It has been known for many years that reduction of polycyclic aromatic hydrocarbons in aprotic media takes place in two discrete one-electron steps, first to a radical anion and then to a dianion. Theoretical calculations at all levels of sophistication predict the two waves to be separated by almost 5 V, yet the spacing between the first and second reduction waves is usually found experimentally to be about 0.5-0.7 V. We have carried out density functional ab initio computations on the thermochemistry of the various chemical processes which take place in the electrochemical experiment. We will show that almost all of the enormous discrepancy between the experimental and computed peak spacings is associated with neglect of solvation in the calculations. Yet other factors must be considered; for example, the potential spacing is also found to be dependent upon the nature of the cation of the supporting electrolyte. Recent computations have clarified the origin of this electrolyte effect and demonstrated the unusual structures of the resulting ion pairs between arene dianions and electrolyte cations. If time permits, we will discuss the extension of these results to the electrochemical behavior of such hydrocarbons in room-temperature ionic liquids.
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Back to The 33rd Northeast Regional Meeting (July 14-17, 2005)