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               Our proposal outlined three related areas of research on the interaction between anions and aromatic p-electron density.  Over the past year we have made significant progress on all three fronts, as summarized below. 

               Correlation Between Anion-Arene Binding Energies and the Polarizability of the Aromatics.  We have very significantly expanded on the number of substituted aromatics considered in our anion-substituted benzene binding energy (E_bind) calculations.  Furthermore, we have performed SAPT calculations to partition the overall binding energy into the contributions from electrostatics, induction, dispersion and exchange.  The work is currently being prepared for publication and the major findings are: (i) there is an excellent correlation between E_bind and the absolute value of the sum of the Hammett substituent constants s_m (S|s_m|) of the substituted aromatics; (ii) adding an additional substituent to the benzene in an anion-substituted benzene complex results in a more attractive E_bind value, regardless of the nature of the substituent; (iii) SAPT calculations show the major contributor to the overall binding energy is induction.  Thus, anion-substituted benzene binding energies can be predicted by the S|s_m| value of the substituted benzene.  The fact that adding any substituent increases the anion-arene binding suggests that substituent induction is an important factor in anion-substituted benzene binding, and this is supported by the SAPT calculations.  The combination of these results brings up the exciting, and puzzling, question of why an electronic parameter, s_m, works so well at predicting anion-substituted benzene E_bind values when the overwhelming evidence points to the major contributing factor being induction.  We hypothesize on some answers to this question in the manuscript we are about to submit, and future work will be directed at this issue. 

               Anion Binding to Transition Metal-Complexed Aromatics.  We have calculated the Cl^– and Br^– binding of Ar-Fe^II-Cp complexes (Ar = substituted aromatic; Cp = cyclopentadienyl anion) where the anion binds to the p-face of the substituted aromatic Ar and compared that to the anion binding to the Fe^II center of the complex.  We determined the preference for anion-p binding by subtracting the binding energy when the anion is complexed to the metal from the binding energy when the anion is complexed to the aromatic p-cloud.  This gave the very surprising result that the preference for anion-p binding increased with an increase in electron donating groups on the aromatic.  This work is nearing completion and will be disseminated in the near future.  This research led to us investigating the nature of Cp binding to transition metals and we have recently had a paper accepted in the Journal of Physical Chemistry A on this topic that acknowledges ACS-PRF support.  The paper shows that adding any substituent to a Cp ring results in a Cp quadrupole moment that is more positive than the parent aromatic, and an interpretation and explanation of this curious result is presented. 

               Preparation of Solid-State Host for Anion-p Binding Studies.  We are currently optimizing the steps in our synthesis and we will soon start preparing derivatives of the parent host so we can study anion-p binding in the solid state.