HYDROGEN-BONDING .53. CORRELATION OF DIFFERENTIAL SCANNING CALORIMETRIC DATA WITH IR AND DISSOCIATION VAPOR-PRESSURE STUDIES OF TRANSITIONSOF HEXAMETHONIUM CHLORIDE AND BROMIDE DIHYDRATES AND HEXAMETHONIUM BROMIDE MONOHYDRATE

Differential scanning calorimetry of hexamethonium chloride dihydrate shows an endothermic transition of 2.70 kcal mol-1 at 36.81-degrees-C. This correlates well with the temperatures observed by IR spectra (36 -degrees-C) and equilibrium dissociation vapor pressure studies (37-de grees-C) for the transition between Type I planar cluster and Type II extended linear HOH ... Cl- hydrogen bonding, and with the value of 2. 77 kcal mol-1 for this transition derived by Hess' law treatment of di ssociation vapor pressure data. Differential scanning calorimetry of h examethonium bromide shows a rapid endothermic transition of 2.38 kcal mol-1 at 35.15-degrees-C and a very slow endothermic transition of ab out 12-13 kcal mol-1 centered near 50-degrees-C. This latter endotherm corresponds to the transition between Type I and Type II HOH ... Br- hydrogen bonding observed by IR and vapor pressure studies at 49-degre es-C. The nature of the 35.15-degrees-C endotherm is not known. Hexame thonium bromide also shows a third endotherm at 142.91-degrees-C, whic h presumably results from melting of hydrate in the sealed DSC cell. C ombined analysis of differential scanning calorimetry and dissociation vapor pressure data predicts a value of about -13 kcal mol-1 for an e xothermic disproportionation at 52-degrees-C of two hexamethonium brom ide monohydrate to Type II dihydrate and anhydrous bromide.