Fe(CO)_4 Flexible as a Two-Level System Avoided Conical Intersection

摘要

This article reports new square-planar Fe(CO)_4 D_(4h),structures that are optimized,using the Hartree-Fock (HF)approach,and multiconfiguration self-consistent field (MCSCF)theory in active space [2b_(2g)2e_g,a_(1g)a_(2g)]~8,and which energy increased in sequence:~3B_(2g),TS < ~1A_(1g)TS < ~1A_(1g)GS.A triple zeta,valence basis set supplemented with 4f for Fe and 3d for C and 6 polarization shells [TZV (DF)] was used.At the HF/TZV (DF)level,~1A_(1g)TS and ~3B_(2g)TS (~3B_(2g)TS energetically more favorable),there are transition states of tetrahedral inversion (defining stereochemical flexibility of Fe(CO)_4)between known equivalent ~1A_1,and ~3B_2 Jahn-Teller distorted tetrahedron C_(2v)structures with activation energy at approx0.96 kcal/mol according to the experimental data.~1A_(1g)TS differs from ~1A_(1g)GS in electronic configuration by occupation of a_(1g)and a_(2u)MOs.At the MCSCF/ TZV (DF)level,~1A_(1g)TS and ~1A_(1g)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_(4h),~1A_(1g)TS < ~1A_(1g)GS is 0.27 kcal/mol.The energy gap between ~1A_(1g)GS and ~1A_1,is 1.28 kcal/mol.It is possible to observe ~3B_2,~1A_1 and ~1A_(1g)GS separately in the course of the experiment.