THE J O U R N A L
O F
PHYSICAL CHEMISTRY Registered in U.S. Patent Office
0 Copyright, 1980, by the American Chemical Society
VOLUME 84, NUMBER 4
FEBRUARY 21,1980
Dielectric and Ultrasonic Relaxation of LiSCN and LiC104 in Dlmethoxymethane David Saar,+ Joseph Brauner, Herman Farber, and Sergio Petrucci’ Departments of Nectrical Engineering and Chemistry, Polytechnic Institute of New York, Brooklyn and Farmingdale Campuses, New York 1120 1 (Received July 16, 1979)
Ultrasonic absorption at radio frequencies, complex dielectric permittivities at microwave frequencies, electrical conductances,and infrared spectra of solutions of LiSCN and LiC104in dimethoxymethaneare reported. The M, indicate that the major species are conductance data, analyzed in the concentration range 7 X 1014neutral pairs of ions although a significant amount of triple ions are present. Band analysis of the IR spectra of solutions with concentrations equal and above 0.05 M reveal the presence of three major species, ion-pairs, quadrupoles and chains, the latter with Lit bound also to the S-end of the SCN ion. The dielectric spectra indicate that there are three dipolar species present, the solvent and two solute species, in accord with information from the IR spectra. Alternate analysis of the dielectric spectra in terms of a Cole-Davidson distribution does not seem to be consistent with the IR findings. Consequently a possible theoretical interpretation of the Cole-Davidson spectrum (based on our earlier proposed mechanism of collisional diffusion and rotation of ion pairs) is considered as unlikely to be relevant. The same applies to recent improvements on our proposal based on a three-dimensional diffusion model. With the aid of the structural information above, the ultrasonic spectra at c 0.1 M for both LiSCN and LiC104are interpreted as due to an ion-pair-quadrupoleequilibrium involving cation desolvation. At higher concentrations a new relaxation process centered at 10 MHz appears.
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Introduction Current energy requirements have caused an acceleration in the design and testing of new batteries as energy sources. Alkali metals are strong candidates for electrode materials. Unfortunately, their reactivity with water and with solvents of high permittivity limits their use to solvents of low permittivity ( 0.5 M. This holds true for both LiSCN and LiS104 in DMM. In the lower concentration range (c 5 0.1 M) one has to devise a mechanism which leads to a first-order or pseudo-first-order equilibrium A B as discussed in the ultrasonic section above. Also any mechanism should contain, as a built-in factor, independence of the nature of the anion as the calculation above suggests. A structural transformation between quadrupoles (apolar symmetrical open chain configurations) would imply breaking of cation-ligand bonds. Therefore a strong specificity on the nature of the anion would be reflected in T-'. The same holds true if one were to postulate an equilibrium such as LiSCN + LiNCS between ion pairs. The same equilibrium could not be applied to the case of the symmetrical C104-; yet comparable relaxation frequencies and activation parameters are calculated from the ultrasonic spectra of both LiSCN and LiC104. Dielectric permittivity data by Chabanel et a1.21 at concentrations below c = 0.1 M can be interpreted quantitatively by assuming the presence of only two species, namely, pairs and quadrupole aggregates. The same salts in THF and other solvents show the 2089-cm-' band only a t concentrations higher than 0.1 M.18 On the basis of the above evidence the logical conclusion would seem that at c 5 0 . 1 M the ultrasonic spectra would be interpreted as an equilibrium between the two major species present: ion pairs and quadrupoles. Further the effect is common to both systems, LiSCN and LiC104 in DMM. One should therefore focus on thb aspects common to both systems, namely, bound Lit ions and solvent. It seems reasonable therefore to picture these partners in the process. We advance the hypothesis that at c 0.1 M the observed ultrasonic process corresponds to an Eigen-type desolvation mechanism2 expressing the equilibrium between ion pairs and quadrupoles as follows: AB
+ AB
kZl
ABe-AB
& A2B2 k32
(16)
where A represents the partially solvated Lit in the contact ion-pair AB, B is the ligand, AB-AB symbolizes a solvent-separated quadrupole, and A2B2 is the contact quadrupole detected by the IR band a t 2035 cm-l. This mechanism is consistent with the IR spectra. IR spectra cannot distinguish between solvent-separated quadrupoles and ion pairs. Both species would appear as contributing to the C=N vibrational band at -2065 cm-'. According to the postulated mechanism, if the first process is diffusion controlled, hence faster than the second step (k12, kzl >> k23, k32) and if the observed relaxation time refers to the slow step, one obtains7
with K12-' = kzl/k12,and 0 = 2c, (cpis the concentration of pairs). In particular if 0 >> KI2-l, then TIC'
=
k23
+ k32
(18)
which leads back to eq 5 and what follows above. To demonstrate that, in the present case 0 >> Kl
