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

Tetramethyl- and tetraethylammonium ions (4MA+ and 4EA+) form primary hydration structures with 4 and 6 mol eq H2O, respectively; this is in accord with stoichiometry predicted by previous NMR dilution studies of 14N to CH coupling in aqueous solutions of halide salts. For 4MA+ and 4EA+ the primary hydrate structures have Td and D2d 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 14N 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.