Abstract
We study both the rare gas hydride anions, RG-H(-) (RG = He-Rn) and Group 2 (Group IIa) metal hydride anions, MIIaH(-) (MIIa = Be-Ra), calculating potential energy curves at the CCSD(T) level with augmented quadruple and quintuple basis sets, and extrapolating the results to the basis set limit. We report spectroscopic parameters obtained from these curves; additionally, we study the Be-He complex. While the RG-H(-) and Be-He species are weakly bound, we show that, as with the previously studied BeH(-) and MgH(-) species, the other MIIaH(-) species are strongly bound, despite the interactions nominally also being between two closed shell species: M(ns(2)) and H(-)(1s(2)). We gain insight into the interactions using contour plots of the electron density changes and population analyses. For both series, the calculated dissociation energy is significantly less than the ion/induced-dipole attraction term, confirming that electron repulsion is important in these species; this effect is more dramatic for the MIIaH(-) species than for RG-H(-). Our analyses lead us to conclude that the stronger interaction in the case of the MIIaH(-) species arises from sp and spd hybridization, which allows electron density on the MIIa atom to move away from the incoming H(-).
Highlights
Since hydrogen has an electron affinity of 0.75 eV2 and the beryllium anion has a negative electron affinity, and so is unstable,2 the negative charge on BeH− would be expected to reside on the hydrogen atom
A valence full-configuration-interaction (FCI) study was conducted, with the surprising conclusion that this molecular ion is strongly bound by 2.1 eV. This was interpreted as being due to the beryllium atom undergoing sp hybridization, and the subsequent formation of a σ bonding orbital between an empty Be sp hybrid orbital and the H 1s orbital
The aim of the present study is to expand the above investigations to the whole family of Group 2 (Group IIa) hydride anions (MIIaH−), to establish whether all are strongly bound and to allow trends to be deduced
Summary
The BeH− system, on first inspection, might be expected to be unremarkable, but it has recently been highlighted. Beryllium, being a closed-shell system in its ground electronic state, has a lower electronegativity on the Pauling scale than that of hydrogen (1.57 vs. 2.20, respectively). Since hydrogen has an electron affinity of 0.75 eV2 and the beryllium anion has a negative electron affinity, and so is unstable, the negative charge on BeH− would be expected to reside on the hydrogen atom. Since hydrogen has an electron affinity of 0.75 eV2 and the beryllium anion has a negative electron affinity, and so is unstable, the negative charge on BeH− would be expected to reside on the hydrogen atom. This would lead to an interaction between two closed-shell 1S systems, which could be expected to yield a weakly bound species; a recent publication by Verdicchio et al. drew attention to the peculiarities of this system. This was interpreted as being due to the beryllium atom undergoing sp hybridization, and the subsequent formation of a σ bonding orbital between an empty Be sp hybrid orbital and the H 1s orbital
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