Geometries and electronic structures of the ground and low-lying excited states of FeCO: an ab initio study.

FeCO is a molecule of astrophysical interest. We report here theoretical calculations of its geometrical parameters, electronic structures, and molecular constants (such as dipole moment and spin-orbit coupling constant) in the electronic ground state X(3)Σ(-) and the low-lying triplet and quintet excited states. The calculations were made at the MR-SDCI+Q_DK3/[5ZP ANO-RCC (Fe, C, O)] and MR-AQCC_DK3/[5ZP ANO-RCC (Fe, C, O)] levels of theory. A multi-reference calculation was required to describe correctly the wavefunctions of all states studied. For all triplet states, the σ-donation through the 10σ molecular orbital (MO) as well as the π-back-donation through the 4π MO are observed, and the dipole moment vector points from O toward Fe as expected. However, in the excited quintet states (5)Π, (5)Φ, and (5)Δ, the almost negligible contribution of Fe 4s to the 10σ MO makes the dipole moment vector point from Fe toward O, i.e., in the same direction as in CO. In the X(3)Σ(-) state, the electron provided by the σ-donation through the 10σ MO is shared between the Fe atom and the C end of the CO residue to form a coordinate-covalent Fe-C bond. In the ã(5)Σ(-) state (the high-spin counterpart of X(3)Σ(-)), the σ-donation through the 10σ MO is not significant and so the Fe-C bond is rather ionic. The π-back-donation through the 4π MO is found to be of comparable importance in the two electronic states; it has a slightly larger magnitude in the X(3)Σ(-) state. The difference in the molecular properties of the low-spin X(3)Σ(-) and the high-spin ã(5)Σ(-) states can be understood in terms of the dynamical electron correlation effects.