MOLECULAR-STRUCTURE AND DYNAMICS OF LICLO4 POLYETHYLENE OXIDE-400 (DIMETHYL ETHER AND DIGLYCOL SYSTEMS) AT 25-DEGREES-C

Infrared spectra of the <(nu)over bar>(4) mode of the ClO4- ion, for the systems LiClO4-polyethylene oxide dimethyl ether of average molar mass 400 (PEG-400) and LiClO4-polyethylene oxide glycol of average molar mass 400 (PEG-400), at 25 degrees C, have been determined and quantitatively described by the sum of two Gaussian-Lorentzian product functions centred at similar to 623 and similar to 635 cm(-1), respectively. Bands are assigned to spectroscopically free ClO4- (at similar to 623 cm(-1)) and to bound ClO4- (at similar to 635 cm(-1)). From the relative amplitudes of the two bands, it is concluded that the majority of the ClO4- exists out of contact with Li+, probably as solvent-separated ion pairs. This conclusion is based on a large ion-pair formation constant K-p = 4.5(2) X 10(3) for LiClO4 in PEO-400, as reported in the literature and reanalyzed in this work. Electrical conductance data for LiClO4 in PEG-400 are also reported and analyzed with current theories furnishing values of K-p for this system. Existence of dimer aggregates (Li+SxClO4-)(2), with S symbolizing an oxygen moiety of a polyether chain, is inferred from ultrasonic relaxation spectra of LiClO4-PEO systems. The presence of some (LiClO4)(2) dimeric contact species is also inferred. The ultrasonic absorption spectra show two relaxation processes followed, at high frequency, by a solvent relaxation process. Matrix analysis has been applied to the two-step dimerization process 2LiClO(4) reversible arrow LiClO4...LiClO4 reversible arrow (LiClO4)(2) (which is used to interpret the electrolyte ultrasonic relaxation processes). The analysis yields all the rate and equilibrium constants for the above steps. In addition, from the amplitude of the ultrasonic spectra, the isoentropic volume changes of the two normal modes, associated with the above equilibria, are calculated. The larger shear viscosity of the PEG solvent precludes the measurement of the solute ultrasonic relaxation in this medium. The molecular dynamics of the above systems have also been explored through microwave and far-infrared dielectric spectra measured in the frequency range from 0.4 GHz to 15 THz (i.e. 500 cm(-1)). The spectra are quantitatively described by the sum of a Cole-Cole distribution, operative mainly in the UHF-microwave region, and of three Powles-Rocard component bands (recently discussed in the literature) that are operative mainly at IR frequencies. In the microwave region, the Cole-Cole distribution describes the ethereal chain segmentational rotation, influenced by Li+ cations that increase the intertangling of the ethereal chains. In the far-IR region the highest frequency Powles-Rocard band at similar to 300 cm(-1) has been assigned, in accordance with the literature, to torsion of ethereal chains. The other two bands are probably due to librations, namely to hindered molecular rotations limited to small angles. Molar refractivities for LiClO4 in PEO-400 and in PEG-400 have been determined in a concentration range comparable to that used in the above work.