Predictable valence excited states of anions.

The excited states of the 1 (1)A' :C≡C-C̈-R(-) family and related anions are investigated. Previous work has shown that 1 (1)A' 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 Cs. 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.