Tuning the ground state symmetry of acetylenyl radicals

The lowest excited state of the acetylenyl radical, HCC, is a ²Π state, only 0.46 eV above the ground state, ²Σ⁺. The promotion of an electron from a π bond pair to a singly occupied σ hybrid orbital is all that is involved, and so we set out to tune those orbital energies, and with them the relative energetics of ²Π and ²Σ⁺ states. A strategy of varying ligand electronegativity, employed in a previous study on substituted carbynes, RC, was useful, but proved more difficult to apply for substituted acetylenyl radicals, RCC. However, π-donor/acceptor substitution is effective in modifying the state energies. We are able to design molecules with ²Π ground states (NaOCC, H₂NCC (²A″), HCSi, FCSi, etc.) and vary the ²Σ⁺-²Π energy gap over a 4 eV range. We find an inconsistency between bond order and bond dissociation energy measures of the bond strength in the Si-containing molecules; we provide an explanation through an analysis of the relevant potential energy curves.