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

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Abstract
Spectroscopic constants and energy levels of the ground 3 Δ i state (X 3 Δ i ) and two low-lying excited states, 1 Δ and 5 ∏, of ironcarbide 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 B 0 (0.6697 cm−1) and spin–orbit coupling constant A SO (−181 cm−1) for the X 3 Δ 2 state agree well with experimental values. The first 1 Δ state which is in isoconfiguration with the X 3 Δ state is predicted to lie at 3528 cm−1 above the X 3 Δ 2 state with B 0 , ω e , and ω e x e values of 0.6861, 923, and 7 cm−1, respectively. The lowest 5 ∏ 2 state described by one electron excitation from nonbonding 1δ orbital to antibonding 4π orbital should be located at 7248 cm−1 above the X 3 Δ 2 state with B 0 , ω e , and ω e x e values of 0.6268, 850, and 5 cm−1, respectively. Thus, considering the coincidence in the predicted excitation energy and spectroscopic constants for the 1 Δ state, the recently observed state at 3460 cm−1 above the X 3 Δ 2 state by Aiuchi et al. [Chem. Phys. Lett. 309, 229 (1999)], though tentatively assigned to the 5 ∏ 2 state, should be reassigned to the 1 Δ state.