M?ssbauer spectroscopy for heavy elements: a relativistic benchmark study of mercury

The electrostatic contribution to the M?ssbauer isomer shift of mercury for the series HgF _n (n?=?1,?2,?4) with respect to the neutral atom has been investigated in the framework of four- and two-component relativistic theory. Replacing the integration of the electron density over the nuclear volume by the contact density (that is, the electron density at the nucleus) leads to a 10% overestimation of the isomer shift. The systematic nature of this error suggests that it can be incorporated into a correction factor, thus justifying the use of the contact density for the calculation of the M?ssbauer isomer shift. The performance of a large selection of density functionals for the calculation of contact densities has been assessed by comparing with finite-field four-component relativistic coupled-cluster with single and double and perturbative triple excitations [CCSD(T)] calculations. For the absolute contact density of the mercury atom, the Density Functional Theory (DFT) calculations are in error by about 0.5%, a result that must be judged against the observation that the change in contact density along the series HgF _n (n?=?1,?2,?4), relevant for the isomer shift, is on the order of 50?ppm with respect to absolute densities. Contrary to previous studies of the ⁵⁷Fe isomer shift (F Neese, Inorg Chim Acta 332:181, 2002), for mercury, DFT is not able to reproduce the trends in the isomer shift provided by reference data, in our case CCSD(T) calculations, notably the non-monotonous decrease in the contact density along the series HgF _n (n?=?1,?2,?4). Projection analysis shows the expected reduction of the 6s _1/2 population at the mercury center with an increasing number of ligands, but also brings into light an opposing effect, namely the increasing polarization of the 6s _1/2 orbital due to increasing effective charge of the mercury atom, which explains the non-monotonous behavior of the contact density along the series. The same analysis shows increasing covalent contributions to bonding along the series with the effective charge of the mercury atom reaching a maximum of around +2 for HgF₄ at the DFT level, far from the formal charge +4 suggested by the oxidation state of this recently observed species. Whereas the geometries for the linear HgF₂ and square-planar HgF₄ molecules were taken from previous computational studies, we optimized the equilibrium distance of HgF at the four-component Fock-space CCSD/aug-cc-pVQZ level, giving spectroscopic constants r _e = 2.007 ? and ?? _e = 513.5?cm^?1.     

Wet chemical synthesis and a combined X-ray and Mössbauer study of the formation of FeSb2 nanoparticles

Understanding how solids form is a challenging task, and few strategies allow for elucidation of reaction pathways that are useful for designing the synthesis of solids. Here, we report a powerful solution-mediated approach for formation of nanocrystals of the thermoelectrically promising FeSb(2) that uses activated metal nanoparticles as precursors. The small particle size of the reactants ensures minimum diffusion paths, low activation barriers, and low reaction temperatures, thereby eliminating solid-solid diffusion as the rate-limiting step in conventional bulk-scale solid-state synthesis. A time- and temperature-dependent study of formation of nanoparticular FeSb(2) by X-ray powder diffraction and iron-57 M?ssbauer spectroscopy showed the incipient formation of the binary phase in the temperature range of 200-250 °C.     

A combined Mössbauer spectroscopy and x-ray diffraction operando study of Sn-based composite anode materials for Li-ion accumulators

The reaction mechanisms of Li with Sn/BPO₄ composites to be used as negative electrode materials for Li-ion batteries were studied during electrochemical cycling by operando Mössbauer spectroscopy and X-ray diffraction using a specifically conceived in situ electrochemical cell. The starting composites consist of three main components: β-Sn particles as the electrochemically active species, an inactive matrix of BPO₄ and an amorphous Sn^II-borophosphate interfacial phase linking the two former components and improving the cohesion of the composite. During the first discharge, the latter Sn(II) species are first reduced to zerovalent tin forming Li-poor Li–Sn alloys. After its complete reduction, the reaction of Li continues with β-Sn leading to Li–Sn alloys increasingly rich in Li, with a final composition between those of Li₇Sn₂ and Li₁₃Sn₅. X-ray diffraction shows a progressive loss of long range order of the composites with the suppression of the diffraction peaks of the initial β-Sn and the formation of an ill-defined mixture of Li–Sn alloys. The evolution of this mechanism is investigated on going from a reference Sn/BPO₄ composite prepared by conventional ceramic methods with common micrometric BPO₄ to a new improved material prepared by carbothermal synthesis starting from nanometric BPO₄. With the new composite prepared by carbothermal synthesis, a significant improvement of the reversible capacity at the first cycle is obtained together with a slight improvement of the cycling behaviour. An additional improvement can be obtained by increasing the rate of the first discharge, and thus hampering the formation of the thermodynamically stable LiSn intermetallic.     

A study of carbonic anhydrase using Mössbauer spectroscopy

The preparation and characterization of an Fe(II) bovine carbonic and human carbonic anhydrase are presented. Results of Mössbauer spectroscopy measurements indicate that the Fe(II) (low spin) is bound at the enzymatically active site. The results are compared with previous studies which had used the technique of perturbed angular correlations of γ-radiation.     

Applications of Mössbauer spectroscopy to the study of electronic materials

A brief introduction to the methodology of the Mössbauer spectroscopy will be given. The main applications to the study of the intrinsic properties of the materials employed in the electronic technology are reviewed.The reported examples prove the effectiveness of this techniques in the study of semiconducting and magnetic materials.Evidence will be given also to the studies of preparation processes of crystalline and amorphous compound.     

The role of alpha-iron and cementite phases in the growing mechanism of carbon nanotubes: a 57Fe Mössbauer spectroscopy study

57Fe M?ssbauer spectroscopy was used to study the reduction behavior at temperatures as high as 1073 K of an iron/silica catalyst, and also the carbonaceous materials isolated after acetylene decomposition over this catalyst at several temperatures (873-1073 K). The products were previously characterized by transmission electron microscopy and it was clearly proven that the concentration of carbon nanotubes increased when reaction reached highest temperatures. This was related with an increment in cementite concentration (generated from initial alpha-iron and the progressive reduction of the remnant Fe+2 caused by acetylene decomposition) as detected by 57Fe M?ssbauer. These results undoubtedly revealed the role of alpha-iron as active center for acetylene decomposition and cementite as main carbide intermediate species in the catalytic growth of CNTs.     

M?ssbauer spectroscopy and structural analysis of solids

Mössbauer spectroscopy is reviewed as a method of analysis of hyperfine interactions in the solid state. It is sensitive both to the atomic scale and to phase structures. It utilizes the interactions between the hyperfine fields and nuclei in solids measured by a nuclear technique. The importance of various Mössbauer isotopes is discussed, the ⁵⁷Fe being still the most important. Principles of the qualitative determination of the structure sites and/or phase attachment are explained on the basis of the measurement of hyperfine structure parameters (i.e. the isomer (chemical) shift, the quadrupole and magnetic splittings). The role of the hyperfine field distribution determination is stressed, especially the magnetic hyperfine induction distribution in magnetically ordered solids. Conditions are explained for the feasibility of quantitative estimations of site occupancy and phase abundance. With respect to the predominant role of the magnetic hyperfine structure predestinating Mössbauer spectroscopy to be considered simultaneously as a special magnetic measuring technique, examples are chosen from the field of new magnetic materials. For the substituted hexagonal (M-type) ferrites (aimed, e.g., for the perpendicular magnetic recording), Mössbauer determination of the cation site occupancy is discussed. Structural changes in ion implanted Fe-B-based amorphous alloys detected by the hyperfine field distribution are shown. For the magnetically extremely soft FeCuNbSiB alloys, produced by the controlled crystallization of an amorphous ribbon, the estimation of their rather complicated phase composition by the Mössbauer phase analysis is demonstrated.Common enterprise of the Department of Low Temperature Physics with the Institute of Physics and Institute of Inorganic Chemistry, Czech Academy of Sciences, Prague     

Formation and structure of Co₂O₄: a combined IR matrix isolation and theoretical study

The formation and structure of dicobalt tetroxide (Co?O?) has been investigated using matrix isolation in solid neon and argon coupled to infrared spectroscopy and quantum chemical methods. It is found that Co?O? can be formed by dimerization of cobalt dioxide without activation energy by diffusion of ground state CoO? molecules at 9 K in the dark. The IR data on eight fundamentals, isotopic effects and quantum chemical calculations are both consistent with a centro-symmetrical structure with two pairs of equivalent oxygen atoms, engaged in a stronger terminal Co-O bond and in a weaker bridging Co-O-Co position. Evidence for other, metastable states is also presented, but the data are not conclusive. The electronic structure and formation pathway has been investigated using the Tao-Perdew-Staroverov-Scuseria/triple-zeta valence polarived basis set (TPSS/TZVP) and broken symmetry unrestricted density functional theory (BS-UDFT) approach and the ground electronic state is predicted to be an open shell 1Ag singlet with the quintet, triplet, septet, and nonet states above by 3.3, 4.9, 9.3, and 27.7 kcal/mol, respectively, but certainly has a complex multireference character that hinders the use of more precise multireference approaches. Different formation pathways have been considered, and the 2(O═Co═O) → Co?O? dimerization reaction is found to be the only barrierless channel and to be strongly exothermic. Comparisons with another transition metal (TM) oxide system (V?O?) suggests that the difference in predicted ground state geometries in TM?O? systems might be due in HOMO-LUMO shapes of the isolated dioxide subunits and optimal overlap configurations.     

Toward identification of the compound I reactive intermediate in cytochrome P450 chemistry: a QM/MM study of its EPR and Mössbauer parameters

Quantum mechanical/molecular mechanical (QM/MM) methods have been used in conjunction with density functional theory (DFT) and correlated ab initio methods to predict the electron paramagnetic resonance (EPR) and Mossbauer (MB) properties of Compound I in P450(cam). For calibration purposes, a small Fe(IV)-oxo complex [Fe(O)(NH(3))(4)(H(2)O)](2+) was studied. The (3)A(2) and (5)A(1) states (in C(4)(v)() symmetry) are found to be within 0.1-0.2 eV. The large zero-field splitting (ZFS) of the (FeO)(2+) unit in the (3)A(2) state arises from spin-orbit coupling with the low-lying quintet and singlet states. The intrinsic g-anisotropy is very small. The spectroscopic properties of the model complex [Fe(O)(TMC)(CH(3)CN)](2+) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) are well reproduced by theory. In the model complexes [Fe(O)(TMP)(X)](+) (TMP = tetramesitylporphyrin, X = nothing or H(2)O) the computations again account for the observed spectroscopic properties and predict that the coupling of the (5)A(1) state of the (FeO)(2+) unit to the porphyrin radical leads to a low-lying sextet/quartet manifold approximately 12 kcal/mol above the quartet ground state. The calculations on cytochrome P450(cam), with and without the simulation of the protein environment by point charges, predict a small antiferromagnetic coupling (J approximately -13 to -16 cm(-)(1); H(HDvV) = - 2JS(A)S(B)) and a large ZFS > 15 cm(-)(1) (with E/D approximately 1/3) which will compete with the exchange coupling. This leads to three Kramers doublets of mixed multiplicity which are all populated at room temperature and may therefore contribute to the observed reactivity. The MB and ligand hyperfine couplings ((14)N, (1)H) are fairly sensitive to the protein environment which controls the spin density distribution between the porphyrin ring and the axial cysteinate ligand.     

Mimicking anaesthetic-receptor interaction: a combined spectroscopic and computational study of propofol···phenol

Propofol is a general anaesthetic that exerts its action by interaction with the GABA(A) receptor. Crystallographic studies suggest that there is a direct interaction between propofol and the phenolic residue of a tyrosine in the channel. In this study we create propofol···phenol clusters by their co-expansion in jets. The complex is probed using a set of mass-resolved spectroscopic strategies: 2-color REMPI, UV/UV hole-burning, IR/UV double resonance and the novel technique IR/IR/UV triple resonance. The existence of at least six different isomers in the expansion is demonstrated. All the isomers are stabilized by interactions between their aromatic rings. Additionally, in some conformers the OH moieties form hydrogen bonds in some of the isomers, with propofol and phenol alternating their donor-acceptor roles, while in others the -OH···OH angle points to a dipole-dipole interaction. Interpretation of the data in the light of dispersion-corrected DFT calculations shows that shallow barriers separate all the isomers, both in the ground and excited electronic states. Comparison of the structures of the complex with the X-ray diffraction data is also offered.     

Argentinean prehistoric pigments’ study by combined SEM/EDX and molecular spectroscopy

Composition of the prehistoric pigments’ (from Carriqueo rock shelter, Rio Negro province, Argentina) has been analysed by means of molecular spectroscopy (Fourier transform infrared (FTIR) and micro-Raman) and scanning electron microscopy (SEM) coupled to an energy-dispersive X-ray spectrometer (EDS). Red and yellow pigments were recognized as red and yellow ochre. The matrix of the pigments is composed of one or more substances. According to the matrix composition yellow and red pigments were also divided into two groups—i.e. those containing kaolinite or sulphates. Green pigment was detected as green earth, made up of celadonite as a chromophore.     

Molecular recognition of endocrine disruptors by synthetic and natural 17β-estradiol receptors: a comparative study

A β-estradiol receptor binding mimic was synthesised using molecular imprinting. Bulk polymers and spherical polymer nanoparticles based on methacrylic acid and ethylene glycol dimethacrylate as the functional monomer and crosslinker, respectively, were prepared in acetonitrile. The selectivity was evaluated by radioligand binding assays. The imprinted polymers were very specific to β-estradiol since the control polymers bound virtually none of the radioligand. The bulk polymer was then employed to screen endocrine disrupting chemicals. Structurally related steroids like α-estradiol, estrone and ethynylestradiol showed, respectively, 14.0, 5.0 and 0.7% of relative binding to the β-estradiol polymer, whereas most unrelated chemicals did not bind at all. These results are compared to those obtained with a bioassay using stably transfected yeast cells in culture bearing the human estrogen receptor. The receptor was activated by several estrogen-like chemicals and to a lesser extent by some structurally related chemicals. Figure A molecularly imprinted polymer that was a synthetic receptor for beta-estradiol was used for the screening of endocrine disrupting chemicals that are structurally related or unrelated to beta-estradiol. The results were compared with the recognition of the compounds by the biological estrogen receptor expressed in yeast cells. Related steroids like alpha-estradiol, estrone and ethynylestradiol showed significant binding to the beta-estradiol imprinted polymer, whereas most unrelated chemicals did not bind. The biological receptor was activated by several estrogen-like chemicals, and to a lesser extent by some structurally related chemicals     

Coordination of adenosylmethionine to a unique iron site of the [4Fe-4S] of pyruvate formate-lyase activating enzyme: a Mössbauer spectroscopic study

Pyruvate formate-lyase activating enzyme (PFL-AE) generates the catalytically essential glycyl radical of PFL. It is a member of the so-called "radical-SAM superfamily" of enzymes that use a [4Fe-4S] cluster and S-adenosylmethionine (AdoMet or SAM) to catalyze diverse radical-mediated reactions. Evidence suggests that this class of enzymes operate by common initial steps involving the generation of an AdoMet-derived adenosyl radical intermediate, of which the mechanism remains unresolved. The three-cysteine CX3CX2C cluster-binding motif common to all members of this superfamily suggests a unique Fe site in the [4Fe-4S] cluster, which presumably interacts with AdoMet to effect the reductive cleavage and radical generation. Here we employ a dual-iron-isotope (56Fe/57Fe) approach to demonstrate the existence of a unique Fe site in the [4Fe-4S] cluster of PFL-AE by M?ssbauer spectroscopy. Coordination of AdoMet to this unique Fe site was made evident by the observation of a substantial increase in the isomer shift (delta) of the M?ssbauer spectrum associated with the unique Fe site: delta = 0.42 mm/s in the absence of AdoMet increases to delta = 0.72 mm/s in the presence of AdoMet. Further, the M?ssbauer data show that the binding of AdoMet to the unique Fe site occurs in the [4Fe-4S]2+ state, prior to the injection of the reducing equivalent required for catalysis. This observation indicates that AdoMet coordination is a necessary prerequisite to adenosyl radical generation.     

Coordination chemistry of silones: Synthesis and reactivity of new silicon-disubstituted (η4-silole) complexes

The synthesis of some new silicon-disubstituted (η⁴-2,5-diphenylsilacyclopentadiene)tricarbonyliron complexes are described. Stable complexes with various functional groups attached at silicon have been isolated. The exo position shows an enhanced reactivity, and cleavage of an SiH bond at this position occurs selectively with retention.     

Phosphorus-fluorine chemistry—XXXVI: Preparation and chemistry of thienyldifluorophosphine

2-Thienyldifluorophosphine has been prepared by chlorine-fluorine exchange in the corresponding dichlorophosphine; the difluorophosphine has been found to undergo redox disproportionation with formation of 2-thienyltetrafluorophosphorane and pentathienylcyclopentaphosphine. Upon reaction with nickel carbonyl, 2-thienyldifluorophosphine forms a tetrasubstituted derivative which may be oxidized by iodine to give tris(thienyldifluorophosphine)nickel(II)iodide. Di- and tri-substituted derivatives of molybdenum carbonyl have been obtained through displacement of the coordinated cycloolefin by the fluorophosphine in bicycloheptadienetetracarbonyl molybdenum and cycloheptatrient-tricarbonyl molybdenum. All compounds were characterized by i.r., n.m.r., and mass spectrometry. A comparison of δ_P values for thienyl-phosphorus compounds with data for the analogous phenyl-phosphorus compounds has shown a high-field shift for the thiophene derivatives in every case. A comparison of δ_F values has shown the opposite effect.     

Coordination asymmetry in μ-oxido divanadium complexes: Development of synthetic protocols

This brief review deals with the development of a general protocol for the synthesis of μ-oxido divanadium(V) compounds [LOV^V-(μ-O)-V^VO(Salen)] (L = L¹–L⁵) (1–5) incorporating coordination asymmetry. One of the vanadium centers in these compounds has an octahedral environment, completed by tetradentate Salen ligand, while the other center has a square pyramidal geometry, made up of tridentate biprotic Schiff-base ligands (H₂L^1–5) with ONO (1–3) and ONS (4, 5) type donor combinations. Single crystal X-ray diffraction, ESI-MS, and multi-nuclear NMR (¹H and ⁵¹V) spectroscopy have been used extensively for the characterization of these compounds. The V₂O₃ core in these compounds, save 3, has a rare type of twist-angular structure. The V(1)?V(2) separations (3.7921(7)–3.3084(6) Å) are by far the largest in these compounds compared to their peers containing a V₂O₃ core. The molecules retain their unsymmetrical binuclear structures also in solution as established by NMR spectroscopy. The mixed-oxidation compound (ImH)[L⁴OV^IV-(μ-O)-V^VOL⁵] 7 containing two dissimilar ligands has a V₂O₃ core with a syn-angular structure and exhibits crystallographically imposed mirror symmetry due to static disorder. In solution of donor solvents, this angular core structure changes into a linear one (anti-linear) by accepting solvents in to the vacant coordination site of the metal centers. Finally, the protocol for the synthesis of heterobimetallic compounds with vanadium(V) and Re(VII) combination flanked by a single μ-oxido bridge has been developed in which the precursor complexes [V^IVOL^6,7] (H₂L^6,7 are Salen type of ligands) are allowed to oxidize aerially in the presence of added perrhenate anion. The oxidized [V^VOL^6,7]⁺ species hold the ReO₄^? anion in the vacant coordination site of the metal ion, trans to the terminal oxido group, thus generating the V^V–O–Re^VII moiety in the heterobimetallic compounds (9 and 10). Both X-ray crystallography and ¹H NMR spectroscopy have been used to establish the identities of these compounds. In compound 9, the Re(1)–O(11)–V(1) bridge angle is barely linear (170.2(3)°) with a Re?V separation of 3.9647(9) Å. The redox behavior of 9 and 10 are quite interesting, each undergoing two reductions both in the positive potential range at E_1/2 = 0.59 and 0.16 V vs. Ag/AgCl reference and have single-electron stoichiometry, confirmed by constant potential coulometry.     

Fe- and Cr-substituted mullites: Mössbauer spectroscopy and rietveld structure refinement

Fe-substituted mullite samples Al_4.5-yFe_ySi_1.5O_9.75 (y = 0.25 and 0.50) have been prepared using a sol-gel route involving Si and Al alkoxides and iron nitrate as precursors. Mössbauer spectroscopy reveals the presence of three different Fe(III) sites but does not allow to conclude concerning the ion environments, tetrahedral or octahedral. Rietveld refinements reveal that Fe ions are distributed between octahedral and tetrahedral positions, the proportion of octahedral iron increasing from y = 0.25 to y = 0.50. On the other hand, for Cr-substituted mullite, Cr ions arc mainly located on octahedral positions. This difference between Fe and Cr has been attributed to the fact that Cr shows a clear trend to occupy octahedral sites compared to Fc and that it is incorporated inside the mullite structure only at high temperature.     

Mössbauer spectroscopy of bucky ferrocenes: lattice dynamics and motional anisotropy of the metal atom

Temperature-dependent (57)Fe M?ssbauer effect spectroscopy has been used to elucidate the metal atom dynamics in three neutral and two cationic bucky ferrocenes. For the three diamagnetic complexes Fe(C(60)H(5))Cp (1), Fe(C(60)Me(5))Cp (2), and Fe(C(60)Ph(5))Cp (3), the metal atom motion is anisotropic and the temperature dependence of the mean-square amplitude of vibration of the metal atom at a number of temperatures is reported. The M?ssbauer lattice temperatures have been determined and compared to the parent ferrocene (6). The synthesis and X-ray crystal structure of 3 have been determined at 153(2) K, and the (1)H and (13)C NMR spectra have been recorded. The cationic complexes derived from 2 and 3 show spin-lattice relaxation. The relaxation rate appears insensitive to the nearest-neighbor environment of the metal atom in this pair.