Toward Electron Encapsulation:  Polynitrile Approach

Abstract

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 co-operatively in polar liquids? Such an "encapsulated electron" would be instrumental for structural and dynamics studies of electron solvation and might be useful for molecular electronics. Two general strategies are outlined for "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 formidable problem: the electron attaches to the nitrile groups forming molecular anions with bent C-C-N fragments. Since the energy cost of this bending is very high, for dinitrile anions in n-hexane, thebinding energies for the electron are very low and for mononitriles, these binding energies are lower still, and the entropy of electron 2. attachment is anomalously small. Density functional theory modeling of electron trapping by mononitriles in n-hexane suggests that the mononitrile molecules substitute for the solvent molecules at the electron cavity, "solvating" the electron by their methyl groups.We argue that such "solvated electrons" resemble multimer radical anions in which the electron density is shared (mainly) between C 2p orbitals in the solute/solvent molecules, instead of existing as 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 diand poly-nitrile anions, such as 1,2,4,5,7,8,10,11-octacyano-cyclododecane -. The work thus reveals limitations of the concept of "solvated electron" for organic liquids: it might be impossible to draw a clear line between such species and a certain class of radical anions. It also demostrates the feasibility of "electron encapsulation."

The Structure of the Hydrated Electron. Part 2. A Mixed Quantum/Classical Molecular Dynamics Embedded Cluster Density Functional Theory: Single−Excitation Configuration Interaction Study

“…In Part 1 of this study, 1 we found that in water cluster anions that trap the electron internally, the excess spin and charge density are localized mainly on the OH groups of the first solvation shell. For the embedded cluster anions examined in the present study, this same type of distribution is also seen, as documented in Figure 9S, which exhibits histograms of the atomic spin ( likely too large is also evident when comparing our calculated radius of gyration g r for the electron to experimental estimates, as noted in section 3.1).…”

“…29 The justification for using this implementation of DFT as opposed to ab initio methods for calculating magnetic resonance information was provided in Part 1 of this study. 1 Unless otherwise specified, for all of our DFT calculations, a 6-31G split-valence double-ζ…”

“…Thus, our results suggest that the optical spectrum alone cannot be used to validate or invalidate models for the − hyd e . Instead, other experimental features, such as hfcc parameters determined using magnetic resonance methods, [1][2][3][4][5][6] or vibrational parameters obtained from resonance Raman 19,20 are needed to refine our theoretical understanding of the hydrated electron.…”

Can a single molecule trap the electron?

“…The electron is expected to be attached to 2 in the same fashion it can be attached to a cluster of alcohol molecules, [6,7] i.e., by dispersal of the electron trap in a polar (e.g., hexamethylphosphoramid) [13] or nonpolar (alkane) [6] liquid/solid with very low binding energy for the excess electron.…”

“…[1][2][3][4] This state is stabilized by attraction to several OH groups pointing towards the center of the cavity and repulsion by hydroxyl protons via the Pauli exclusion. Our recent results [5,6] suggest that this familiar picture of the solvated/trapped electron might be incomplete: in addition to the electron density inside the cavity, there is a substantial (ca. 18% for hydrated electron, − hyd e [5]) density in the O 2p orbitals of the oxygen atoms in the solvating OH groups.…”