Solvation of the Li+-Br--Li+ triple ion in the gas phase

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 Lif with loss of LiBr. 15-Crown-5-ether yields a 1:1 (15C5) Lif 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 Lif 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 were measured, and the binding energies were determined from the temperature dependence of the equilibrium constants. The mechanisms of stepwise solvations are discussed in light of the structures and energetics of the 1:1 complexes of Li+ and LiBrLi+ with dioxanes and THF calculated at the Hartree-Fock level with a standard 6-311G(d,p) basis set.