An ab initio study of excited states of C₄H₃ radical

Carbon-rich radicals are important for understanding chemistry of flames as well as astrochemistry. However, they are elusive for both experiments and computations, due to the peculiarity of radicals. Here, we calculated excited states of the 1-buten-3-yn-2-yl (HCCCCH2, i-C₄H₃) and n-C₄H₃ (HCCCHCH) radicals with the state-of-the-art complete active space self-consistent field (CASSCF) and multi-reference configuration interaction (MRCI) methods using the 6-311 + G(3df,2p) and aTVZ basis sets. The vertical energies for the seven excited states of each of the two isomers were obtained with the MRCI method at the geometries optimized at the CASSCF/6-311G(d,p) level. The result using the aTVZ basis set has been more reliable than predicted with the 6-311 + G(3df,2p) basis set with the relative stability of the ground states of the two isomers better predicted in comparison to existing data. We believe that our results will be useful for understanding the chemistry of carbon-rich radicals.