Solvation of the Li+−Br-−Li+ Triple Ion in the Gas Phase

Abstract

The solvation of the Li+−Br-−Li+ triple ion with oxygen-donor Lewis bases was studied in the gas phase with use of Fourier-transform ion cyclotron resonance spectrometry. The Li+−Br-−Li+ triple ions were prepared in an ICR cell by matrix-assisted laser desorption ionization of a lithium bromide/dibenzo-18-crown-6-ether matrix pasted on a Teflon substrate. The stepwise solvations of LiBrLi+ were examined with oxygen-containing Lewis bases: 15-crown-5-ether (15C5), 12-crown-4-ether (12C4), 1,4-dioxane, 1,3-dioxane, and tetrahydrofuran (THF). Crown ethers solvate Li+ with loss of LiBr. 15-Crown-5-ether yields a 1:1 (15C5)·Li+ complex as the end product, whereas 12-crown-4-ether forms a 2:1 (12C4)2·Li+ complex. On the other hand, low dielectric constant solvents, such as dioxanes and THF, solvate the Li+−Br-−Li+ triple ion rather than abstracting Li+ with loss of the neutral ion pair. The maximum coordination numbers are 3 for dioxanes and 4 for THF at 293 K. The rate constants for the stepwise solvations we...