Author:

Keywords:

quantum chemistry, density-functional calculations, computational chemistry, heme-nitrosyl bonding, multiconfigurational perturbation-theory, CASPT2, Science & Technology, Physical Sciences, Chemistry, Physical, Chemistry, NITRIC OXIDE COMPLEXES, SPIN-STATE ENERGETICS, ANO BASIS-SETS, TRANSITION-METAL, PERTURBATION-THEORY, NO COMPLEXES, CORRELATION-ENERGY, DFT CALCULATIONS, LIGAND-BINDING, IRON, Electrons, Heme, Iron, Models, Molecular, Molecular Conformation, Nitrogen Oxides, Quantum Theory, Thermodynamics, 02 Physical Sciences, 03 Chemical Sciences, 09 Engineering, 34 Chemical sciences, 40 Engineering, 51 Physical sciences

Abstract:

The multiconfigurational CASSCF/CASPT2 approach, along with various functionals of Density Functional Theory, is applied to selected iron(II)--nitrosyl {FeNO}^7 complexes, both with heme and non-heme groups. The energetics of the lowest doublet and quartet spin states at the correlated ab initio (CASPT2) level is presented for the first time. Comparison of the CASSCF and (unrestricted) DFT spin densities indicates that the non-hybrid functionals yield the spin densities most closely to the ab initio ones. The analysis of the multiconfigurational CASSCF wave function in terms of the localized active orbitals allows to resolve the nature of Fe--NO bonding as a~mixture of Fe(II)--NO(0) and Fe(III)--NO(-) resonance structures (in comparable contributions) for both spin states and different ligands.