This work reports density functional calculations of geometric, electronic and magnetic properties of freestanding iron-sulfur Fe 2 S 2 , Fe 3 S 4 and Fe 4 S 4 clusters which are the ones most frequently contained in proteins. We investigate neutral, anionic and cationic clusters using a method that employs linear combinations of atomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials and a generalized gradient approximation to exchange and correlation. The results are discussed in connection with available experimental data. We mainly show that the ground-state geometries of these free clusters are consistent with their structures in core proteins and they are the same in the neutral, anionic and cationic states, but with small distortions. In all cases, an antiferromagnetic order between Fe atoms is always preferred to ferromagnetic and paramagnetic ones. The geometric distortions induced by magnetism decrease with cluster size and the maximum deviation between Fe-Fe distances is 11% in Fe 2 S 2 , but only 4% in Fe 3 S 4 and 3% in Fe 4 S 4 clusters.
The structural, electronic, and magnetic properties of neutral and charged Fe n S 2 0/± (n = 1−6) clusters have been calculated in the framework of the density functional theory in the generalized gradient approximation for the exchange and correlation. The calculated adiabatic electron affinity and the vertical detachment energy are found to be in good agreement with the available experimental data. The impact of disulfide-doping of small iron clusters on the atomic structure, stability, magnetic moment, and reactivity is determined through the analysis of the binding energy per atom, electronic charge transfer, spin-polarized electronic densities of states, and global reactivity indicators like the electronegativity and chemical hardness. Our results provide an exhaustive characterization of these small iron-sulfide particles under vacuum, which is the first step to completely understand their role as components of proteins.
We report results, based on density functional theory− generalized gradient approximation calculations, that shed light on how NO, CO, and O 2 interact with Fe 2 S 2 , Fe 3 S 4 , and Fe 4 S 4 clusters and how they modify their structural and electronic properties. The interest in these small iron sulfide clusters comes from the fact that they are at the protein cores and that elucidating fundamental aspects of their interaction with those light molecules which are known to modify their functionality may help in understanding complex behaviors in biological systems. CO and NO are found to bind molecularly, leading to moderate relaxations in the clusters, but nevertheless to changes in the spin-polarized electronic structure and related properties. In contrast, dissociative chemisorption of O 2 is much more stable than molecular adsorption, giving rise to significant structural distortions, particularly in Fe 4 S 4 that splits into two Fe 2 S 2 subclusters. As a consequence, oxygen tends to strongly reduce the spin polarization in Fe and to weaken the Fe−Fe interaction inducing antiparallel couplings that, in the case of Fe 4 S 4 , clearly arise from indirect Fe−Fe exchange coupling mediated by O. The three molecules (particularly CO) enhance the stability of the iron−sulfur clusters. This increase is noticeably more pronounced for Fe 2 S 2 than for the other iron−sulfur clusters of different compositions, a result that correlates with the fact that in recent experiments of CO reaction with Fe m S m (m = 1−4), the Fe 2 S 2 CO product results as a prominent one.
Small free-standing Ni clusters have been widely investigated during the last decade, but not many of their derived chalcogenides, despite their interest in technology and the new prospects that the nanoscale may open. The present work uncovers the effects of the S-doping on the structural, electronic and magnetic properties of Nin, n=1-10 clusters. Density functional theoretic calculations within the generalized gradient approximation for the exchange and correlation were conducted to explore the structural, electronic, and magnetic properties of the resulting NinS chalcogenide nanoparticles. The sulfur impurity is always adsorbed on the 3-fold holow sites available on the nickel host, in qualitative agreement with recent results of S adsorption on Ni(111) surfaces. S-doping tends to enlarge the average Ni-Ni inter-atomic distance but enhaces the thermodinamical stability of Ni clusters. It also increases the vertical ionization energy and electron affinity. However, S-doping has a small effect on the magnetism of small Ni clusters. According to the spin-dependent HOMO-LUMO gap, most of these clusters are good candidates as molecular junctions for spin filtering at low bias voltage.
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