Hydrogen bonding, Part 72.: Infrared and thermodynamic study of the lower hydrates of N,N-dimethyl-1-adamantylammonium chloride and bromide, and tri-n-pentylammonium chloride

The monohydrate of N,N-dimethyl-1-adamantylammonium chloride (DMAACl), like that of tri-i-pentylammonium chloride (TIPACl), is extraordinarily stable and contains a planar (H2O.Cl-)(2) cluster sandwiched between DMAA(+) cations with NH Cl- hydrogen bonds to each Cl- in the cluster. The thermodynamic parameters determined by dissociation vapor pressure measurements for DMAACl.H2O are essentially identical to those previously determined for TIPACl.H2O, and again demonstrate a positive Delta H-lattice change on dehydration. DMAABr.H2O, unlike TIPABr.H2O, does not contain a similar planar cluster; rather the bromide forms a dihydrate with an extended hydrogen bonded water-bromide ion structure. The thermodynamics of dissociation of this bromide dihydrate are remarkable. Although it loses H2O with alacrity under ambient conditions, the Delta H of dissociation per H2O is one of the highest we have ever recorded for a bromide hydrate, which indicates a large positive Delta H-lattice change on dehydration. The instability of DMAABr.H2O results from its very large positive Delta S of dissociation. In a search for other tertiary amine monohydrates that would resemble DMAACl.H2O, TIPACl.H2O and TIPABr.H2O we have investigated tri-n-pentylammonium (TNPA(+)) and tri-n-butylammonium chlorides and bromides, and tri-n-butylammonium and quinuclidinium chlorides. Only TNPACl formed a stable hydrate; however, this monohydrate does not contain a (H2O.Cl-)(2) cluster. (C) 2000 Elsevier Science B.V. All rights reserved.