Singlet Excited States of Anions with Higher Main Group Elements

TL;DR

This study uses high-level coupled cluster theory to explore the excited state properties of anions containing silicon and other higher main group elements, revealing the existence of bound and valence excited states in several such systems.

Contribution

It is the first comprehensive computational investigation demonstrating valence singlet excited states in small, closed-shell anions with higher main group elements beyond silicon.

Findings

Nine anions possess bound excited states.

Two anions have clear valence excited states.

Valence singlet excited states are shown only in group 14 anions.

Abstract

Previous studies have shown that dipole-bound excited states exist for certain small anions. However, valence excited states have been reported for some closed-shell anions, but those with singlet valence excited states have, thus far, contained a single silicon atom. This work utilizes high-level coupled cluster theory previously shown to reproduce excited state energies to better than 0.1 eV compared with experiment in order to examine the electronic excited state properties of anions containing silicon and other higher main group atoms as well as their first row analogues. Of the fourteen anions involved in this study, nine possess bound excited states of some kind: CH$_2$SN$^-$, C$_3$H$^-$, CCSiH$^-$, CCSH$^-$, CCNH$_2^-$, CCPH$_2^-$, BH$_3$PH$_2^-$, AlH$_3$NH$_2^-$, and AlH$_3$PH$_2^-$. Two possess clear valence states: CCSiH$^-$ and its first row analogue C$_3$H$^-$. Substantial mixing appears to be present in the valence and dipole-bound characters for the first excited state wavefunctions of many of the systems reporting excited states, but the mixing is most pronounced with the ammonia borane-like AlH$_3$NH$_2^-$, and AlH$_3$PH$_2^-$ anions. Inclusion of second row atoms in anions whose corresponding radical is strongly dipolar increases the likelihood for the existence of excited states of any kind, but among the systems considered to date with this methodology, only the nature of group 14 atoms in small, closed-shell anions has yet been shown to allow valence singlet excited states.