Atmospheric pressure gas-phase ammonium/alkyl ammonium exchange studies of some crown ethers complexes using ion mobility spectrometry: A thermodynamic investigation and collision cross section measurements

A systematic evaluation of the intermolecular binding forces responsible for the association of crown ethers (hosts) with alkyl amines (guests) is presented by exchanging the ammonium and alkyl ammonium in the ion source of atmospheric pressure ion mobility spectrometry. The hydrogen bonding interactions of crown ether/alkyl ammonium ion complexes are examined with respect to the macrocyclic size effects of five crown ethers (12-crown-4, 15-crown-5, benzo-15-crown-5, 18-crown-6 and aza-18-crown-6) and the gas-phase basicities of five homologous series of alkyl amines. Our approach is based on a formalism assuming an equilibrium: [host + NH4](+) + [guest + H](+) <-> [host + guestH](+) + [NH4](+). The exchange equilibrium constant for this process was calculated using the relative peak intensities of the corresponding species in the 1MS spectra. Determination of the equilibrium constants at different temperature via Van't Hoff plots led to the standard molar enthalpy change Delta H degrees that is equivalent to the guest-exchange affinity of different crown ethers. The collision cross-sections are measured for the formation of complexes ions and different alkyl ammonium ions, which are produced. Complexes of 15C5 with two different normal and iso-alkyl amine showed a gas phase selectivity binding to normal alkyl amines relative to iso-alkyl amines. (C) 2011 Elsevier B.V. All rights reserved.