Citation for the original published paper (version of record):Zheng, G., Witek, H A., Bobadova-Parvanova, P., Irle, S., Musaev, D G. et al. (2007) Parameter calibration of transition-metal elements for the spin-polarized self-consistent-charge density-functional tight-binding (DFTB) method: Sc, Ti, Fe, Co, and Ni. Journal of Chemical Theory and
We investigate the structures and magnetic properties of small Mn(n) clusters in the size range of 2-13 atoms using first-principles density functional theory. We arrive at the lowest energy structures for clusters in this size range by simultaneously optimizing the cluster geometries, total spins, and relative orientations of individual atomic moments. The results for the net magnetic moments for the optimal clusters are in good agreement with experiment. The magnetic behavior of Mn(n) clusters in the size range studied in this work ranges from ferromagnetic ordering (large net cluster moment) for the smallest (n=2, 3, and 4) clusters to a near degeneracy between ferromagnetic and antiferromagnetic solutions in the vicinity of n=5 and 6 to a clear preference for antiferromagnetic (small net cluster moment) ordering at n=7 and beyond. We study the details of this evolution and present a picture in which bonding in these clusters predominantly occurs due to a transfer of electrons from antibonding 4s levels to minority 3d levels.
First-principles density-functional-theory investigations of small Mn n (nϭ2 -7,13) clusters reveal a competition between ferromagnetic and antiferromagnetic ordering of atomic magnetic moments. For smaller sizes (nр6), this competition results in a near degeneracy between the two types of orderings, whereas AF arrangements are clearly favored for larger clusters. The calculations thus predict a size-dependent transition in the magnetic ordering of Mn clusters.The study of magnetism in transition-metal clusters is motivated largely by the desire to understand how magnetic properties change when the dimensions of a material are reduced to nanometer length scales, a question of potentially great technological importance. A variety of interesting magnetic behavior has been discovered, ranging from enhanced magnetic moments in clusters of ferromagnetic metals such as Fe ͓1͔, to the prediction of net magnetic moments in clusters of nonmagnetic bulk materials ͓2͔. Generally, the magnetic properties of clusters show a dependence on cluster size ͓3-5͔ and systematic studies of these systems hold the promise of yielding new insight into magnetic ordering in materials.Manganese clusters are particularly interesting. An early electron-spin-resonance study ͓6͔ on small Mn clusters in an inert matrix suggested ferromagnetic ordering with atomic moments of ϳ5 B , the Hund's rule value for the free atom. More recently, Stern-Gerlach ͑SG͒ molecular-beam experiments ͓5͔ were carried out on larger clusters (Mn 11 -Mn 99 ). Analysis of the data assuming superparamagnetic behavior ͓7͔ in the clusters found small, but nonzero, average atomic magnetic moments. This result can be interpreted in one of two ways. If ferromagnetic ͑FM͒ ordering is assumed, the atoms must all have small individual moments. A second possibility is that the atomic moments remain large, but their orientation flips from site to site, so that the net cluster moments are small. The latter possibility has been found to be the preferred one for small Fe n (nϭ2 -4) clusters of low spin ͓8͔. This antiferromagnetic ͑AF͒ interpretation is compelling, since ␣-Mn, the most stable form of bulk manganese, is AF. It is also supported by new density-functional theory ͑DFT͒ calculations that found AF solutions to be more stable than FM solutions in intermediate size Mn n clusters (nϭ13, 15, 19, and 23͒ ͓9͔.Given these results, it appears that Mn clusters undergo a change in magnetic behavior from FM ordering for the smallest sizes to AF ordering for intermediate sizes and beyond. We address the nature of this transition in this paper. Our DFT calculations on Mn n clusters (nϭ2 -7,13) show that the smaller clusters are characterized by a close competition between FM and AF solutions. FM ordering is clearly favored for nϭ2 and 4, but AF and FM states are nearly degenerate for nϭ3, 5, and 6. A radical change occurs at n ϭ7 where the AF solution is much more stable than the FM. A similar behavior is found for nϭ13, suggesting that AF ordering is a general feature of the larger c...
Ab initio molecular orbital calculations at the HF/6-31+G(d,p) level were used to investigate the hydrogen bonding between hydrogen fluoride and two series of molecules, nitrile and carbonyl compounds of the type R−CN and R−CHO, respectively, where R= −H, −OH, −SH, −OCH3, −NH2, −NO2, −C⋮N, −F, −Cl, −CH3, and −CF3. Geometry optimization and vibrational frequency calculations at the optimized geometry were performed for isolated and hydrogen-bonded systems. The estimated energies of hydrogen-bond formation were corrected for zero-point vibrational energy and basis set superposition error (including the relaxation correction). Linear relations between the energy of hydrogen-bond formation (ΔE) and the H−F stretching frequency shift (ΔνHF) are obtained for the two series studied. Linear dependencies are also found between ΔE and the change of H−F bond length (Δr HF). An excellent linear dependence is found between ΔE R-CN and the ab initio calculated molecular electrostatic potential at the nitrile nitrogen (V N) in isolated nitrile molecules. A linear dependence is also found between E R-CHO and the ab initio calculated molecular electrostatic potential at the carbonyl oxygen (V O) in isolated carbonyl molecules. These relations show that the molecular electrostatic potential can be successfully used to predict the reactivity of the molecules studied with respect to hydrogen bonding. Significantly, a dependence that unifies the two series of proton-acceptor molecules was also found. It can be used with confidence in predicting the energy of hydrogen-bond formation when different substituents are added to the simplest member of a series.
A series of twenty-two BODIPY compounds were synthesized, containing various meso-phenyl and meso-thienyl groups, and their spectroscopic and structural properties were investigated using both experimental and computational methods. Further functionalization of the BODIPY framework via iodination at the 2,6-pyrrolic positions was explored in order to determine the effect of these heavy atoms on the photophysical and cytotoxicity of the meso-aryl-BODIPYs. BODIPYs bearing meso-thienyl substituents showed the largest red-shifted absorptions and emissions and reduced fluorescence quantum yields. The phototoxicity of the BODIPYs in human carcinoma HEp2 cells depends on both the presence of iodines and the nature of the meso-aryl groups. Six of the eleven 2,6-diiodo-BODIPYs investigated showed at least a sevenfold enhancement in phototoxicity (IC50 = 3.5–28 μM at 1.5 J/cm2) compared with the non-iodinated BODIPYs, while the others showed no cytotoxicity, while their singlet oxygen quantum yields ranged from 0.02 to 0.76. Among the series investigated, BODIPYs 2a and 4a bearing electron-donating meso-dimethoxyphenyl substituents showed the highest phototoxicity and dark/phototoxicity ratio, and are therefore the most promising for application in PDT.
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