Molecular properties of FeCO as derived from AB initio molecular orbital calculations

Abstract

We have performed ab initio all-electron and pseudopotential MO calculations on FeCO with varying degrees of sophistication in the basis sets as well as in the calculational procedures. Basis sets with at least double zeta representation for valence orbitals as well as inclusion of correlation corrections in the MO calculations are the minimum requirements in order to arrive at a clear decision about the electronic ground state of FeCO. In contradiction to the suggestion given by Guenzburger et al. [23] we derive from total energy considerations and from comparing experimental and calculated C−O stretching force constants that the ground state is a spin-quintet (5∑−) and not a spin-triplet. Our calculated quadrupole splitting for MO equilibrium geometry of FeCO5∑− deviates by about 25% from the value which was observed by Peden et al. [1] for FeCO in solid noble gas. However, the calculated electronic configuration of iron within FeCO5∑− of about 3d6.44s0.8 is far from 3d64s2, which approximately is expected for FeCO when comparing isomer shifts of FeCO and Fe, both matrix-isolated in solid noble gas, i.e. −0.60 mm/s and −0.75 mm/s, respectively. Thus, at the moment the question remains open to what extent the molecular properties of FeCO are affected by the solid noble gas matrix.