Hydrogen bonding part 65. IR study of hydration of trimethonium and tetramethonium halides and comparison of halide hydration for six methonium cations

Article abstract of DOI:10.1016/0022-2860(96)09246-0

The infrared spectra of bound water in trimethonium bromide monohydrate, chloride dihydrate, and fluoride dihydrate support the presence of planar (H₂O·X^-)₂ clusters. Trimethonium fluoride also forms a hexahydrate which apparently contains several types of bound H₂O, and a trihydrate which contains both F^- ···HOH···F^- and H₂O···HOH···OH₂ bridging H₂O. Tetramethonium bromide dihydrate (Br^- ion monohydrate) forms a known structure with Br^- ···HOH···OH₂ bridging H₂O in a ladder-like arrangement; this is different from those of other quaternary ammonium bromides we have studied previously. The bromide also forms an inexplicably stable monohydrate with an (H₂O·Br^-)₂ cluster. The IR spectrum of tetramethonium chloride dihydrate (Cl^- ion monohydrate) differs from other chloride monohydrates studied previously, and suggests the structure may be similar to that of the bromide dihydrate. Tetramethonium fluoride forms a trihydrate quite similar to trimethonium fluoride trihydrate, and also a dihydrate. This dihydrate (F^- monohydrate) is the first quaternary ammonium fluoride monohydrate among the many we have studied that does not contain a C(2h) (H₂O·F^-)₂ cluster. The IR spectrum supports the presence of a ladder-like fluoride-water structure similar to the arrangement known for tetramethonium bromide dihydrate. FT-NMR spectra and modeling demonstrate that in aqueous solution the trimethonium ion is locked into C(2v) symmetry, while the tetramethonium ion, which crystallizes with C(2h) symmetry, is free to rotate about the C₂-C₃ bond. Structures for the halide hydrates of six methonium ions are compared.