Abstract
Molecular systems containing magnetically interacting (exchange-coupled) manganese ions are important in catalysis, biomimetic chemistry, and molecular magnetism. The reliable prediction of exchange coupling constants with quantum chemical methods is key for tracing the relationships between structure and magnetic properties in these systems. Density functional theory (DFT) in the broken-symmetry approach has been employed extensively for this purpose and hybrid functionals with moderate levels of Hartree–Fock exchange admixture have often been shown to perform adequately. Double-hybrid density functionals that introduce a second-order perturbational contribution to the Kohn–Sham energy are generally regarded as a superior approach for most molecular properties, but their performance remains unexplored for exchange-coupled manganese systems. An assessment of various double-hybrid functionals for the prediction of exchange coupling constants is presented here using a set of experimentally characterized dinuclear manganese complexes that cover a wide range of exchange coupling situations. Double-hybrid functionals perform more uniformly compared to conventional DFT methods, but they fail to deliver improved accuracy or reliability in the prediction of exchange coupling constants. Reparametrized double-hybrid density functionals (DHDFs) perform no better, and most often worse, than the original B2-PLYP double-hybrid method. All DHDFs are surpassed by the hybrid-meta-generalized gradient approximation (GGA) TPSSh functional. Possible directions for future methodological developments are discussed.
Highlights
Oligonuclear complexes with magnetically interacting manganese ions have a long and rich history in chemistry, as synthetic analogues of essential biological clusters, such as the oxygen-evolving complex of photosystem II, and in their own right, because of their magnetic, spectroscopic, and catalytic properties [1,2,3,4,5,6,7,8,9,10,11,12,13,14]
All double-hybrid density functionals (DHDFs) are surpassed by the hybrid-meta-generalized gradient approximation (GGA) TPSSh functional
The trends are largely anticipated from past studies: functionals that do not incorporate any exact exchange strongly favor the broken-symmetry over the high-spin state, yielding exchange coupling constants that are too negative compared to experiment
Summary
Oligonuclear complexes with magnetically interacting manganese ions have a long and rich history in chemistry, as synthetic analogues of essential biological clusters, such as the oxygen-evolving complex of photosystem II, and in their own right, because of their magnetic, spectroscopic, and catalytic properties [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. From the perspective of quantum chemistry, a fundamental challenge in the description of the electronic structure of these systems is to model reliably the magnetic energy levels, that is, the energy levels associated with the magnetic “interaction” between the open-shell Mn ions [15] This forms the basis for obtaining the full range of spin-dependent observables for a given system. The challenge of directly computing the full spectrum of energy states often can or has to be reduced for practical purposes to the more modest target of extracting pairwise exchange coupling constants that parameterize the magnetic energy levels in the framework of an effective spin Hamiltonian. This is often achieved at the level of Kohn–Sham density functional theory (DFT) using the broken-symmetry approach
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