Osmium Atoms Research Articles

Sr4Os3O12 – A Layered Osmate(V,VI) that is Magnetic Close to Room Temperature

AbstractBlack crystals of the mixed‐valence osmate(V,VI) Sr4Os3O12 were grown via a gas phase reaction in an evacuated silica tube. The material is attracted to a permanent magnet at room temperature (295±5 K) but loses this property when heated or cooled. In this temperature interval, soft magnetic behavior with vanishing coercivity is observed, but the saturation magnetization is only 0.05 μB per formula unit. Resistivity measurements show that Sr4Os3O12 is a semiconductor with a small band gap of about 0.26 eV at room temperature. X‐ray diffraction on a single‐crystal revealed a rhombohedral structure with the space group R m and lattice parameters ar=554.64(2) pm and cr=2700.1(1) pm at 300(1) K. The compound crystallizes in the La4Ti3O12 structure type and is isostructural to Sr4CoRe2O12. The structure is a rhombohedral stack of layered blocks, each of which can be considered as a section of three layers of the cubic perovskite structure cut perpendicular to one of its threefold axes. The Os–O−Os bond angle of about 175° favors superexchange that leads to antiferromagnetic coupling between the osmium atoms with 5d3 and 5d2 configurations. Sr4Os3O12 exhibits strong lattice dynamics. One aspect is the antiferro‐rotative lattice modes of the corner‐sharing [OsO6] octahedra, which freeze upon cooling. At 100 K, an ordered, yet twinned triclinic superstructure is formed. Strong spin‐orbit coupling and other effects cause deformations of the [OsO6] octahedra that differ significantly at 300 K and 100 K, suggesting a change in the electronic configuration. Competing ground states could also explain the temperature‐dependent band gap and the magnetic fluctuations manifested in the magnetization peak at room temperature.

Study of dual osmium and boron co-doped SWCNTs for reversible hydrogen storage

The dual osmium/boron doped/co-doped armchair single walled carbon nanotubes (SWCNTs) have been explored for hydrogen storage applications via ab-initio method. We thoroughly screen dual osmium/boron doping at two different opposite positions in SWCNTs and analyze bond distances, binding energy, band gaps, electrophilicity, density of states, adsorption energies, adsorption enthalpy, Gibbs free energy and storage capacity. The results summit Osmium doped CNTs as hopeful nominees for hydrogen storage applications. The findings indicates that Osmium atoms doping in opposite sides of CNT along with boron atoms at ortho position (2Os-2BCNT) show 1.95 wt% gravimetric hydrogen storage capacity at 298.15 Kelvin temperature and 1 atm pressure. Furthermore, the feasibility of doped SWCNTs have been explored for storage applications by performing average Van't Hoff desorption temperature calculations and molecular dynamics simulations for 2Os-2BCNT at maximum desorption temperature and 1 atm pressure.

Triosmium cluster bearing a metalated dibenzylideneacetone ligand: Synthesis, structure, redox and electronic properties

Dibenzylideneacetone (DBA) reacts with [Os3(CO)10(NCMe)2] in boiling benzene to afford the triosmium cluster [Os3(CO)10{κ2-(C,O)-C17H13O}(µ-H)] (1) formed through activation of one of the β-C–H bonds of the DBA ligand after initial coordination of the ketonic oxygen. The molecular structure of 1 has been determined unequivocally by single crystal X-ray diffraction analysis which reveals that the metalated DBA ligand formed a five-membered osmacycle by coordinating to a single osmium atom using its both donor atoms. Cluster 1 exhibits two closely spaced irreversible reduction peaks at –1.20 V and –1.30 V, respectively, together with an irreversible oxidation peak at 0.57 V. DFT calculations reveals the frontier orbitals are primarily either metal-based (HOMO) or ligand-based (LUMO) which suggests that the oxidation occurs from the metallic core while the first reduction is ligand-centered. Cluster 1 displays two absorption peaks at 337 nm and 477 nm in the UV-visible absorption spectrum. The lowest energy absorption at 477 nm can be attributed to the transition from HOMO to LUMO (MLCT) based on its frontier orbitals.

Stable room-temperature ferromagnetism and gate-tunable quantum anomalous Hall effect of two-dimensional 5d transition-metal trihalide OsX3 (X = Cl, Br, I) monolayers.

5d transition-metal compounds are usually not expected to exhibit distinct magnetic ordering owing to their substantial binding energy associated with 5d electrons. In this study, we demonstrate that two-dimensional (2D) 5d transition-metal Os trihalide OsX3 monolayers can exhibit room-temperature ferromagnetism and quantum anomalous Hall effect (QAHE) by utilizing density functional theory and Monte Carlo simulation. Our calculation results of coexisting Raman and infrared activities of lattice vibration reveal the structural stability of 2D OsX3 (X = Cl, Br, I) and structural instability of 2D OsX3 (X = F). Furthermore, all 2D OsX3 trihalides (X = Cl, Br, I) are half-metals, and their ferromagnetism remains stable under ambient temperature, where 2D OsCl3 and OsBr3 have an in-plane easy axis while 2D OsI3 has an out-of-plane easy axis. Notably, when spin-orbit coupling is included, the gate-tunable QAHE could emerge in ferromagnetic 2D OsI3, while 2D OsCl3 and OsBr3 are topologically trivial. Additionally, the magnon bands of 2D OsX3 (X = Cl, Br, I) possess two spin-wave branches with dispersion similar to that of the Dirac cone in the electronic structure of graphene, which are attributed to the unique ferromagnetic honeycomb sublattice of osmium atoms.

A DFT investigation of Osmium decorated single walled carbon nanotubes for hydrogen storage

The investigation for hydrogen storage in armchair single walled carbon nanotubes (SWCNTs) with decoration of osmium atom on the surface of tube has been done using ab-initio method utilizing Density Functional Theory (DFT). In the present work, single/dual osmium atoms decoration on the surface of SWCNTs (5,5) has been investigated regarding the effects on structural, electronic and thermodynamic properties. Frontier molecular orbital (FMO) i.e. HOMO and LUMO structures and electrostatic potential surfaces (ESP) of all geometrically optimized and stable species are studied. Investigations regarding metal-metal clustering are carried out by calculating diffusion barrier energy for osmium atoms and atomistic molecular dynamic simulations performed at 300°K temperature for 1ps. It is found that osmium decoration on SWCNTs enhances the hydrogen storage capacity via spillover mechanism. It is observed that the adsorption energy per hydrogen molecule in case of Pristine SWCNT is -0.001eV, which is increased with decoration of osmium atoms and finally found to be -0.575eV/H2 in case of single osmium decorated SWCNT (Os-CNT) and -0.681eV/H2 due to double osmium decoration on SWCNT (2Os-CNT). This study reveals gravimetric hydrogen storage capacity of 1.32 wt% and 2.53 wt% for Os-CNT and 2Os-CNT respectively at 298.15 K temperature and 10 atm pressure. Average Van't Hoff desorption temperature for osmium decorated SWCNTs are also calculated.

The First Quaternary Phase of the NbRuB Structure Type: Experimental and Computational Investigations of TiFe1.3Os3.9B2.8

AbstractThe novel composition – TiFe1.3Os3.9B2.8 – is the first quaternary phase of the NbRuB structure type. The layer‐like structure consists of boron (mixed with 8 % Fe) dumbbells in the same layer as osmium atoms, some sites with mixed occupancies. The second layer has isolated boron, osmium/titanium (51 % Os) and iron atoms. The presence of iron enables the study of magnetism as Fe‐chains are present, with intra‐ and interchain distances of 3.02 Å and 5.76 Å, respectively. Density functional theory (DFT) predicts magnetic ordering to be likely with the antiferromagnetic ground state only 13 meV more favorable than the ferromagnetic state. This discovery opens the possibilities of studying various quaternary compositions in the superconducting NbRuB structure type.

Noble metal single-atom catalyst supported by C[formula omitted]N[formula omitted] monolayer as a promising reduction electrocatalyst for low-density environmental NO

Here, we show that the noble metal atoms decorated over the porous monolayer of C8N8 demonstrate superior performance in the reduction reaction of low-density environmental NO gas. Among different considered single-atom catalysts (SAC), the osmium atom exhibits the most energetically favored NO-reduction pathway with the free energy change of −12.71 eV even at low NO coverage. Both nitrogen- and carbon-dominated large rings are capable of adsorbing NM atoms to form NM@C8N8 configurations. Fortunately, no clustering resulted for the noble metal atoms anchored onto the pristine C8N8 due to the substantial binding energy of −12.87 eV. The considered SACs exhibit their electrocatalytic activities by the dominant contribution to the total DOS around the Fermi level, especially for the Os, Ir, and Rh adatoms. At low NO coverage, NOER over Os@C8N8 proceeds the downhill profile diagram and demonstrates promising electrocatalytic performance in NO-reduction. Similarly, at high NO coverage, N2O2 can be adsorbed by NM@C8N8, due to the negative value of the free energy change.

Syntheses, Crystal structures and magnetic properties the tetrahedral [FeX4]− (X=Cl, Br) in the Os-based chelating diphosphine anticancer compounds

Compound 1,trans-[OsII(dppe)2Cl2], was obtained by refluxing OsII(PPh3)3Cl2 with dppe (dppe = bis (diphenylphosphino) ethane). Based on the oxidation potential of 1, Cp2FeIII(PF6) was chosen as oxidant for the preparation of its one-electron product, trans-[OsIII(dppe)Cl2](PF6) (2, Cp = cyclopentadienyl anion). Compounds trans-[OsIII(dppe)2Cl2](FeIIIX4) (X = Cl, 3; X = Br, 4) were achieved in a straightforward way by reaction of compound 1 with FeX3 (X = Cl, Br), respectively. Compounds 1–4 were characterized by IR, electronic absorption spectra and elemental analysis, respectively. Single-crystal X-ray diffraction studies indicate that the osmium atoms in compounds 1–4 all define a slightly distorted octahedron with a [OsP4Cl2] coordination core, and the iron atoms in anion FeX4− of compounds 3 and 4 displays the expected tetrahedron with a [Fe1X4] coordination core. The electronic absorption spectra exhibit metal-to-ligand charge-transfer (MLCT) transitions in compound 1, and both MLCT and ligand-to-metal charge-transfer (LMCT) transitions in compounds 2–4. Also the magnetic properties of compounds 3 and 4 have been investigated. The relative cell viability of compounds 1–4 on HeLa cancer cells is in the order of 3>1>4 >2 by CCK-8 method. Compound 1 and 3 have better activity than that of cisplatin in higher concentration.

Hydrogen adsorption on osmium and boron co-doped single walled carbon nanotubes for energy storage: A DFT study

The osmium and boron co-doped armchair single walled carbon nanotubes (SWCNTs) have been studied for hydrogen storage using ab-initio method. The calculations have been carried out using Gaussian software with Density functional theory. We have replaced a carbon atom by osmium atom in SWCNT & neighboring carbon atoms are replaced by boron atoms sequentially. In the present work, the hydrogen adsorption ability of osmium/boron co-doped CNTs has been investigated on the basis of bond distances, variation of binding energy, chemical hardness, band gaps, charge distribution and adsorption energies. It is found that osmium/boron co-doping in SWCNTs enhanced the hydrogen storage capacity but increase of boron atom concentration in osmium doped SWCNT reduces the storage ability. The maximum hydrogen adsorption energy was calculated in case of osmium atom decorated single boron atom doped SWCNT (Os-BCNT) i.e. E ads - 0.490 eV. And study reveal that osmium doped CNT (Os-CNT) and osmium/boron co-doped CNT (Os-BCNT) both can efficiently adsorb five hydrogen molecules at 298.15 Kelvin temperature/1 atm pressure. • Modified structural and electrical properties of osmium/boron co-doped SWCNT • Enhanced hydrogen storage capacity of osmium/boron co doped SWCNT. • Effect of boron concentration on hydrogen storage capacity on SWCNT

Study on the Densification of Osmium by Experiment and First Principle Calculations.

The sintering of osmium is critical for the preparation of raw material targets for film coating, which is the main application area of osmium. In order to get a better understanding of the intrinsic mechanism of densification of osmium, a serial study on the sintering behavior of osmium has been made in this study. By the master sintering curve (MSC) and constant heating rate (CHR) method, the sintering activation energy of nanosized osmium is evaluated to be about 340 kJ/mol, which is higher than most other metals. The density-functional theory calculation indicates the higher energy barrier of the surface atom and vacancy migration and lacking migration tunnel of inner point vacancies. For example, the diffusion of osmium atoms on the surface of particles is mainly limited by Os (1010), which has an energy barrier as high as 1.14 eV, that is higher than the W atom on W (110) of 0.99 eV. The vacancy migration energy barrier inside osmium's grains is higher than 3.0 eV, while that of W is only 1.7 eV. This means that it is more difficult for osmium to achieve a high density compared with W, which is consistent with the experimental results. Accordingly, the proposed strategy provides a new opportunity to design a sintering process for target fabrication with excellent properties for various applications.

Density functional theory study of carbon monoxide adsorption on transition metal doped armchair graphene nanoribbon

Density Functional Theory calculations are used to study adsorption of carbon monoxide gas on transition metal doped armchair graphene nanoribbon. Sensing behaviour of pristine armchair graphene nanoribbon (Pr-AGNR) and different geometries of osmium doped AGNR such as one-edge doped AGNR, center doped AGNR and both-edge doped AGNR have been investigated. Most stable adsorption geometry, adsorption energy, binding distance, band structure and density of states of different geometries using in our work have been discussed. Electronic properties of different sensing materials can be interpreted in terms of band structure and density of states. It was studied that a weak type of physical adsorption process took place when CO molecule interacts with pristine AGNR with a very small amount of adsorption energy of amount −0.22 eV. This suggests that pristine graphene is not a good adsorber for CO adsorption. In contrast, doping of graphene with osmium atom enhances the adsorption energy of Pr-AGNR to greater extent. Computational results concluded that osmium doping in graphene nanoribbon at one-edge and both-edge position increases the adsorption energies with values −6.77 eV and −9.53 eV respectively which is 30 times and 43 times greater than in comparison to Pr-AGNR. Energy gap calculations proves that large band gap variations are observed for both-edge doped graphene nanoribbon. Sharp and intense peaks at various energy levels from density of states analysis of both-edge doped geometry confirms adsorption of CO. Our results predicts that both-edge osmium doped sensing material can be considered as promising sensing material for CO adsorption.

Hydrogen sulfide gas sensor using osmium doped graphene nanoribbon: An insights from DFT study

First Principles adsorption phenomenon of Hydrogen Sulfide (H2S) gas on surface of undoped armchair graphene nanoribbon (AGNRs) and Osmium doped AGNRs (Os-AGNRs) is reported. Adsorption energies and electronic properties of AGNRs doped with osmium atom in three possible configurations like one-edge doped, center doped and both-edge doped has been investigated theoretically using DFT. Results of adsorption energies revealed that undoped AGNR is insensitive to H2S gas with adsorption energy of about −0.25 eV. Whereas, osmium doped AGNRs at one-edge and both–edge doped position is having adsorption energies −6.67 eV and −9.67 eV, which is nearly 26 times and 38 times more than undoped AGNR. The both edges doped configuration shows significant variation in band gap after adsorption of gas. The study suggests that this novelboth edges doped Os-AGNRmaterial can be highly suitablefor H2S gas adsorption and sensing applications.

Chirality and Relativistic Effects in Os3(CO)12.

The energy and structural parameters were obtained for all forms of the carbonyl complex of osmium Os3(CO)12 with D3h and D3 symmetries using density functional theory (DFT) methods. The calculations took into account various levels of relativistic effects, including those associated with nonconservation of spatial parity. It was shown that the ground state of Os3(CO)12 corresponds to the D3 symmetry and thus may be characterized either as left-twisted (D3S) or right-twisted (D3R). The D3S↔D3R transitions occur through the D3h transition state with an activation barrier of ~10–14 kJ/mol. Parity violation energy difference (PVED) between D3S and D3R states equals to ~5 × 10−10 kJ/mol. An unusual three-center exchange interaction was found inside the {Os3} fragment. It was found that the cooperative effects of the mutual influence of osmium atoms suppress the chirality of the electron system in the cluster.

Copper carbene complexes. Synthesis and structural analysis of a chloro-bridged dicopper cation and the triosmium-copper carbene cluster complex HOs3(CO)11[µ-Cu(IPr)

The chloro-bridged carbene-substituted dicopper salt [{(IPr)Cu}2(µ-Cl)][PF6], 1, IPr = [N,N′-bis(2,6-diisopropylphenyl)imidazolin-2- ylidene], was synthesized by the reaction of (IPr)CuCl with Tl[PF6]. Reaction of 1 with the anion, [HOs3(CO)11]− of the salt [PPN][HOs3(CO)11], 2 yielded the Cu - Os heterometallic cluster complex, HOs3(CO)11[µ-Cu(IPr)], 3, the first example of Cu - Os bimetallic cluster complex containing a N-heterocyclic carbene ligand. Compounds 1 and 3 were characterized by IR, 1H NMR and structurally by single-crystal X-ray diffraction analysis. Compound 3 contains a triosmium carbonyl cluster with a bridging Cu(IPr) group on one edge of the cluster and a terminally-coordinated hydrido ligand on one of the Cu-bridged osmium atoms.

Физический механизм работы осмированных катодов СВЧ приборов

Emission properties of the traditional metal porous cathodes (MPC) are determined by oxygen vacancies in BaO crystallites and the surface located oxygen vacancies forms the acceptor type surface states in accordance with it take place the distorting of energy zones to up. In cathodes with osmium the osmium atoms are dissolved in BaO crystallites and then formed the donor type surface states in accordance with it take place the distorting of energy zones to down and the decreasing of work function. All investigations were made by using of optical spectroscopy, electron spectroscopy for chemistry analysis, spectroscopy of the characteristic electron energy loses.

Optical properties of half-metallic ferrimagnetic double perovskite Sr2CaOsO6 compound

The crystal, electronic, magnetic and optical properties of the Sr2CaOsO6 double perovskite compounds have been investigated in this work, using density functional theory as implemented in the Full-Potential Linearized Augmented Plane Wave (FP-LAPW) method, employing the Generalized Gradient Approximation (GGA-PBE), GGA + U (Hubbard Coulomb correction) and GGA plus Trans-Blaha-modified Becke–Johnson (TB-mBJ-GGA) as the exchange correlation. According to the structural results, the lattice parameter is in agreed well with the previous experimental data. Spin polarization calculations has shown half-metallic behavior with ferrimagnetic ground state coupling for Ca and Os spins for Sr2CaOsO6 compound. Our calculated electronics results within the TB-mBJ-GGA show that our studied material has direct half-metalic (HM) gaps energy in (Γv-ΓC) directions with a value equal 2.08 eV and net magnetic moment of 2.000μB. The most contribution of total magnetic moment is by Osmium atom. For optical properties, it can be concluded that Sr2CaOsO6 material has a high absorption and can be used in future Spintronics and Optoelectronic applications in a large energy range of the light spectrum which extends from visible to ultraviolet.

Thermolysis of [HOs3(CO)8{µ3-Ph2PCH2P(Ph)C6H4}]: New Os2- and Os3- cluster products based on multiple C[sbnd]H bond activation of the bis(diphenylphosphino)methane ligand

Thermolysis of [HOs3(CO)8{µ3-Ph2PCH2P(Ph)C6H4}] in refluxing xylene has been investigated, and three new polynuclear osmium compounds containing an activated dppm ligand(s) as a result of multiple CH bond cleavages have been isolated. The new complexes are [H2Os3(CO)8{µ3-Ph2PCHP(Ph)C6H4}] (1), [H2Os3(CO)6(µ3-Ph2PCHPPh2){µ3-Ph2PCH2P(Ph)C6H4}] (2) and [Os2(CO)6{µ-H4C6(Ph)PCH2P(Ph)C6H4}] (3), each of which exhibits a different coordination mode involving the activated diphosphine ligand. Cluster 1 is formed by CH bond oxidative addition of the backbone methylene spacer and an ortho site at an ancillary aryl ring. Control experiments confirm that cluster 2 is formed from cluster 1 and dppm. The dinuclear compound 3 is formed via metalation of two phenyl rings on the dppm ligand and is accompanied by the release of an osmium atom. The molecular structure of each new complex has been established by single-crystal X-ray diffraction analysis, and the bonding in compounds 1–3 has been examined by DFT calculations.

Kinetics of Phosphorescence Decay of the New Paramagnetic Osmium(IV) Tetra-Crown Porphyrinate in Polystyrene at Temperatures of 77 and 298 K

The reaction of osmium(II) tetra-15-crown-5-porphyrin (Os(II)TCP) with molecular iodine in high yield yielded the corresponding osmium(IV) porphyrin containing iodide ions in the axial positions of the osmium(IV) atom and possessing paramagnetic properties according to 1H NMR. The iodine anions in the complex create an additional heavy atom effect in addition to such an effect from the osmium(IV) cation, which is of certain interest from the point of view of increasing the probability of phosphorescence transitions in osmium(IV) crown porphyrin molecules. The paramagnetism of osmium(IV) crown porphyrin also affects the probability of phosphorescence transitions, and this effect depends on temperature. These two effects (paramagnetism and the heavy atom effect) create some interest in terms of the possibility of creating a phosphorescent temperature sensor based on the new osmium(IV) crown porphyrin. A comparative analysis of the kinetics of phosphorescence decay of osmium(IV), osmium(VI) and osmium(II) porphyrins in the polystyrene matrix at temperatures of 298 and 77 K. It was found that the new osmium(IV) crown porphyrin has the highest phosphorescence lifetime with temperature in this series and can be considered as a luminescent temperature sensor for changes in phosphorescence lifetime.

Facile Os-Os bond cleavage in the reactions of [Os3(CO)10(NCMe)2] and [Os3(CO)10(μ-H)2] with tetramethylthiuram disulfide (tmtd): Syntheses and crystal structures of new polynuclear osmium carbonyl complexes containing a dimethyldithiocarbamate ligand(s)

The reaction of the labile triosmium cluster [Os3(CO)10(NCMe)2] with tetramethylthiuram disulfide [(Me2NCS2)2, tmtd] has been investigated at room temperature. Conducting the reaction under mild conditions has permitted the isolation of the three new polynuclear osmium complexes [Os3(CO)10{κ2(S,S)-S2CNMe2}2] (1), [Os4(CO)12{κ2(S,S)-S2CNMe2}2{μ3-η1(C),κ2(O,O)-CO2)}(μ3-S)] (2) and [Os3(CO)9{μ3-η1(C),κ1(S)-SCNMe2}{μ-κ1(S)-SC(O)NMe2}] (3) in 20, 16 and 10% yield, respectively, in addition to the known mononuclear complex [Os(CO)2{κ2(S,S)-S2CNMe2}2] (4) in 10% yield. The triosmium complex 1 has a linear arrangement of osmium atoms with each terminal osmium center containing a chelating Me2NCS2 ligand. The tetraosmium complex 2 also possesses a μ3-CO2 ligand in addition to capping sulfido and chelating Me2NCS2 ligands. Complex 3 contains an open triosmium core with the open Os···Os edge bridged by a Me2NC(O)S ligand and a capping Me2NCS ligand. A similar reaction between the unsaturated cluster [Os3(CO)10(μ-H)2] and [(Me2NCS2)2] yielded the triosmium complexes [Os3(CO)10{μ-κ1(S)-S2CNMe2}(μ-H)] (5) and [Os3(CO)9{μ3-κ2(S,S)-S2CNMe2}(μ-H)] (6) in addition to compounds 1 and 4 and the known hydroxyl cluster [Os3(CO)10(μ-OH)(μ-H)] (7) in 7, 14, 14, 10, and 6% yield, respectively. Both 5 and 6 possess a triosmium core, and the Me2NCS2 ligand acts in edge-bridging capacity in the former while it serves as a face-capping ligand in the latter. All new complexes have been characterized by combustion analyses and IR and NMR spectroscopies, and the solid-state structures of 1–3, and 6 have been established by X-ray crystallography. The bonding in clusters 1-3 and 6 has been examined by electronic structure calculations, and the thermodynamics for the formation of 1 and MeCN (two equiv) from [Os3(CO)10(MeCN)2] and tmtd, relative to other chelated and bridged isomers of [Os3(CO)10(tmtd)] are discussed.

Relativity or aromaticity? A first-principles perspective of chemical shifts in osmabenzene and osmapentalene derivatives.

We have studied the magnetic response properties and aromaticity of osmium metallacycles by means of scalar-relativistic (1c) and fully relativistic (4c) density functional theory computations. For osmabenzene, whose aromatic character is controversial, a topological analysis of the current density has revealed the presence of a unique σ-type Craig-Möbius magnetic aromaticity. We show that the partially filled osmium valence shell induces a large paratropic current, which may interfere with certain methods commonly used to analyze aromaticity, in particular NICS. Further, we show that the extreme deshielding of the light atoms in the vicinity of the osmium atoms in osmapentalene derivatives is not a consequence of aromaticity but can be explained by paramagnetic couplings between σOs-C bonding orbitals and the π*Os orbitals. We demonstrate that variations in the orientation of the induced magnetic currents through the molecule dictates the alternating signs of the spin-orbit contribution to the NMR chemical shift.