Local Effective Crystal Field Combined with Molecular Mechanics. Improved QM/MM Junction and Application to Fe(II) and Co(II) Complexes

Abstract

The quantum mechanical effective Hamiltonian of crystal field (EHCF) methodology (previously developed for describing electronic structure of transition metal complexes) is combined with the Gillespie-Kepert version of molecular mechanics (MM) in order to describe multiple potential energy surfaces (PES) of the Werner-type complexes corresponding to different spin states of the latter. The procedure thus obtained is a special version of the hybrid quantum mechanics/molecular mechanics approach. The MM part is responsible for representing the whole molecule, including ligand atoms and metal ion coordination sphere, but leaving aside the effects of the d shell. The quantum mechanics part (EHCF) is restricted to the metal ion d shell. The method reproduces with considerable accuracy geometry and spin states of a wide range of Fe(II) and Co(II) complexes of various total spin and coordination polyhedra and containing both monodentate and polydentate ligands with aliphatic and aromatic nitrogen donor atoms. In this setting, a single MM parameters set is shown to be sufficient for dealing with all spin states and coordination numbers of the complexes.

Effective Hamiltonian Crystal Field As Applied to Magnetic Exchange Parameters in μ-Oxo-Bridged Cr(III) Dimers

“…This general approach has turned out to be enormously successful when applied to TMCs: the EHCF method correctly reproduces not only the ground-state symmetries and spin multiplicities of all important classes of the Werner complexes, as well as their d–d-excitation spectra at the experimental geometries. − In addition, their dependence on geometry variations − including a successful modeling of the spin-transitions in Fe(II) complexes of nitrogen-containing ligands has been achieved, inaccessible for both ab initio and DFT-based methods.…”

Resonance theory of catalytic action of transition‐metal complexes: Isomerization of quadricyclane to norbornadiene catalyzed by metal porphyrins

“…If the electronic structure of a TMC alone is to be described, a statically correlated treatment is necessary for the d ‐shell, while the rest can be calculated using a simpler one‐electron approximation. This concept is a basis for the Effective Hamiltonian Crystal Field (EHCF) method which has been thoroughly tested . In this method, the trial wave function is taken in the form where ϕ d and ϕ l are wave functions for electrons in the d ‐shell and in the ligands, respectively; the symbol ⊗ has to be understood as an antisymmetrized product of the wave functions of the electrons in two subsystems.…”

“…The electronic variables of the subsystems are separated using the effective Hamiltonian approach based on concerted usage of the Löwdin partitioning technique and the McWeeny group‐functions theory . That simple setting, however, does not prevent the EHCF method from being capable to reproduce extremely subtle features of electronic structure of TMC's like spin crossover transitions in quasioctahedral complexes of Fe(II) and Co(II) and the 3 E ground state of Fe(II) porphyrin which represents a serious problem even for very advanced methods of ab initio quantum chemistry.…”

Effective hamiltonian crystal field: Present status and applications to iron compounds

“…[14] Second, the ground states of spinactive complexes of iron (II) had been correctly described at the respective experimental geometries: they come out high-spin at the high-spin geometry and low-spin at the low-spin one. [15] Moreover, the QM/MM extension of the EHCF approach [16] turned out to be very successful as shown in Figure 2. There we depicted numerous iron(II) complexes with rather involved organic ligands the ground state spins of which we were capable to reproduce.…”