Solvation Structures of Tetraethylammonium Bromide and Tetrafluoroborate in Aqueous Binary Solvents with Ethanol, Trifluoroethanol, and Acetonitrile.

Abstract

The solvation structures of tetraethylammonium bromide and tetrafluoroborate (TEABr and TEABF4) in aqueous binary solvents with ethanol (EtOH), 2,2,2-trifluoroethanol (TFE), and acetonitrile (AN) have been clarified by molecular dynamics (MD) simulations. In addition, 1H and 13C NMR chemical shifts of the H and C atoms within TEA+ in the binary solvents have been measured as a function of the mole fraction of the organic solvent, xOS. The variations of the chemical shifts with an increase in xOS were interpreted according to the solvation structures of TEA+, Br-, and BF4- obtained from the MD simulations. It has been found that TEABF4 at 130 mmol dm-3 cannot be dissolved into the EtOH and TFE solvents above xOS ≈ 0.7 and 0.6, respectively, while TEABr can be done in both solvents. Interestingly, TEABr and TEABF4 at the concentration can be dissolved in the AN solvents over the entire xOS range. The solvation of TEA+, Br-, and BF4- in each solvent has been discussed in terms of the electrostatic force, the weak hydrogen bond of C-H···F-C, and the dipole-dipole interaction.