Ammoniated Electron as a Solvent Stabilized Multimer Radical Anion

Abstract

The excess electron in liquid ammonia (“ammoniated electron”) is commonly viewed as a cavity electron in which the s-type wave function fills the interstitial void between 6 and 9 ammonia molecules. Here we examine an alternative model in which the ammoniated electron is regarded as a solvent stabilized multimer radical anion in which most of the excess electron density resides in the frontier orbitals of N atoms in the ammonia molecules forming the solvation cavity. The cavity is formed due to the repulsion between negatively charged solvent molecules. Using density functional theory calculations, we demonstrate that such core anions would semiquantitatively account for the observed pattern of Knight shifts for ¹H and ¹⁴N nuclei observed by NMR spectroscopy and the downshifted stretching and bending modes observed by infrared spectroscopy. We speculate that the excess electrons in other aprotic solvents might be, in this respect, analogous to the ammoniated electron, with substantial transfer of the spin density into the frontier N and C orbitals of methyl, amino, and amide groups.