Fe(CO) 4 flexible as a two‐level system avoided conical intersection

This article reports new square‐planar Fe(CO) 4 D 4 h structures that are optimized, using the Hartree–Fock (HF) approach, and multiconfiguration self‐consistent field (MCSCF) theory in active space [2 b 2 g 2 e g a 1 g a 2 u ] 8 , and which energy increased in sequence: 3 B 2 g TS < 1 A 1 g TS < 1 A 1 g GS. A triple ζ valence basis set supplemented with 4 f for Fe and 3 d for C and O polarization shells [TZV (DF)] was used. At the HF/TZV (DF) level, 1 A 1 g TS and 3 B 2 g TS ( 3 B 2 g TS energetically more favorable), there are transition states of tetrahedral inversion (defining stereochemical flexibility of Fe(CO) 4 ) between known equivalent 1 A 1 and 3 B 2 Jahn–Teller distorted tetrahedron C 2 v structures with activation energy at ∼0.96 kcal/mol according to the experimental data. 1 A 1 g TS differs from 1 A 1 g GS in electronic configuration by occupation of a 1 g and a 2 u MOs. At the MCSCF/ TZV (DF) level, 1 A 1 g TS and 1 A 1 g GS are optimized as near‐pure states in different potential energy surfaces (PES) avoided conical intersection with near‐equal interatomic distances, and define electronic flexibility of Fe(CO) 4 . Estimation of the energy separation in a two‐level system that avoids a conical intersection from vibrational spectrum is based on the effective Hamiltonian of the perturbation theory. The energy gap between two square‐planar Fe(CO) 4 D 4 h 1 A 1 g TS < 1 A 1 g GS is 0.27 kcal/mol. The energy gap between 1 A 1 g GS and 1 A 1 is 1.28 kcal/mol. It is possible to observe 3 B 2 , 1 A 1 and 1 A 1 g GS separately in the course of the experiment. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006