Hydrogen bonding Part 48. IR and thermodynamic study of the lower hydrates of N-methylquinuclidinium iodide, bromide, chloride, fluoride, and hydroxide; evidence for a temperature-dependent rearrangement of hydrogen bonds in the chloride monohydrate

Abstract

Below 28°C the IR spectrum of N-methylquinuclidinium chloride shows the characteristic absorptions of a C 2h (H 2O · Cl −) 2 planar cluster; however, above this temperature this spectrum is replaced with an entirely different spectrum tentatively assigned to extended “linear” hydrogen bonding. The bromide and iodide show only the linear type of IR spectra. Dissociation vapor pressure measurements of the low temperature form of the chloride give an unreasonably high value for Δ H dis° (27.56 kcal mol −), while the high temperature form shows a normal value (14.70 kcal mol −). The high value for the low temperature form is ascribed to an unfavorable lattice enthalpy change. The bromide gives a normal Δ H dis° of 11.58 kcal mol −1. The first material to separate on dehydration of an aqueous fluoride solution is a pentahydrate; there is also a fluoride 3.50 H 2O hydrate and a fluoride monohydrate. The IR spectrum of the fluoride monohydrate supports the presence of a C 2h (H 2O · F −) 2 cluster similar to that found in tetramethylammonium fluoride monohydrate, N-Methylquinuclidinium hydroxide forms a dihydrate and a monohydrate; the monohydrate appears to contain planar (H 2O · OH −) 2 clusters.