Hydrogen bonding. Part 87: DFT MO study of hydration of tetramethyl- and tetraethylammonium ions – Correlation of hydrate symmetry with 14N to CH NMR coupling

Abstract Tetramethyl- and tetraethylammonium ions (4MA + and 4EA + ) form primary hydration structures with 4 and 6 mol eq H 2 O, respectively; this is in accord with stoichiometry predicted by previous NMR dilution studies of 14 N to CH coupling in aqueous solutions of halide salts. For 4MA + and 4EA + the primary hydrate structures have T d and D 2d symmetries, similar to those of the cations. The result is symmetrical electric field gradients from solvent about symmetrical ions. This may account for the onset of 14 N to CH coupling at primary hydrate stoichiometry, and casts doubt on our previous postulate of ion tumbling in solution as the sole cause for the onset of such coupling on dilution.