen bonding between the molecules of the first coordination shell. However, the Li+ complexes with five and six ammonia molecules show the formation of hydrogen bonds between the coordinated molecules of first and second shells. From the results, it follows that both Li+ and K+ ions form similar complexes with up to four ammonia molecules. However, in the Li+ complexes, the first coordination shell saturates with four molecules and leads to conformations differing from those of the K+ complexes. In accordance with recent neutron diffraction experimental analysis of the structure of Li+ and K+ in NH3 solutions [J. Chem. Phys. 112 (2000) 7147], the lithium is tetrahedrally coordinated while potassium results octahedrally coordinated by solvent molecules. The theoretical results are acceptably consistent with the experiments and can furnish insight into the identification of some features of the clustering process.