Hydrogen bonding in the gas phase. Part 5.—Infrared spectroscopic investigation of O—H ⋯ N complexes formed by water: ammonia monohydrate and amine and pyridine monohydrates

Abstract

An infrared investigation of hydrogen bonding by water in gaseous mixtures establishes the formation of gaseous monohydrates of ammonia, amines and pyridines. A model has been developed which relates the infrared shifts Δν₁ and Δν₃ for the H₂O molecule in such complexes to the change in stretching force constant of the O—H bond involved in complex formation. The model has been tested against existing experimental results and used to relate the shifts observed for the gas phase complexes of water to those for N ⋯ H—O complexes formed by methanol. From the empirical relationship between Δν and the enthalpy of complex formation for these complexes, it has been possible to obtain enthalpies of complex formation (with an estimated uncertainty of ± 2 kJ mol^–1) of water with ammonia and with the amines and pyridines investigated. The main results for these gas phase N ⋯ H—O complexes formed by water may be summarised as follows: [graphic omitted] Comparison of the dissociation energy obtained for H₃N ⋯ HOH with values obtained from ab initio calculations shows satisfactory agreement with calculations employing the larger basis sets.