HYDROGEN-BONDING .63. IR STUDY OF HYDRATION OF DIMETHONIUM AND PENTAMETHONIUM HALIDES AND NMR-STUDY OF CONFORMATION OF PENTAMETHONIUM ION IN SOLUTION

We have investigated hydrate formation by dimethonium and pentamethoni um - (CH3N+-(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 hypob arogenic dihydrate which is crystalline at reduced pressure but dispro portionates to anhydrous material and liquid hexahydrate. Dimethonium fluoride forms three hydrates, a hexahydrate, tetrahydrate, and dihydr ate. These hydrates, which represent fluoride ion trihydrate, dihydrat e, and monohydrate respectively, have water-fluoride structures simila r to those observed for a variety of other quaternary ammonium fluorid e hydrates of corresponding stoichiometry; the dihydrate contains C-2h (H2O . F-)(2) clusters. Pentamethonium bromide and chloride form cryst alline dihydrates (halide ion monohydrates) which do not contain plana r (H2O . X(-))(2) clusters. Pentamethonium fluoride forms a trihydrate with unknown structure and, unlike other quaternary ammonium fluoride s, does not form a fluoride ion monohydrate with (H2O . F-)(2) cluster s. The fact that the odd-numbered pentamethonium ion (C-2v) fails to f orm halide dihydrates with planar (H2O . X(-))(2) clusters while the e ven-numbered dimethonium, hexamethonium, and decamethonium ions (C-2h) all do may be a function of ion symmetry; however, further studies ar e required to clarify this point. FT-NMR study of the pentamethonium i on in aqueous solution and molecular modeling shows that rotation abou t the C1-C2 and C4-C5 sigma-bonds is not possible, while rotation abou t the C2-C3 or C3-C4 bonds does take place.