HYDROGEN-BONDING .65. IR STUDY OF HYDRATION OF TRIMETHONIUM AND TETRAMETHONIUM HALIDES AND COMPARISON OF HALIDE HYDRATION FOR 6 METHONIUM CATIONS

The infrared spectra of bound water in trimethonium bromide monohydrat e, chloride dihydrate, and fluoride dihydrate support the presence of planar (H2O . X(-))(2) clusters. Trimethonium fluoride also forms a he xahydrate which apparently contains several types of bound H2O, and a trihydrate which contains both F- ... HOH ... F- and H2O ... HOH ... O H2 bridging H2O. Tetramethonium bromide dihydrate (Br- ion monohydrate ) forms a known structure with Br- ... HOH ... OH2 bridging H2O in a l adder-like arrangement; this is different from those of other quaterna ry ammonium bromides we have studied previously. The bromide also form s an inexplicably stable monohydrate with an (H2O . Br-)(2) cluster. T he IR spectrum of tetramethonium chloride dihydrate (Cl- ion monohydra te) differs from other chloride monohydrates studied previously, and s uggests the structure maybe similar to that of the bromide dihydrate. Tetramethonium fluoride forms a trihydrate quire similar to trimethoni um fluoride trihydrate, and also a dihydrate. This dihydrate (F- monoh ydrate) is the first quaternary ammonium fluoride monohydrate among th e many we have studied that does not contain a C-2h (H2O . F-)(2) clus ter. The IR spectrum supports the presence of a ladder-like fluoride-w ater structure similar to the arrangement known for tetramethonium bro mide dihydrate. FT-NMR spectra and modeling demonstrate that in aqueou s solution the trimethonium ion is locked into C-2v symmetry, while th e tetramethonium ion, which crystallizes with C-2h symmetry, is free t o rotate about the C-2-C-3 bond. Structures for the halide hydrates of six methonium ions are compared.