Structural and spectral consequences of ion pairing. 4. Theoretical study of BF4 -M+ (M = Li, Na, K, and 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.