MOLECULAR RELAXATION DYNAMICS OF LICLO4 IN ACYCLIC POLYETHER SOLVENTS

Ultrasonic relaxation spectra are reported for LiClO4 dissolved in acyclic polyethers of increasing chain length including 1,2-dimethoxyethane (1,2-DME), CH3(OCH2CH2)nOCH3 With n = 1; diglyme with n = 2; and tetraglyme with n = 4. Spectra were measured at 25-degrees-C at various concentrations in the frequency range approximately 1 to approximately 500 MHz. The data are analyzed in terms of the dimerization equilibrium 2LiClO4 reversible (LiClO4)2, determining rate constants, equilibrium constants of dimerization, K(d), and isoentropic volume changes for the process, DELTAV(s). Infrared spectra for the nuBAR4 mode Of ClO4- at approximately 623 cm-1 reveal a satellite band at wavenumbers ranging from 633 to 638 cm-1 that is attributed to contact ion pairs. Microwave dielectric relaxation spectra of LiClO4 in the above polyethers reveal a relaxation process involving the solute (that is absent in the pure solvent) which is attributed to the rotational relaxation of LiClO4 ion pairs. The relaxation process shifts in frequency from around 2 GHz for LiClO4 in 1,2-DME to 0.21 GHz for LiClO4 in tetraglyme at 25-degrees-C. This behavior correlates well with the change in the viscosity of the solvents which has been determined separately. The apparent dipole diameter is about a0 = 4 X 10(-8) cm. Comparable values have been calculated from the Bottcher function determining the apparent dipole moment of the ion pairs in solution. The molar refraction of lithium perchlorate, R(LiClO4), has been calculated from measured refractive indices and densities of solutions of LiClO4 in polyethers at various electrolyte concentrations. The values of R(LiClO4) vary with the solvent and can be correlated with the gas-phase value of the dipole moment of the solvent. This is interpreted as reflecting electrostatic interactions of the solvent dipoles with ClO4-, the more polarizable ion (compared to Li+).