Molecular relaxation of lithium hexafluoroarsenate (LiAsF6) in 1,2-dimethoxyethane

Abstract

Audiofrequency electrical conductivity, Raman spectra, radiofrequency ultrasonic absorption, and microwave dielectric permittivity of LiAsFg in the solvent 1,2-dimethoxyethane (DME) are reported. An analysis of the electrical conductivity data reveals the electrolyte to be associated to ion pairs (KA = 1 X 105 M"1). Raman spectra of the symmetrical stretching mode of AsFg" suggest the ion to be "spectroscopically free", namely, either unpaired or solvent separated from the cation. Ultrasonic relaxation data are interpreted in terms of the equilibrium Li+, S, AsFg" <=* LiAsFg, (S being a solvent molecule). The equilibrium appears to be heavily shifted toward solvent-separated ion pairs in accord with the information obtained from Raman spectra. In other words, the majority of the electrolyte is associated in the form of a solvent-separated ion pair symbolized by Li+, S, AsFg". Microwave dielectric relaxation data can be interpreted as due to the diffusion rotation relaxation of ion pairs. The estimate of the charge-to-charge distance in the ion pairs reinforces the model that the solvent resides between the cation and the anion in the majority of the pairs. The equilibrium is shifted to increase contact pairs, by increasing the electrolyte concentration, in accordance with the law of mass action.

Ionic Association of Some Lithium Salts in 1,2-Dimethoxyethane. A Raman Spectroscopic and Conductivity Study

“…We also tried to identify triple ions and quadrupoles by analyzing the conductivity data on the basis of Newton's method, as done by Hojo et al 23,24 However, our data did not fit well with the equation. This was also true for the conductance data of the salts in 1,2-dimethoxyethane 19 and 2-methyltetrahydrofuran 16 taken from the works of Petrucci et al…”

“…15,17 LiAsF 6 solution exhibits a very strong band centered at ∼681 cm -1 due to the symmetrical stretching mode of AsF 6 -ion. 18,19 Raman spectrum of LiBF 4 solution shows two new bands peaking around 476 and 2335 cm -1 due to an internal vibration of the anion, 13 the latter of which being the overtone of the former. The ap-pearence of these new bands for these salts can be assigned to the "spectroscopically free" anion B -(B ) ClO 4 , AsF 6 , and BF 4 ) in DME, that is, to the solvent-separated ion pair Li + SB -(S is solvent molecule) and/or to the solvent-separated dimer, Li + SB -‚‚‚Li + SB -, spectroscopically indistinguishable from each other.…”

“…Three other weaker bandssat 995 cm -1 for LiClO 4 , at 513 cm -1 for LiBF 4 , and at 705 cm -1 for LIAsF 6 sare presumed to be due to contact ion pairs, the anions having lower symmetries, based on the collateral evidence in 1,2-dimethoxyethane 19 and 2-methyltetrahydrofuran. 17 The relative intensity continues to decrease in the order LiClO 4 > LiBF 4 > LiAsF 6 , suggesting that the proportion of the contact species decrease in the same order.…”

A Raman spectral study of solvation and ion association in the systems LiAsF6/CH3CO2CH3 and LiAsF6/HCO2CH3

“…l(b). Farber et al (17) attributed this weak band to the Raman forbidden v3 mode, made active b y perturbations from the solvent.…”

Changes in the composition of the solvation shell of lithium ions in γ-butyrolactone upon addition of 15-crown-5 as studied by quantum chemistry

“…Studies of LiClO 4 and LiAsF 6 solutions have shown that different conductivities of these systems are due to different degrees of ion association, which is higher for LiClO 4 . 5,6 Association (formation of, e.g., neutral dimers and trimers) at salt concentrations of at least 1 mol L -1 causes an increase in viscosity and a decrease in con ductivity.…”