Cation−Ether Complexes in the Gas Phase:  Bond Dissociation Energies and Equilibrium Structures of Li⁺(1,2-dimethoxyethane)<i>_x</i>, <i>x</i> = 1 and 2, and Li⁺(12-crown-4)

Abstract

Bond dissociation energies, equilibrium structures, and harmonic vibrational frequencies are reported for Li + (DXE), where DXE = CH 3 O(CH 2 ) 2 OCH 3, 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 et al. 1,2