Solvation state of sodium tetraphenylborate in 3-methylpyridine and its aqueous solutions

The solvation states of the sodium ion (Na⁺) and the tetraphenylborate ion (BPh₄⁻) in 3-methylpyridine (3MP) and its mixture with deuterium oxide (D₂O) were investigated by Raman spectroscopy and DFT calculations. Raman spectra observed for the 3MP solutions systematically varied with increase in the amount of NaBPh₄; the Raman band at 633 cm⁻¹ weakened and that at 639 cm⁻¹ strengthened with increase in the salt concentration. According to DFT calculations, the former was assigned to the 3MP molecule in the bulk and the latter to that bound to the Na⁺ ion. The number of 3MP molecules bound to Na⁺, n, was estimated to be 6.1, that is, a [Na(3MP)₆]⁺ complex was formed in the 3MP/NaBPh₄ solution. On the other hand, n was estimated to be 1.6 in the D₂O/3MP/NaBPh₄ mixtures with χ_3MP = 0.50, where χ_3MP denotes the mole fraction of 3MP in the D₂O/3MP mixtures. Furthermore, both integrated intensities at 633 cm⁻¹ (free 3MP) and 640 cm⁻¹ (hydrated 3MP by D₂O) were almost independent of the salt concentration in the case of D₂O/3MP mixtures with χ_3MP = 0.09. Additionally, the position of the other Raman bands at 609, 616, 626, and 672 cm⁻¹, arising from BPh₄⁻, were almost independent of χ_3MP between 1 and 0.09. These results suggest that Na⁺ is preferentially solvated with D₂O, whereas BPh₄⁻ is preferentially solvated with 3MP in such mixtures.