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

Abstract

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.