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

Spectroscopic constants and energy levels of the ground (3)Delta (i) state (X (3)Delta (i)) and two low-lying excited states, (1)Delta and (5)Pi, 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 B-0 (0.6697 cm(-1)) and spin-orbit coupling constant A(SO) (-181 cm(-1)) for the X (3)Delta (2) state agree well with experimental values. The first (1)Delta state which is in isoconfiguration with the X (3)Delta state is predicted to lie at 3528 cm(-1) above the X (3)Delta (2) state with B-0, omega (e), and omega (e)x(e) values of 0.6861, 923, and 7 cm(-1), respectively. The lowest (5)Pi (2) state described by one electron excitation from nonbonding 1 delta orbital to antibonding 4 pi orbital should be located at 7248 cm(-1) above the X (3)Delta (2) state with B-0, omega (e), and omega (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)Delta state, the recently observed state at 3460 cm(-1) above the X (3)Delta (2) state by Aiuchi [Chem. Phys. Lett. 309, 229 (1999)], though tentatively assigned to the (5)Pi (2) state, should be reassigned to the (1)Delta state. (C) 2001 American Institute of Physics.