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

Abstract

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 633cm−1 weakened and that at 639cm−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 χ3MP=0.50, where χ3MP denotes the mole fraction of 3MP in the D2O/3MP mixtures. Furthermore, both integrated intensities at 633cm−1 (free 3MP) and 640cm−1 (hydrated 3MP by D2O) were almost independent of the salt concentration in the case of D2O/3MP mixtures with χ3MP=0.09. Additionally, the position of the other Raman bands at 609, 616, 626, and 672cm−1, arising from BPh4−, were almost independent of χ3MP 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.