Noncovalent inclusion complexes of protonated amines with crown ethers

Abstract

The formation of stable noncovalent inclusion complexes in the gas phase between crown ethers and protonated amines has been investigated by the use of quadrupole ion trap mass spectrometry. The effect of varying the substituents on protonated amines of the type [RNH 3] + (R = CH 3(CH 2) 2, C 6H 5(CH 2) 2, O 2NC 6H 4(CH 2) 2 or C 6H 5CH 2C(H)COOH) was found to have a strong influence on the relative intensities of the inclusion complexes formed with crown ethers in competitive gas-phase reactions. Relative intensity measurements indicate that the introduction of an aromatic ring on the protonated amine or crown ether results in the formation of a less stable complex than the nonaromatic protonated propylamine/18-crown-6 adduct. The relative affinities of protonated propylamine and phenethylamine for 18-crown-6 and of phenylalanine for phenyl-18-crown-6 and 18-crown-6 are supported by tandem mass spectrometric and ligand exchange data. The overall order of stabilities for the noncovalent complexes investigated by ion trap mass spectrometry can be assigned tentatively as: [p + H + 18-crown-6] + > [p + H + benzo-18-crown-6] + ∼ [(r)-phenyl + H + 18-crown-6] + = [(s)-phenyl + H + 18-crown-6] + > [(s)-phenyl + H + (s)-phenyl-18-crown-6] + > [nitro + H + 18-crown-6] + > [phen + H + 18-crown-6] + ∼ [phen + H + benzo-18-crown-6] +, where p = propylamine, phen = phenethylamine, phenyl = phenylalanine, and nitro = 4-nitrophenylalanine.