Cation-ether complexes in the gas phase: Bond dissociation energies and equilibrium structures of Li+(1,2-dimethoxyethane)(x), x=1 and 2, and Li+(12-crown-4)

Bond dissociation energies, equilibrium structures, and harmonic vibrational frequencies are reported for Li+(DXE), where DXE = CH3O(CH2)(2)OCH3, Li+(DXE)(2), and Li+(12-crown-4). The bond dissociation energies are determined experimentally by analysis of the thresholds for collision-induced dissociation of the cation-ether complexes by xenon (measured using guided ion beam mass spectrometry) and computationally by ab initio electronic structure calculations. For Li+(DXE)(x); x = 1 and 2, the primary and lowest energy dissociation channel observed experimentally is endothermic loss of one dimethoxyethane molecule. For Li+(12-crown-4), the primary dissociation channel is endothermic loss of the intact crown ether, although ligand fragmentation is also observed. The cross section thresholds are interpreted to yield 0 and 298 K bond energies after accounting for the effects of multiple ion-molecule collisions, internal energy of the complexes, and unimolecular decay rates. The calculated and experimentally-derived bond energies are in good agreement for Li+(DXE), are in reasonable agreement for Li+(12-crown-4), and differ by 32 +/- 12 kJ/mol for Li+(DXE)(2). On average, the experimental bond dissociation energies differ from theory by 9 +/- 6 kJ/mol per metal-oxygen interaction. The equilibrium structures are determined primarily by strong electrostatic and polarization interactions between Li+ and the ligands. Charge transfer interactions are also important, as indicated by a natural energy decomposition analysis. Correlations between the bond dissociation energies and the equilibrium structures demonstrate that the orientation of the C-O-C subunits in the ethers relative to the metal cation is more important than the Li+... O bond length in determining the stability of the complexes as predicted by Hay ct al.(1,2)