Electronic spectra of ion pairs of picrate ion with alkali-metal polyether complexes in non-aqueous solvents

Abstract

Absorption spectra of ion pairs of picrate ion with the alkali-metal ion complexes of poly(oxyethylene)monododecyl ethers [C ₁₂ H ₂₅ (OC ₂ H ₄ ) _n OH, DEOn; (n=4, 6 and 8)] and tetraalkylammonium ions, have been measured in water-saturated non-polar solvents, 1,2-dichloroethane (1,2-DCE) and xylene (Xy). The wavenumber of absorption maxima (ν _max ) of the ion pairs in 1,2-DCE was evaluated by taking into consideration the dissociation of the ion pairs. Those values show a good correlation with the reciprocal of the interionic distance of the ion pair except for the hydrated lithium complexes and with the ν _max in Xy. The values of the ν _max in various non-polar solvents were determined for the ion pair of the DEO6–potassium ion complex. These values were inversely correlated with the E _T value, which is the parameter of solvent polarity. The electronic structures and ion pair structures of picrate ion forming the ion pair with alkali-metal ion complexes were scrutinized by using ab initio calculations. The results obtained experimentally and theoretically revealed: (i) that the change in the electronic excitation energy depends on the interionic distance between the picrate ion and the counter cation in the ion pair and (ii) that the interaction between the picrate ion and the counter cation is predominantly electrostatic.