STRUCTURAL AND SPECTRAL CONSEQUENCES OF ION-PAIRING .4. THEORETICAL-STUDY OF BF4-LI+, BF4-NA+, BF4-K+, BF4-RB+

Ion pairs of BF4- anion with the cations Li+, Na+, K+, and Rb+ have been studied by ab initio quantum-chemical methods. Optimized geometries for monodentate, bidentate, and tridentate structures have been determined at the HF/3-21G and HF/6-31+G,3-21G levels of theory, and the energetics of these modes of coordination have been calculated with the inclusion of electron correlation by the MP2 and MP4 methods. The bidentate and tridentate structures for BF4-Li+ have been studied at various levels of calculation up to MP2/6-311+G*//MP2/6-31G*. Vibrational frequencies, 10B isotope frequency shifts, and infrared intensities have been calculated at the HF/3-21G level for each ion pair and at the HF/6-31+G,3-21G level for the tridentate BF4-K+ ion pair. The tridentate structure is preferred for Na+, K+, and Rb+, but the bidentate structure is preferred for Li+. The topological, structural, energetic, and spectral consequences of the changing nature of the cation in the ion pairs are discussed. © 1990 American Chemical Society.