HYDROGEN-BONDING .51. THERMODYNAMIC, IR, AND SOLUBILITY STUDIES OF THE IODINE HYDRATES OF N-METHYLHEXAMETHYLENETETRAMINE CATION, N-METHYLQUINUCLIDINIUM ION, N-METHYLTRIETHYLENEDIAMINE CATION, HEXAMETHONIUM ION, AND N,N'-DIMETHYLTRIETHYLENE-DIAMMONIUM ION - EVIDENCE FOR A POSSIBLE (H2O-CENTER-DOT-I-)(2) PLANAR CLUSTER

We have carried out dissociation vapor pressure measurements on N-meth ylhexamethylenetetramine cation iodide monohydrate, N-methylquinuclidi nium iodide monohydrate and hemihydrate, N-methyltriethylenediamine ca tion iodide monohydrate and hemihydrate, N,N'-dimethyltriethylenediamm onium iodide monohydrate and hexamethonium iodide dihydrate and monohy drate. Six of these hydrates have similar values for Delta H(dis)degre es with an average of 10.2 kcal mol(-1) per mol of H2O lost, or a mini mum value of about 5.1 kcal mol(-1) per HOH...I- hydrogen bond broken. N-methylquinuclidinium and N-methyltriethylenediamine cation iodide h emihydrates have essentially identical Delta H(dis)degrees, values of 15.8 kcal mol(-1), or 7.9 kcal mol(-1) per HOH I- hydrogen bond. These two hemihydrates also have similar and much higher values of Delta S( dis)degrees, than the other hydrates, and their IR spectra suggest tha t they may contain Type I planar (H2O.I-)(2) cluster ions. The IR spec tra of all of the other iodide hydrates studied show Type II extended linear HOH...I- hydrogen bonding. The solubilities of quaternary ammon ium iodides in H2O vary over an extremely large range, even for cation s of similar size and charge. Possible reasons for these solubility ef fects are discussed.