Electron attachment to representative cations composing ionic liquids

Double Rydberg anions, Rydberg radicals and micro-solvated cations with ammonium-water kernels

Highly accurate ab initio electron-propagator and coupled-cluster methods are employed to predict the vertical electron attachment energies (VEAEs) of NH₄⁺(H₂O)_n (n = 1-4) cationic clusters. The VEAEs decrease with increasing n and the corresponding Dyson orbitals are diffused over peripheral, non-hydrogen bonded protons. Clusters formed from NH₄^- double Rydberg anions (DRAs) and stabilized by hydrogen bonding or electrostatic interactions are studied through calculations on NH₄^-(H₂O)_n complexes and are compared with more stable H^-(NH₃)(H₂O)_n isomers. Structures that have cationic and anionic congeners have notable changes in geometry. For all values of n, the hydride-molecule complex H^-(NH₃)(H₂O)_n is always the most stable, with large vertical electron detachment energies (VEDEs). NH₄^-(H₂O)_n DRA isomers are predicted to have VEDEs that correspond to energetically well-separated peaks in an anion photoelectron spectrum. Less stable DRA isomers display proton donation from the tetrahedral NH₄^- fragment to water molecules and VEDEs close to those of previously discovered DRAs. The most stable DRA isomers feature tetrahedral NH₄^- fragments without H bridges to water molecules and VEDEs that increase with n. Dyson orbitals of NH₄^-(H₂O)_n DRAs occupy regions beyond the exterior non-bridging O-H and N-H bonds. Thus, the Rydberg electrons in the uncharged Rydberg radicals and DRAs are held near the outer protons of the water and ammonia molecules. Several bound low-lying excited states of the doublet Rydberg radicals have single electrons occupying delocalized Dyson orbitals of s-like, p-like, d-like, or f-like nodal patterns with the following Aufbau principle: 1s, 1p, 1d, 2s, 2p, 1f.