Hydrogen bonding. Part 63. IR study of hydration of dimethonium and pentamethonium halides and NMR study of conformation of pentamethonium ion in solution

Article abstract of DOI:10.1016/0022-2860(95)08898-6

We have investigated hydrate formation by dimethonium and pentamethonium - (CH3)3N⁺-(CH2)n-⁺N(CH3)3 where n = 2 and 5 - bromide, chloride, and fluoride.Dimethonium bromide forms a dihydrate which contains a C_2h(H2O*Br^-)2 planar cluster.Dimethonium chloride forms a hypobarogenic dihydrate which is crystalline at reduced pressure but disproportionates to anhydrous material and liquid hexahydrate.Dimethonium fluoride forms three hydrates, a hexahydrate, tetrahydrate, and dihydrate.These hydrates, which represent fluoride ion trihydrate, dihydrate, and monohydrate respectively, have water-fluoride structures similar to those observed for a variety of other quaternary ammonium fluoride hydrates of corresponding stoichiometry; the dihydrate contains C_2h(H2O*F^-)2 clusters.Pentamethonium bromide and chloride form crystalline dihydrates (halide ion monohydrates) which do not contain planar (H2O*X^-)2 clusters.Pentamethonium fluoride forms a trihydrate with unknown structure and, unlike other quaternary ammonium fluorides, does not form a fluoride ion monohydrate with (H2O*F^-)2 clusters.The fact that the odd-numbered pentamethonium ion (C_2v) fails to form halide dihydrates with planar (H2O*X^-)2 clusters while the even-numbered dimethonium, hexamethonium, and decamethonium ions (C_2h) all do may be a function of ion symmetry; however, further studies are required to clarify this point.FT-NMR study of the pentamethonium ion in aqueous solution and molecular modeling shows that rotation about the C1-C2 and C4-C5 ?-bonds is not possible, while rotation about the C2-C3 or C3-C4 bonds does take place.