HYDROGEN-BONDING .50. IR AND THERMODYNAMIC STUDY OF THE HYDRATES OF HEXAMETHONIUM IODIDE, BROMIDE, CHLORIDE AND FLUORIDE

We have examined the solid hydrates of the dihalides of the hexamethon ium dictation, (CH3)2N+-(CH2)6-+N(CH3)3, a dipositive analog of simple monopositive quaternary ammonium ions. The iodide forms a dihydrate w hich shows the IR spectrum associated with linear Type II halide monoh ydrate hydrogen bonding. At 25-degrees-C the bromide forms a dihydrate and a monohydrate; both show IR spectra of Type I planar (H2O.Br-)2 c lusters. Below 49-degree-C equilibrium vapor pressure measurements on the bromide dihydrate give an abnormally high DELTAH(diss)0 34.37 kcal mol-1. At 49-degrees-C a transition occurs to yield a species with an IR spectrum for linear Type II hydrogen bonding and a normal DELTAH(d iss)0 of 15.41 kcal mol 1; this transition reverses very slowly on coo ling. The chloride forms a dihydrate and monohydrate, and possibly a s esquihydrate. Below 36-degrees-C the dihydrate shows the IR spectrum o f Type I C2h (H2O . Cl-)2 clusters; above 40-degrees-C the spectrum ch anges to that of Type II linear hydrogen bonding. The low and high tem perature forms of the chloride dihydrate show relatively normal DELTAH (diss)0 values of 20.95 and 18.18 kcal mol 1 respectively. Both the ch loride sesquihydrate and monohydrate give IR spectra of Type I C2h (H2 O . Cl-)2 clusters; these spectra do not show a transition to Type II on heating. The fluoride forms three hydrates; a hexahydrate, a tetrah ydrate and a dihydrate. These hydrates, which are formally a fluoride ion trihydrate, dihydrate and monohydrate, show IR spectra for their w ater fluoride ion structures which are identical to those of the corre sponding monopositive quaternary ammonium fluoride hydrates, and are a ssumed to have hydrogen bonding arrangements which are very similar to those found in tetraethylammonium and tetramethylammonium fluoride hy drates. IR observations on the chloride and bromide dihydrates are cor related with known crystal structures.