Electronic structure and structure of ion pairs of anionic sigma adducts of polynitroarenes with acetonate ion

The anionic Yanovskii sigma adducts of 1,3,5-trinitro- and 1,3-dinitrobenze nes, 1, 3-dinitronaphthalene, and 3,5-dinitrobenzoic acid with sodium, pota ssium, and tetrabutylammonium acetonates in low-polarity solvents exist mos tly as contact ion pairs, while in polar solvents, as solvent- separated io n pairs and free ions. Lowering the temperature increases the fraction of s olvent-separated ion pairs in low-polarity solvents and of free ions in pol ar solvents, by shifting the equilibria to stronger solvated ionic species. By quantum-chemical calculations of the geometry and electronic structure of the anions and of the electronic absorption spectra of the free ions and ion pairs, as well as by a spectrophotometric study of cy adducts with sol vents of various polarity it was established that the cation on ion-pair fo rmation coordinates with the 4-nitro group with respect to the pyramidal no de.