SOLVENT EFFECTS ON THE INTERIONIC STRUCTURE OF ION-PAIRS - ELECTRIC-DIPOLE MOMENTS AND INFRARED-SPECTRA OF N-ALLYL-N-ETHYL-N-METHYLANILINIUM PARA-TOLUENESULFONATE ION-PAIRS IN A SERIES OF SOLVENTS

Molar dielectric increments for N-allyl-N-ethyl-N-methylanilinium p-toluenesulfonate ion pairs (AEM+Ts-), measured at concentrations up to 3.5 mM, gave the following apparent dipole moments: 4.9 ± 0.2 D in benzene (ϵ 2.275); 8.1 ± 0.1 D in octanoic acid (HOct; ϵ 2.46); 7.0 ± 0.5 D in anisole (ϵ 4.33); 11.2 ± 0.3 D in chloroform (ϵ 4.72); 7.4 ± 0.5 D in chlorobenzene (ϵ 5.61); and 8.6 ± 0.2 D, both in 0.95 M HOct in benzene and in 1.00 M benzene in HOct. In non-hydrogen-bonding solvents ranging from benzene to dimethyl sulfoxide (ϵ 46.7), the S-O stretching absorption of Ts' consisted of two nearly coalesced bands with maxima at 1220 Ω 5 and 1198 ± 5 cm-1. In formic, acetic, and octanoic acids this band shifted to 1145-1160 cm-1 owing to acceptance of a hydrogen bond by the SO3- group. Analysis of the carboxyl C̳O stretching vibration in benzene indicated the formation of a 1:1 complex, formally AEM+Ts-·HOct, with an association constant of 280 ± 40 M-1. In CHC13 the centroid of the S-O stretching absorption shifted little or not at all and gave no real evidence for hydrogen bonding. The marked variability of the ion-pair dipole moment is attributed at least in part to specific solvent-induced changes in the average interionic geometry, the ion pair being regarded as a solvent-dependent mixture of interionic structural isomers that differ in interionic tightness. Such changes are facilitated by the asymmetric structure of the AEM+ cation, and the variation of tightness calls to mind the “intimate” and “solvent-separated” ion-pair isomers proposed6 as intermediates in solvolysis. © 1990, American Chemical Society. All rights reserved.