Toward electron encapsulation: polynitrile approach.
This study seeks an answer to the following question: Is it possible to design a supramolecular cage that would "solvate" the excess electron in the same fashion in which several solvent molecules do that cooperatively in polar liquids? Two general strategies are outlined for this "electron encapsulation", viz. electron localization using polar groups arranged on the (i) inside of the cage or (ii) outside of the cage. The second approach is more convenient from the synthetic standpoint, but it is limited to polynitriles. We demonstrate, experimentally and theoretically, that this second approach faces a problem: the electron attaches to the nitrile groups, forming molecular anions with bent C-C-N fragments. Because the energy cost of this bending is high, for dinitrile anions in n-hexane, the binding energies for the electron are low and, for mononitriles, these binding energies are lower still, and the entropy of electron attachment is anomalously small. Density functional theory modeling of electron trapping by mononitriles in n-hexane suggests that the solute molecules substitute for the solvent molecules at the electron cavity, "solvating" the electron by their methyl groups. We argue that such species would be more correctly viewed as multimer radical anions in which the electron density is shared (mainly) between C 2p orbitals in the solute/solvent molecules, rather than cavity electrons. The way in which the excess electron density is shared by such molecules is similar to the way in which this sharing occurs in large di- and polynitrile anions, such as 1,2,4,5,7,8,10,11-octacyanocyclododecane(-). Only in this sense is the electron encapsulation possible. The work thus reveals limitations of the concept of "solvated electron" for organic liquids: it is impossible to draw a clear line between such species and a certain class of radical anions.
Shkrob
Ilya A
IA
Chemistry Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA. shkrob@anl.gov
Sauer
Myran C
MC