Computational studies of ion pairing. 7. Ion-pairing and association effects between tetraalkylammonium ions and nitrobenzene redox species. "Ion pairing" to neutral substances.

The structures of few ion pairs are known with any degree of certainty. In this work, the structures and energies of attraction in acetonitrile of nitrobenzene and its anion radical and dianion with a series of tetraalkylammonium ions of increasing size were computed by density functional theory. The resulting associated molecular pairs all exhibit the same unusual but chemically reasonable structure resulting from attraction between hydrogen atoms of the ammonium cation and the oxygen atoms of the nitro group. The cations form weak complexes with neutral nitrobenzene, stronger ion pairs with the monoanion, and strongly bound ion pairs with the dianion. The results delineate subtle issues governing the formation of such complexes that are directly relevant to issues of molecular recognition, self-assembly, and supramolecular chemistry.