Electron attachment to representative cations composing ionic liquids

Using ab initio electronic structure methods with flexible atomic orbital basis sets, we investigated the electronic structure and stability of reduction products of selected representative cations (C+) constituting ionic liquids. We found that an electron attachment to such cations leads to the neutral radicals, whereas a subsequent attachment of another (i.e., excess) electron leads to adiabatically stable anions only in two cases {[P(CH3)4]− and [MeMePyr]−}. The possibility of the formation of various dimers (such as CC+, CC, and CC−) was also considered, and the resulting systems were characterized by predicting their lowest energy structures, ionization potentials, electron affinities, and susceptibilities to the fragmentation process. Among the cations studied, only the [MeMePyr]+ was found to form a typical Rydberg radical (MeMePyr) and double-Rydberg anion ([MeMePyr]−), whereas the remaining cations were predicted to form neutral radicals of a primarily valence (MeMeIm and MePy) or mixed Rydberg-valence [P(CH3)4] character. Our calculations confirmed the stability of all CC+ and CC dimers against fragmentation yielding the corresponding monomers (the binding energies of 12.2–20.5 kcal/mol and 11.3–72.3 kcal/mol were estimated for CC+ and CC dimers, respectively). [(MeMePyr)2]− was identified as the only adiabatically stable CC− dimeric anion having its vertical electron detachment energy of 0.417 eV. We also found that in the [(MeMePyr)2]− anionic state, three outermost electrons are described by Rydberg orbitals, which results in the (σ)2(σ*)1 configuration.