HYDROGEN-BONDING .60. THERMODYNAMICS OF DISSOCIATION OF DECAMETHONIUMHALIDE DIHYDRATES

Infrared studies have demonstrated that decamethonium iodide dihydrate , bromide dihydrate, and chloride dihydrate, all of which contain plan ar C-2h(H2O.X(-))(2) clusters, are apparently isomorphous, as are anhy drous decamethonium iodide and bromide. Contrary to our previous repor t, decamethonium chloride has a structure different from the iodide an d bromide. Since lattice enthalpy changes should be similar in the deh ydration of the iodide and bromide dihydrates, the difference in the d issociation enthalpy should directly reflect differences in HOH...X(-) hydrogen bond strength. The value of 5.15 kcal mol(-1) per HOH...I- h ydrogen bond broken in dehydration of the iodide dihydrate is too high relative to the other dihydrates to be accounted for by electronegati vity, proton affinity, or ionic potential considerations. Vapor pressu re vs. H2O content studies demonstate that the dihydrate is the only h ydrate formed by decamethonium iodide and bromide. The infrared spectr um of decarnethonium chloride containing 1 mole H2O shows a mixture of the dihydrate and anhydrous material; thus the chloride also only for ms a dihydrate.