Structural and spectral consequences of ion pairing. 5. Theoretical study of Li+BH4 - and Na+BH4 -

Ion pairs of BH₄ ^- anion with the cations Li⁺ and Na⁺ have been studied by ab initio quantum-chemical methods. Optimized geometries for monodentate, bidentate, and tridentate structures have been determined at the HF and MP2 levels of theory using the 6-31G* and 6-311G** bases, and the energetics of these modes of coordination have been determined. Vibrational frequencies and infrared intensities have been calculated at the MP2/6-311G** level for the free anion for the various ion-pair structures. Only the tridentate structures correspond to minima on the potential energy surfaces; scaled, calculated vibrational frequencies for several isotopomers are presented to assist in the future spectroscopic assignment of these ion pairs.