Ab initio study of the ground and two low-lying electronic excited states of FeC

Spectroscopic constants and energy levels of the ground Δi3 state (X 3Δi) and two low-lying excited states, Δ1 and ∏,5 of iron carbide FeC have been calculated by the ab initio multireference singles and doubles configuration interaction (MR-SDCI) molecular orbital method taking relativistic and spin–orbit coupling effects into account. Predicted rotational constant B0 (0.6697 cm−1) and spin–orbit coupling constant ASO (−181 cm−1) for the X 3Δ2 state agree well with experimental values. The first Δ1 state which is in isoconfiguration with the X 3Δ state is predicted to lie at 3528 cm−1 above the X 3Δ2 state with B0, ωe, and ωexe values of 0.6861, 923, and 7 cm−1, respectively. The lowest ∏25 state described by one electron excitation from nonbonding 1δ orbital to antibonding 4π orbital should be located at 7248 cm−1 above the X 3Δ2 state with B0, ωe, and ωexe values of 0.6268, 850, and 5 cm−1, respectively. Thus, considering the coincidence in the predicted excitation energy and spectroscopic constants for the Δ1 state, the recently observed state at 3460 cm−1 above the X 3Δ2 state by Aiuchi et al. [Chem. Phys. Lett. 309, 229 (1999)], though tentatively assigned to the ∏25 state, should be reassigned to the Δ1 state.