Original scientific paper

Speciation of A 1-Alkyl-4-Cyanopyridinium Iodide Using the Charge Transfer Band

David A. Binder ; Chemical Dynamics Program, Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, MN 55455 USA
C. Allen Mead ; Chemical Dynamics Program, Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, MN 55455 USA
Maurice M. Kreevoy ; Chemical Dynamics Program, Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, MN 55455 USA

Abstract

The intensity of the charge transfer band of 4-cyano-l-(3,7-dimethyloc- tyDpyridinium iodide (Py+I~) has been used to estimate the contact ion pair (CIP) concentration in a number of solvents. In several nonhydroxylic solvents with dielectric constant between 4 and 8 the molar integrated intensity of the low frequency charge transfer band is very similar, and these values have been averaged to estimate the intrinsic molar intensity of the CIP.
A nonlinear concentration dependence of this intensity in 2-propanol shows the presence of a substantial concentration of free ions in that solvent, and an ion association constant, Kp = 3.9 x 103, was found. Tributylphos- phate gives similar evidence of dissociation, but only at lower concentrations. A value of 3.7 x 106 was estimated for Kp. In the other solvents studied, dissociation is hard to distinguish from other effects which reduce the molar intensity at low concentrations. Both tributyl phosphate and 2-propanol also give molar intensities below the average of less polar solvents at high Py+I_ concentration. This is regarded as evidence for the presence of solvent separated ion-pairs (SSIP). Solvent separated ion-pairs are not abundant, however, in the solvents studied. The CIP:SSIP concentration ratio seems to be 4-6 in tributylphosphate and 2-propanol, and is still higher in the other solvents.
With increasing concentration the charge transfer band shifts to higher frequency. The magnitude of the shift depends on the solvent, and is most notable in chlorobenzene. A theory is developed which attributes this shift to the increase in the dielectric constant of the medium brought about by the ion-pairs. The effect could be attributed to higher aggregation, but the degree of aggregation would have to increase continuously with concentration, even at concentrations —10"4 M. The idea of dielectric modification can also be folded together with aggregation, with the former predominant at low concentration and the latter at higher concentration (~10-3 M).

Keywords

Hrčak ID:

137134

URI

https://hrcak.srce.hr/137134

Publication date:

15.10.1992.

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