COMPARISON OF GAS-PHASE PROTON AND AMMONIUM ION AFFINITIES OF CROWN ETHERS AND RELATED ACYCLIC ANALOGS

In order to evaluate the intrinsic binding interactions involved in host-guest complexation of crown ethers with ammonium ions or protons, the relative orders of gas-phase ammonium ion and proton affinities of crown ethers and acyclic analogs have been measured by the kinetic method. Thc polyether/cation complexes were generated by ion-molecule reactions between ammonium ions and neutral ether substrates in the chemical ionization source of a triple quadrupole mass spectrometer. The proton or ammonium ion complexes of interest, (M1 + H + M2)+ or (M1 + NH4 + M2)+, were selectively activated, and the abundances of the resulting fragment ions (i.e. single polyethers attached to a proton or ammonium ion) were measured and used to establish a relative order of affinities. The order of ammonium ion affinities of the polyethers was determined to follow the trend 12-crown-4 < triethylene glycol dimethyl ether < tetraethylene glycol < 15-crown-5 < pentaethylene glycol < tetraethylene glycol dimethyl ether < 18-crown-6 < 21-crown-7. Compared to the affinities of the acyclic ethers, the crown ethers with large cavity sizes demonstrate dramatically higher relative affinities for the ammonium ion than for the proton. This preference is attributed to the favorable ability of the crown ethers to form multiple hydrogen-bonding interactions to the bulky tetrahedral ammonium ion with an overall lower entropy of complexation. The order of relative ammonium ion affinities is similar to the orders of relative affinities for rubidium or potassium ions, both of which are cations of similar size to the ammonium ion. The structures of the crown ether/ammonium ion complexes were characterized by collisionally activated dissociation techniques, and evidence was found to suggest that the crown ethers maintain their cyclic nature upon complexation.