It is demonstrated herein that poly(ethylene glycol) (PEG) oligomers can form stable complexes with the chlorine anion in the gas phase as evidenced by results from electrospray ionization mass spectrometry (ESI-MS) and molecular dynamics simulation. While the formation of crown-ether-like structures by acyclic polyethers in their complexes with alkali metal cations coordinated by the ether oxygen atoms has been extensively studied, the possibility of forming 'inversed' quasi-cyclic structures able to bind a monoatomic anion has not been proved till now. We have observed the formation of stable gas-phase complexes of oligomers of PEG-400 with the Cl(-) anion experimentally by ESI-MS for the first time. It is suggested that a necessary precondition for obtaining the polyether-chlorine anion clusters is the prevention of the formation of neutral ion pairs. Molecular dynamics simulation has demonstrated the wrapping of the Cl(-) anion by the PEG chain, to stabilize the PEG(n)•Cl(-) clusters in the gas phase. The conformation of the polyether chain in such quasi-cyclic or quasi-helical complexes is 'inversed' compared with that in the complexes with cations: that is its hydrogen atoms are turned towards the central anion. Awareness of the possibility of the Cl(-) anion being trapped in quasi-cyclic PEG structures may be of practical importance when considering the intermolecular interactions of PEGs.
Copyright © 2011 John Wiley & Sons, Ltd.