The Ionic Hydrogen Bond. 2. Multiple NH+...O and CH?+...O Bonds. Complexes of Ammonium Ions with Polyethers and Crown Ethers

Article abstract of DOI:10.1021/ja00353a012

Complexes of ammonium ions RNH3⁺ (R = CH3, c-C6H11), (CH3)3NH⁺, and pyridineH⁺ with polyethers and crown ethers are observed in the gas phase in the abscence of the solvent effects.The dissociation energies, ΔH⁰_D, of the RNH3⁺ polyether complexes range from 29.4 kcal mol^-1 (for RNH3⁺*CH3OCH2CH2OCH3) to 46 kcal mol^-1 (RNH3⁺*18-crown-6).The large ΔH⁰_D values for complexes of polydentate ligands indicate multiple -NH⁺...O-hydrogen bonding.Such mutiple bonding can contribute up to 18 kcal mol^-1 to the bonding in RNH3⁺*CH3(OCH2CH2)3OCH3 and 21 kcal mol^-1 in RNH3⁺*18-crown-6.Multiple interactions are also evident in the (CH3)3NH⁺*polyether complexes where -CH^?+...O-hydrogen bonding seems to occur; and consecutive -CH^?+...O-bonds contribute approximately 6, 4, and 2 kcal/mol^-1 respectively for up to three such bonds.Total ΔH⁰_D values in the (CH3)3NH⁺*polyether complexes thus range from 26.7 kcal mol^-1 in (CH3)3NH⁺*CH3O(CH2)2OCH3 to 41 kcal mol^-1 in (CH3)3NH⁺*18-crown-6.Multiple interaction effects, possibly including van der Waals dispersion forces, are observed also in pyridineH⁺*polyether complexes.Large negative entropies in RNH3⁺*acyclic polyether complexes vs.RNH3⁺*cyclic crown ethers make the acyclic polyethers less efficient ligands.