First-principle computation of zero-field splittings: application to a high valent Fe(IV)-oxo model of nonheme iron proteins.

We report the computational implementation of a combined spin-density-functional theory and perturbation theory (SDFT-PT) methodology for the accurate calculation of zero-field splittings (ZFS) in complexes of the most diverse nature including metal centers in proteins. We have applied the SDFT-PT methodology to study the cation of the recently synthesized complex [Fe(IV)(O)-(TMC)(NCCH(3))](OTf)(2), [J. Rohde et al., Science 299, 1037 (2003)] which is an important structural and functional analog of high-valent intermediates in catalytic cycles of nonheme iron enzymes. The calculated value (D(Theory)=28.67 cm(-1)) is in excellent agreement with the unusually large ZFS reported by experiment (D(Exp)=29+/-3 cm(-1)). The principal component D(zz) of the ZFS tensor is oriented along the Fe(IV)=oxo bond indicating that the oxo ligand dominates the electronic structure of the complex.