Predictable Valence Excited States of Anions

The excited states of the 1 1A′ :CC–C̈–R– family and related anions are investigated. Previous work has shown that 1 1A′ C3H– and CCSiH– each possess a rare valence excited state in addition to their dipole-bound excited states. A similar methodology to that employed previously shows that related anions (C3OH–, C3NC–, C4N–, C5H–, and HBCN–) also possess valence excited states. The valence states are the result of breaking the symmetry from C ∞v to C s . The half-filled π or π-type highest occupied molecular orbital (HOMO) is split into a′ and a″ pieces. The valence excitation takes place between these two pieces. If the anion HOMO is not a half-filled π-type orbital because of an increase in symmetry, cyclization, or both, the anion most likely does not exhibit the signs of a valence excited state even if the anion is an isomer of or isoelectronic to an anion that does possess a valence excited state. However, the :CC–C̈–R– set is not the only classification of anions shown to possess valence excited states even though it is the most predictable grouping exhibiting this behavior found to date.