Albert J. Fry

Very little data is available on the detailed structures of ion pairs in solution, since few general experimental methods are available for obtaining such information. For this reason, computational methods have emerged as the method of choice for determining the structures of organic ion pairs in solution. The present study examines the ion pairs between a series of tetraalkylammonium ions and several redox forms of nitrosobenzene and a series of substituted benzaldehydes. The structures, though previously unexpected, are chemically reasonable and fit into a previous pattern of ion pairing described in previous publications in this series. To date in these studies, a total of 73 ion pairs and related species have in fact been identified having exactly the same unusual orientation of the tetraalkylammonium component with respect to the donor species. The results are pertinent to topics of general current interest, including self-assembly, molecular recognition, and supramolecular assembly.

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.

Cyclic voltammograms of several polycyclic aromatic hydrocarbons (PAH's) in highly purified N,N-dimethylformamide are known to exhibit two reversible reduction waves. To a good approximation, the potential of the first wave is independent of the nature of the supporting electrolyte, but the potential of the second wave is highly dependent upon the nature of the electrolyte. The spacing ΔE° between the first and second waves increases as the size of the cation of the electrolyte is increased from Et₄N⁺ through Pr₄N⁺ to Bu₄N⁺ . This is typically interpreted as due to decreasing strength of ion-pairing between the cation and the dianion of the PAH with increasing size of the electrolyte cation. However, it has been known for many years that Me₄N⁺ exhibits anomalous behavior: even though Me₄N⁺ is much smaller than Et₄N⁺ , ΔE° is greater with Me₄N⁺ than with Et₄N⁺ for anthracene and in fact greater than any of the larger electrolytes with perylene. It is now shown that this behavior arises out of the fact that Me₄N⁺ ion is present in solution as a tetrasolvate [Me₄N⁺ /(DMF)₄ ]. The PAH dianion (Ar⁻² ) reacts with Me₄N⁺ /(DMF)₄ to displace a molecule of DMF and produce the species Me₄N⁺ /(DMF)₃ /Ar⁻² . The computed pairing association constant K_ion-pairing for the anthracene species is 35 M⁻¹, compared with a value of 50 000 M⁻¹ for association of the bare Me₄N⁺ ion with the dianion; the corresponding values for perylene are computed to be 4400 and 3.5 M⁻¹, respectively.

Introduction: General Considerations: Alkenes ( C  C Compounds): C  N Compounds: Carbonyl ( C  O ) Compounds: Acknowledgments:

Introduction: General Considerations: Substituted Nitroaromatics: Aromatic Dinitro Compounds: Aliphatic Nitro Compounds: Related Processes: Nitroso Compounds: Acknowledgments:

I am deeply appreciative to ECS for the Manuel Baizer Award in Organic Electrochemistry and also for the opportunity and incentive this award address has offered me to look into and ponder some questions I have had for some time.