A simple method to estimate relative stabilities of polyethers cationized by alkali metal ions

Dissociation of doubly cationized polyethers, namely [P + 2X](2+) into [P + X](+) and X(+), where P = polyethylene glycol (PEG), polypropylene glycol (PPG) and polytetrahydrofuran (PTHF) and X = Na, K and Cs, was studied by means of energy-dependent collision-induced dissociation tandem mass spectrometry. It was observed that the collision voltage necessary to obtain 50% fragmentation (CV(50)) determined for the doubly cationized polyethers of higher degree of polymerization varied linearly with the number of degrees of freedom (DOF) values. This observation allowed us to correlate these slopes with the corresponding relative gas-phase dissociation energies for binding of alkali ions to polyethers. The relative dissociation energies determined from the corresponding slopes were found to decrease in the order Na(+) > K(+) > Cs(+) for each polyether studied, and an order PPG ≈ PEG > PTHF can be established for each alkali metal ion.