Electron Trapping by Polar Molecules in Alkane Liquids:  Cluster Chemistry in Dilute Solution

Abstract

Experimental observations are presented on condensed-phase analogues of gas-phase dipole-bound anions and negatively charged clusters of polar molecules. Both monomers and small clusters of such molecules can reversibly trap conduction band electrons in dilute alkane solutions. The dynamics and energetics of this trapping have been studied using pulse radiolysis-transient absorption spectroscopy and time-resolved photoconductivity. Binding energies, thermal detrapping rates, and absorption spectra of excess electrons attached to monomer and multimer solute traps are obtained, and possible structures for these species are discussed. "Dipole coagulation" (stepwise growth of the solute cluster around the cavity electron) predicted by Mozumder in 1972 is observed. The acetonitrile monomer is shown to solvate the electron by its methyl group, just as the alkane solvent does. The electron is dipole-bound to the CN group; the latter points away from the cavity. The resulting negatively charged species has a binding energy of 0.4 eV and absorbs in the infrared. Molecules of straight-chain aliphatic alcohols solvate the excess electron by their OH groups; at equilibrium, the predominant electron trap is a trimer or a tetramer, and the binding energy of this solute trap is ca. 0.8 eV. Trapping by smaller clusters is opposed by the entropy that drives the equilibrium toward the electron in a solvent trap. For alcohol monomers, the trapping does not occur; a slow proton-transfer reaction occurs instead. For the acetonitrile monomer, the trapping is favored energetically, but the thermal detachment is rapid (ca. 1 ns). Our study suggests that a composite cluster anion consisting of a few polar molecules imbedded in an alkane "matrix" might be the closest gas-phase analogue to the core of solvated electron in a neat polar liquid.

Toward Electron Encapsulation: Polynitrile Approach

“…15 and the Appendix, because the 1064 nm pulse is much shorter than the 248 nm pulse (an example of such analysis for glutarontrile solution is shown in Figures 2b and 2c). …”

“…15,16 These observations led us to suggest that acetonitrile monomer does not form a molecular anion in n-hexane. 15 Rather, the methyl group of the acetonitrile molecule is at the wall of the electron cavity together with the methyl/methylene groups of the solvent molecules. The cavity electron is dipole-bound to the CN dipole (with the C-C-N angle of 180 o ), which results in a more stable trap.…”

Interface tailoring for charge injection control in polyethylene

“…But this is in a context of nonpolar polymers whereas chemical changes achieved here consist in introducing polar groups. It was shown for example that polar groups into liquid alkanes may constitute clusters capable of stabilizing electrons [76]. The fate of holes in such situation is not addressed.…”

Ammoniated Electron as a Solvent Stabilized Multimer Radical Anion

“…Large Knight shifts for 14 N and 13 C nuclei for excess electron in dilute Na/methylamine solutions 31 suggest that electron solvation by the amino-and methyl-groups is qualitatively similar to that for the ammoniated electron. We have already suggested that electron solvation in alkanes 65 and acetonitrile 66 involves a solvent stabilized multimer anion with a fraction of the spin density transferred onto the frontier orbitals of C atoms in the methyl groups. It is very likely that a similar situation occurs in ethers, as such liquids also solvate the electron by their methyl and methylene groups.…”

“…66,67 Observation of the predicted large Knight shift on 13 C nuclei in this liquid would provide direct evidence as to the occurrence of spin sharing by methyl groups. Alternatively, it might be possible to determine spin densities on 13 C nuclei in 13 CH 3 labeled glass-forming branched alkanes that are known to trap electrons below 77 K. 65,69 So far, the emphasis of the EPR and electron spin echo studies has been to determine anisotropic hfcc on the cavity protons. 69 Our models suggest that a measurement of isotropic hfcc on 13 C nuclei would be a more direct probe of the mode of electron trapping.…”