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.