Electron Localization in Liquid Acetonitrile

Abstract

Time-resolved one- and two-pulse laser dc photoconductivity has been used to observe two kinds of reducing species, anion-1 and anion-2, in liquid acetonitrile. At 25 °C, the standard enthalpy of conversion from anion-1 to anion-2 is −44.3 ± 3.6 kJ/mol and the conversion time is ∼3 ns. The high-temperature form, anion-1, absorbs in the IR and migrates >3.3 times faster than any other ion in acetonitrile. This rapid migration has a low activation energy of 3.2 kJ/mol (vs ∼7.6 kJ/mol for other ions). Anion-1 rapidly transfers the electron to acceptors with high electron affinity, with rate constant up to 1011 M-1 s-1. The low-temperature form, anion-2, absorbs in the visible and exhibits normal mobility and electron-transfer rates, ca. 1.5 × 1010 M-1 s-1. It reacts, by proton transfer, with two hydrogen-bonded molecules of water and/or aliphatic alcohols. Laser photoexcitation of these two solvent anions in their respective absorption bands leads to the formation of CH3 and CN-. We present arguments indicating that anion-2 is a dimer radical anion of acetonitrile, whereas anion-1 is a multimer radical anion that may be regarded as a “solvated electron”.

Photoinduced Electron Transfer and Solvation in Iodide-doped Acetonitrile Clusters

“…Two transient features were found centered at around 500 and 1500 nm, and these were attributed to two solvation motifs of the ejected electron; the 500 nm band was assigned to formation of a valence anion, in which the electron is localized on one or two solvent molecules, and the 1500 nm band was ascribed to a solvated electron in a solvent cavity, more akin to the hydrated electron. Shkrob and Sauer 21 independently found similar results by pulse radiolysis-transient absorption and time-resolved photoconductivity.…”

Toward Electron Encapsulation: Polynitrile Approach

“…16,26 These two pathways of electron localization are also found in large acetonitrile clusters. 27 The electron is solvated by methyl groups of acetonitrile, as 10.…”

“…32b Such a dissociation can also be stimulated by absorption of visible light, and it was observed for all acetonitrile anions, both in solid 31,33 and in neat liquid. 16 Crystalline adiponitrile is also known to yield dimer radical anion in low-temperature radiolysis. 32a Apart from these observations, there have been no reports of molecular anions of nitriles and dinitriles, in any media.…”

“…suggested by QM/MD simulations 28a and density functional theory (DFT) 16 and ab initio calculations. 28b In the liquid acetonitrile, the dimer anion is 450 meV more stable than the "solvated electron," but the formation of this anion requires bending of the C-C-N group, which results in a large barrier towards C-C bonding and dimerization.…”

“…-15 J/mol.K. 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.…”