Multinuclear nuclear magnetic resonance studies of aqueous solutions of tetrafluoroborate salts

Abstract

New n.m.r. data are reported for the tetrafluoroborate anion present at low concentration (0.03 mol dm^–3) in a range of host salt solutions. Host anions appear to have no direct influence on the BF₄ ^– parameters though if they ion pair with the host cation then the latter's effect is reduced. Cations have a marked and varied influence both on ¹J(B–F) and on the ¹⁹F chemical shift δ_F. If ¹J increases, δ_F may move up or downfield whereas if ¹J decreases, only downfield shifts are observed. The results are compared with the electric fields generated by collision of a cation with BF₄ ^– in its aqueous environment. The changes in ¹J correlate well with the calculated fields. The changes in chemical shift are more difficult to accommodate within an electric field theory though the gross features of the shifts are reproduced. Several types of ion pairing may be distinguished using the BF₄ ^– ion as a probe. Constant anion-cation collisions occur but which cannot apparently be distinguished from collisions among other molecules and ions present. Only for certain pairs of ions, usually involving a small or highly charged anion, can the collisions be said to result in a specific ion pairing interaction.