Computational studies of EPR parameters for paramagnetic molybdenum complexes. II. Larger MoV systems relevant to molybdenum enzymes

The careful validation of modern density functional methods for the computation of electron paramagnetic resonance (EPR) parameters in molybdenum complexes has been extended to a number of low-symmetry MoV systems that model molybdoenzyme active sites. Both g and hyperfine tensors tend to be reproduced best by hybrid density functionals with about 30-40% exact-exchange admixture, with no particular spin contamination problems encountered. Spin-orbit corrections to hyperfine tensors are mandatory for quantitative and, in some cases, even for qualitative agreement. The g11 (g||) component of the g tensor tends to come out too positive when spin-orbit coupling is included only to leading order in perturbation theory. Compared to single-crystal experiments, the calculations reproduce both g- and hyperfine-tensor orientations well, both relative to each other and to the molecular framework. This is significant, as simulations of the EPR spectra of natural-abundance frozen-solution samples frequently do not allow a reliable determination of the hyperfine tensors. These may now be extracted based on the quantum-chemically calculated parameters. In a number of cases, revised simulations of the experimental spectra have brought theory and experiment into substantially improved agreement. Systems with two terminal oxo ligands, and to some extent with an oxo and a sulfido ligand, have been confirmed to exhibit particularly large negative Deltag33 shifts and thus large g anisotropies. This is discussed in the context of the experimental data for xanthine oxidase.     

New molybdenum compound suitable for determination by electron paramagnetic resonance

Molybdenum reacted with diethyldithiocarbamate showed a single line at g=1.980 in electron paramagnetic resonance spectrometry at room temperature after reduction by oxalic acid. Molybdenum in 10 μl of urine could be quantitated within 5 min with the detection limit of 50 pg.     

Thermal and electron paramagnetic resonance studies on pyridine perchromate

A number of physical techniques including scanning calorimetry and electron paramagnetic resonance have been used in an attempt to elucidate the course of the decomposition reaction of pyridine perchromate.     

Synthesis, structural characterization, and multifrequency electron paramagnetic resonance studies of mononuclear thiomolybdenyl complexes

Reaction of Tp*MoVSCl2 with a variety of phenols and thiols in the presence of triethylamine produces mononuclear, thiomolybdenyl complexes Tp*MoVSX2 [Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate; X = 2-(ethylthio)phenolate (etp), 2-(n-propyl)phenolate (pp), phenolate; X2 = benzene-1,2-dithiolate (bdt), 4-methylbenzene-1,2-dithiolate (tdt), benzene-1,2-diolate (cat)]. The complexes have been characterized by microanalysis, mass spectrometry, IR, EPR, and UV-visible spectroscopic data, and X-ray crystallography (for the etp, pp, bdt, and cat derivatives). The mononuclear, six-coordinate, distorted-octahedral Mo centers are coordinated by terminal sulfido (MoS = 2.123(1)-2.1368(8) A), tridentate facial Tp*, and monodentate or bidentate O/S-donor ligands. Multifrequency (S-, X-, Q-band) EPR spectra of the complexes and selected molybdenyl analogues were acquired at 130 K and 295 K and yielded a spin Hamiltonian of Cs symmetry or lower, with gzz < gyy < gxx < ge and Az'z' > Ax'x' approximately Ay'y', and a noncoincidence angle in the range of beta = 24-39 degrees . Multifrequency EPR, especially at S-band, was found to be particularly valuable in the unambiguous assignment of the spin Hamiltonian parameters in these low-symmetry complexes. The weaker pi-donor terminal sulfido ligand yields a smaller SOMO-LUMO gap and reduced g-values for the thiomolybdenyl complexes compared with molybdenyl analogues, supporting existing crystallographic and EPR data for an apically coordinated oxo group in the active site of xanthine oxidase.     

Spin hamiltonian parameters for Cu(II)-prion peptide complexes from L-band electron paramagnetic resonance spectroscopy

Cu(II) is an essential element for life but is also associated with numerous and serious medical conditions, particularly neurodegeneration. Structural modeling of crystallization-resistant biological Cu(II) species relies on detailed spectroscopic analysis. Electron paramagnetic resonance (EPR) can, in principle, provide spin hamiltonian parameters that contain information on the geometry and ligand atom complement of Cu(II). Unfortunately, EPR spectra of Cu(II) recorded at the traditional X-band frequency are complicated by (i) strains in the region of the spectrum corresponding to the g(∥) orientation and (ii) potentially very many overlapping transitions in the g(⊥) region. The rapid progress of density functional theory computation as a means to correlate EPR and structure, and the increasing need to study Cu(II) associated with biomolecules in more biologically and biomedically relevant environments such as cells and tissue, have spurred the development of a technique for the extraction of a more complete set of spin hamiltonian parameters that is relatively straightforward and widely applicable. EPR at L-band (1-2 GHz) provides much enhanced spectral resolution and straightforward analysis via computer simulation methods. Herein, the anisotropic spin hamiltonian parameters and the nitrogen coordination numbers for two hitherto incompletely characterized Cu(II)-bound species of a prion peptide complex are determined by analysis of their L-band EPR spectra.     

Electron paramagnetic resonance for everybody--MICROspec-X--a new class of electron paramagnetic resonance spectrometer

The electron paramagnetic resonance spectroscopy is the only method for detecting free radicals. Free radicals have an increased importance in our daily life. A small transportable EPR spectrometer is presented for the popularisation of the EPR method. The technical construction and some applications are illustrated which show the usability of the spectrometer.     

Some recent multi-frequency electron paramagnetic resonance results on systems relevant for dosimetry and dating

Electron Paramagnetic Resonance (EPR) applications like e.g. EPR dosimetry and dating, are usually performed at X-band frequencies because of practical reasons (cost, sample size, etc.). However, it is increasingly recognized that the radiation-induced EPR signals are strongly composite, what might affect dose/age estimates. A few recent examples from both the dosimetry and dating field, illustrating the problems, will be presented. The involved spectra are mainly due to carbonate-derived radicals (CO2-, CO3(3-), etc.). Measurements at higher microwave frequencies are often recommended to improve the insight into the spectra and/or the practical signal quantification. Recent results at Q- and W-band frequencies will show that a multi-frequency approach indeed opens many interesting perspectives in this field but also that each frequency may have specific (dis)advantages depending on the EPR probe and application involved. The discussion will concern carbonate-containing apatite single crystals, shells, modern and fossil tooth enamel.     

Density functional restricted-unrestricted approach for nonlinear properties: application to electron paramagnetic resonance parameters of square planar copper complexes

The density functional restricted-unrestricted approach for treatments of spin polarization effects in molecular properties using spin restricted Kohn-Sham theory has been extended from linear to nonlinear properties. It is shown that the spin polarization contribution to a nonlinear property has the form of a quadratic response function that includes the zero-order Kohn-Sham operator, in analogy to the lower order case where the spin polarization correction to an expectation value has the form of a linear response function. The developed approach is used to formulate new schemes for computation of electronic g-tensors and hyperfine coupling constants, which include spin polarization effects within the framework of spin restricted Kohn-Sham theory. The proposed computational schemes are in the present work employed to study the spin polarization effects on electron paramagnetic resonance spin Hamiltonian parameters of square planar copper complexes. The obtained results indicate that spin polarization gives rise to sizable contributions to the hyperfine coupling tensor of copper in all investigated complexes, while the electronic g-tensors of these complexes are only marginally affected by spin polarization and other factors, such as choice of exchange-correlation functional or molecular structures, will have more pronounced impact on the accuracy of the results.     

Comparative density-functional study of the electron paramagnetic resonance parameters of amavadin

Electronic g tensors and hyperfine coupling tensors have been calculated for amavadin, an unusual eight-coordinate vanadium(IV) complex isolated from Amanita muscaria mushrooms. Different density-functional methods have been compared, ranging from local via gradient-corrected to hybrid functionals with a variable Hartree-Fock exchange admixture. For both electron paramagnetic resonance (EPR) properties, hybrid functionals with an appreciable exact-exchange admixture provide the closest agreement with experimental data. Second-order spin-orbit corrections provide non-negligible contributions to the 51V hyperfine tensor. The orientation of g and A tensors relative to each other also depends on spin-orbit corrections to the A tensor. A rationalization for the close resemblance of the EPR parameters of amavadin to those of the structurally rather different vanadyl complexes is provided, based on the nature of the relevant frontier orbitals.     

Vibrational and electron paramagnetic resonance properties of free and MgO supported AuCO complexes

The bonding, spin density related properties, and vibrational frequency of CO bound to single Au atom in the gas-phase or supported on MgO surfaces have been investigated with a variety of computational methods and models: periodic plane waves calculations have been compared with molecular approaches based on atomic orbital basis sets; pseudopotential methods with all electron fully relativistic calculations; various density functional theory (DFT) exchange-correlation functionals with the unrestricted coupled-cluster singles and doubles with perturbative connected triples [CCSD(T)]. AuCO is a bent molecule but the potential for bending is very soft, and small changes in the bond angle result in large changes in the CO gas-phase vibrational frequency. At the equilibrium geometry the DFT calculated vibrational shift of CO with respect to the free molecule is about -150 cm(-1), whereas smaller values -60-70 cm(-1) are predicted by the more accurate CCSD(T) method. These relatively large differences are due to the weak and nonclassic bonding in this complex. Upon adsorption on MgO, the CO vibrational shift becomes much larger, about -290 cm(-1), due to charge transfer from the basic surface oxide anion to AuCO. This large redshift is predicted by all methods, and is fully consistent with that measured for MgOAuCO complexes. The strong influence of the support on the AuCO bonding is equally well described by all different approaches.     

Thermally stimulated luminescence and electron paramagnetic resonance studies on uranium doped calcium phosphate

Thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) studies on uranium doped calcium phosphate yielded mechanistic information on the observed glow peaks at 365, 410 and 450 K. TSL spectral studies of the glow peaks showed that UO₂ ²⁺ acts as the luminescent center. Electron paramagnetic resonance studies on gamma-irradiated samples revealed that the predominant radiation induced centers are H⁰, PO₄ ^2-, PO₃ ^2- and O^- ion. Studies on the temperature dependence studies of the EPR spectra of samples annealed to different temperatures indicate the role of H⁰ and PO₄ ^2- ions in the main glow peak at 410 K.     

Electron paramagnetic resonance studies on copper(II)-cyclodextrin systems

Three copper(II) complexes containing beta-cyclodextrin (betaCD) derivatives as ligands [mono-6-methylamino-6-deoxy-betaCD dithiocarbamate (CDTC), mono-6-histamino-6-deoxy-betaCD (CDHM) and mono-6-Nalpha-arginyl-6-deoxy-betaCD (CDARG)] have been studied by electron paramagnetic resonance. The spectra have been recorded at X- and S-bands and different temperatures and simulated to obtain the best set of magnetic parameters. In particular, the 300 K spectra are typical of the slow motion regime, as expected considering the high molecular weight of the ligands. Some structural characteristics of the complexes are proposed on the basis of dynamic and covalency parameters obtained from simulation.     

Computational modeling of isotropic electron paramagnetic resonance spectra of doublet state main group radicals

The combined use of theoretical and mathematical methods in the analysis of electron paramagnetic resonance data has greatly increased the ability to interpret even the most complex spectra reported for doublet state inorganic main group radicals. This personal account summarizes the theoretical basis of such an approach and provides an in-depth discussion of some recent illustrative examples of the utilization of this methodology in practical applications. The emphasis is on displaying the enormous potential embodied within the approach.     

Supramolecular metal-polypyridyl and Ru(II) porphyrin complexes: photophysical, electron paramagnetic resonance, and electrochemical studies

A series of mixed-metal supramolecular porphyrin arrays in which the geometry of the central metal-polypyridyl moiety defines the spatial arrangement of two or more Ru(II)-porphyrin units through axial coordination have been prepared by employing self-assembly based protocols, and their photophysical and electrochemical properties have been studied. The electrochemical properties of the constituent parts of these arrays depend only on their own chemical environment, regardless of the nuclearity and the overall charge of the compound; in this way species with predetermined redox patterns can be obtained via the synthetic control of the self-assembly process. Interestingly, several of these arrays are luminescent both at room and at low temperatures, and in many cases core-to-periphery or periphery-to-core intramolecular energy transfer processes take place according to the nature of the central metal template.     

Identification of diluted small defect clusters in silicon by electron paramagnetic resonance

The possibilities to identify small diluted paramagnetic cluster defects in solids by Electron Paramagnetic Resonance are demonstrated in two examples: the Mn ₄ ⁰ cluster and a Cu-Au cluster in silicon crystals. A comprehensive picture of the clusters can be given containing the chemical nature of the constituents, the spatial arrangement, the charge states and the electronic structure.