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Spin Density (spin + density)
Selected AbstractsSpin densities in two-component relativistic density functional calculations: Noncollinear versus collinear approachJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 8 2002Christoph Van WüllenArticle first published online: 3 APR 200 Abstract With present day exchange-correlation functionals, accurate results in nonrelativistic open shell density functional calculations can only be obtained if one uses functionals that do not only depend on the electron density but also on the spin density. We consider the common case where such functionals are applied in relativistic density functional calculations. In scalar-relativistic calculations, the spin density can be defined conventionally, but if spin-orbit coupling is taken into account, spin is no longer a good quantum number and it is not clear what the "spin density" is. In many applications, a fixed quantization axis is used to define the spin density ("collinear approach"), but one can also use the length of the local spin magnetization vector without any reference to an external axis ("noncollinear approach"). These two possibilities are compared in this work both by formal analysis and numerical experiments. It is shown that the (nonrelativistic) exchange-correlation functional should be invariant with respect to rotations in spin space, and this only holds for the noncollinear approach. Total energies of open shell species are higher in the collinear approach because less exchange energy is assigned to a given Kohn-Sham reference function. More importantly, the collinear approach breaks rotational symmetry, that is, in molecular calculations one may find different energies for different orientations of the molecule. Data for the first ionization potentials of Tl, Pb, element 113, and element 114, and for the orientation dependence of the total energy of I and PbF indicate that the error introduced by the collinear approximation is ,0.1 eV for valence ionization potentials, but can be much larger if highly ionized open shell states are considered. Rotational invariance is broken by the same amount. This clearly indicates that the collinear approach should not be used, as the full treatment is easily implemented and does not introduce much more computational effort. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 779,785, 2002 [source] Spin densities in parabolic quantum wires with Rashba spin-orbit interactionPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2006Sigurdur I. Erlingsson Abstract Using canonical transformations we diagonalize approximately the Hamiltonian of a gaussian wire with Rashba spin-orbit interaction. This proceedure allows us to obtain the energy dispersion relations and the wavefunctions with good accuracy, even in systems with relatively strong Rashba coupling. With these eigenstates one can calculate the non-equilibrium spin densities induced by applying bias voltages across the sample. We focus on the z -component of the spin density, which is related to the spin Hall effect. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Theory of chemical bonds in metalloenzymes.INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2010Abstract Reaction mechanisms of oxygen evolution in native and artificial photosynthesis II (PSII) systems have been investigated on the theoretical grounds, together with experimental results. First of all, our previous broken-symmetry (BS) molecular orbitals (MO) calculations are reviewed to elucidate the instability of the d,-p, bond in high-valent (HV) Mn(X)O systems and the d,-p,-d, bond in HV MnOMn systems. The triplet instability of these bonds entails strong or intermediate diradical characters: ,Mn(IV)O, and ,MnOMn,; the BS MO resulted from strong electron correlation, leading to the concept of electron localizations and local spins. The BS computations have furthermore revealed guiding principles for derivation of selection rules for radical reactions of local spins. As a continuation of these theoretical results, the BS MO interaction diagrams for oxygen-radical coupling reactions in the oxygen evolution complex (OEC) in the PSII have been depicted to reveal scope and applicability of local singlet diradical (LSD) and local triplet diradical (LTD) mechanisms that have been successfully utilized for theoretical understanding of oxygenation reactions mechanisms by p450 and methane monooxygenase (MMO). The manganese-oxide cluster models examined are London, Berlin, and Berkeley models of CaMn4O4 and related clusters Mn4O4 and Mn3Ca. The BS MO interaction diagrams have revealed the LSD and/or LTD mechanisms for generation of molecular oxygen in the total low-, intermediate and high-spin states of these clusters. The spin alignments are found directly corresponding to the spin-coupling mechanisms of oxygen-radical sites in these clusters. The BS UB3LYP calculations of the clusters have been performed to confirm the comprehensive guiding principles for oxygen evolution; charge and spin densities by BS UB3LYP are utilized for elucidation and confirmation of the LSD and LTD mechanisms. Applicability of the proposed selection rules are examined in comparison with a lot of accumulated experimental and theoretical results for oxygen evolution reactions in native and artificial PSII systems. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] A DFT study of two diiron (II) synthetic model compounds and their diiron(III) peroxide oxygenation productsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2009R. C. Binning Jr. Abstract Unrestricted density functional theory calculations have been conducted on two diiron(II) synthetic model compounds. Calculations employed the BPW91 and BOP density functionals with both high-spin and broken symmetry low-spin representations of weakly coupled high-spin irons. Comparison of the calculated and crystallographic structures is made, and good agreement is found with both spin representations. Raman spectral data are available for the diiron(III) product of the reaction with O2 to form a bridged peroxide. Calculated harmonic frequencies confirm the experimental assignments. Small geometry differences between the high spin and broken symmetry results are seen in bond lengths, angles, Raman frequencies, and spin densities associated with oxo and peroxo bridges in the diiron(III) oxidation products. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] MaSK: A visualization tool for teaching and research in computational chemistryINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2009Yevgeniy Podolyan Abstract The number of researchers using computational chemistry tools is growing every year. There are multiple programs used for calculation of various molecular and electronic properties such as optimized geometry, energy, vibrational spectra, and so forth. Another set of programs is used for the visualization of these properties. However, such programs are either too complex for a beginner or too simple for an intermediate user for everyday use. Molecular Modeling and Simulation Kit (MaSK) is designed to fill this gap by presenting an easy-to-use intuitive interface to quantum chemical programs such as GAMESS and Gaussian with an array of advanced tools. The program can be used as a postprocessor to visualize calculated properties or as a preprocessor to prepare the input files for quantum chemical programs. In addition, some properties such as the surfaces of molecular orbitals, electron and spin densities, and molecular electrostatic potentials are actually calculated by MaSK. If MaSK is combined with PC GAMESS, the preparation of the input, running of the calculations, and the display of the results can all be done without leaving the program's interface. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Spin densities in parabolic quantum wires with Rashba spin-orbit interactionPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2006Sigurdur I. Erlingsson Abstract Using canonical transformations we diagonalize approximately the Hamiltonian of a gaussian wire with Rashba spin-orbit interaction. This proceedure allows us to obtain the energy dispersion relations and the wavefunctions with good accuracy, even in systems with relatively strong Rashba coupling. With these eigenstates one can calculate the non-equilibrium spin densities induced by applying bias voltages across the sample. We focus on the z -component of the spin density, which is related to the spin Hall effect. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] X-ray constrained unrestricted Hartree,Fock and Douglas,Kroll,Hess wavefunctionsACTA CRYSTALLOGRAPHICA SECTION A, Issue 1 2010Martin Hudák The extension of the X-ray constrained (XC) wavefunction approach to open-shell systems using the unrestricted Hartree,Fock formalism is reported. The XC method is also extended to include relativistic effects using the scalar second-order Douglas,Kroll,Hess approach. The relativistic effects on the charge and spin density on two model compounds containing the copper and iron atom are reported. The size of the relativistic effects is investigated in real and reciprocal space; in addition, picture-change effects are investigated and discussed for the isolated Cu atom. It is found that the relativistic terms lead to changes in the densities that are much smaller than those from the X-ray constraint. Nevertheless, the use of the relativistic corrections in the ab initio model always leads to an improvement in the agreement statistics. An interesting result of the unrestricted XC technique is the possibility of obtaining experimentally derived spin densities from X-ray data. [source] Magnetic Properties of a Ni2+ Kagome SystemCHEMPHYSCHEM, Issue 2 2007J. N. Behera Magnetic frustration: The diverse nature of interactions occurring within kagome structures arise due to the interplay between the frustrated kagome geometry and the integer spin moment of the magnetic ions (see figure showing spin densities). Distortion in the kagome plane leads to Dzyaloshinsky-Moriya (DM) interactions at low temperatures giving rise to partial magnetic polarization. [source] Theoretical Design of High-spin Organic Molecules with ,·N,N, as a Spin-containing Fragment and Heterocycles as an End GroupCHINESE JOURNAL OF CHEMISTRY, Issue 11 2006De-Qing Chu Abstract Novel stable high spin molecules possessing three different arranged fashions are designed with ,·N,N< as a spin-containing (SC) fragment, various aromatic, such as benzene (1), pyridine (2), pyridazine (3), pyrimidine (4), pyrazine (5), triazine (6) as end groups (EG) and phenyl as a ferromagnetic coupling (FC) unit. The effects of a different end groups on the spin multiplicities of the ground states and their stabilities were investigated by means of AM1-CI approach. It has been found that the spin densities on the two atoms of the SC fragment are different from delocalization resulting in the specific stability of ,·N,N<. In these molecules, the stabilities of the triplet states decrease when the distance between the atoms of central SC (,N,) increases. The orders of the stability of triplet states for 1an, 1bn, 1cn[They are isomers in which SC is connected with FC in different way (1an, N1NNN1; 1bn, N1N N1N; 1cn, NN1N1N) and six heterocycles are EG] show that the stability of triplet states with heterocycles as end groups is higher than that with phenyl as end groups, and in the order:triazine (EG)>pyrimidine, pyrazine>pyridine, pyridazine. [source] Aerosols and gaseous contrast agents for magnetic resonance imaging of the lungCONTRAST MEDIA & MOLECULAR IMAGING, Issue 5 2008Karim Mosbah Abstract Magnetic resonance imaging of lungs and the investigation of pulmonary pathologies with this technique are limited by low proton spin density, degraded magnetic homogeneity and motion. Inhaled contrast agents (gases or aerosols) can improve the diagnostic value of MRI for lung. Paramagnetic contrast agents such as gadolinium chelates aerosol or dioxygen gas increase the relaxivity of proton in lung parenchyma and can be used to assess the ventilated fraction of the bronchoalveolar space. Similarly, inhalation of non proton-MRI nuclei such as perfluorinated gas or hyperpolarized gases (3He or 129Xe) can provide functional ventilation image. In this review paper, the principles, the practical implementation, the limitations and possible safety issues of these different techniques are summarized. The main pre-clinical and clinical applications of these approaches based on oral contrast agents are reviewed and illustrated with cutting-edge lung MRI studies. Copyright © 2008 John Wiley & Sons, Ltd. [source] Mono- and Binuclear Arylnickel Complexes of the ,-Diimine Bridging Ligand 2,2,-Bipyrimidine (bpym)EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 6 2010Axel Klein Abstract The mono- and binuclear organometallic NiII complexes [(,-bpym){Ni(Mes)Br}n] (bpym = 2,2,-bipyrimidine; n = 1 or 2; Mes = mesityl = 2,4,6-trimethylphenyl) were prepared and characterised electrochemically and spectroscopically (NMR, UV/Vis/NIR) in detail. The long-wavelength absorptions for the binuclear complex reveal a marked electronic coupling of the two metal centres over the ligand bridge via their low-lying ,*-orbitals. While the mononuclear complex undergoes rapid dissociation of the bromido ligand after one-electron reduction the binuclear derivative exhibits reversible reductive electrochemistry and both of them yield stable radical anionic complexes with mainly bpym ligand centred spin density as shown by EPR spectroscopy of the free ligand bpym and the nickel complexes. The molecular structure of the binuclear bpym complex [(,-bpym){Ni(Mes)Br}2] was studied by EXAFS in comparison to the mononuclear analogue [(bpym)Ni(Mes)Br] revealing markedly increased Ni,C/N distance of the first coordination shell for the binuclear derivative suggesting an optimum overlap for the mononuclear complex, while two nickel complex fragments {Ni(Mes)Br} are seemingly too large to fit into the bis-chelate coordination site. [source] EPR Insensitivity of the Metal-Nitrosyl Spin-Bearing Moiety in Complexes [LnRuII -NO·]kEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 14 2004Stéphanie Frantz Abstract A survey of 18 paramagnetic species [LnRu(NO)]k, including seven new examples studied by in situ electrolysis, reveals a surprisingly narrow range of EPR parameters despite a wide variety of ligands such as pyridine, polypyridines, imines, amines, nitriles, phosphanes, carbonyl, cyclopentadienides, halides, hydride, hydroxide, thiocyanate or cyanide: g1 = 2.015 ± 0.02, g2 = 1.990 ± 0.015, g3 = 1.892 ± 0.03, gav = 1.968 ± 0.02, ,g = g1 , g3 = 0.122 ± 0.037, A2(14N) = 3.3 ± 0.5 mT. This rather small variability, smaller still if the organometallic compounds are excluded, differs from the wider range of EPR data reported for nitrosyliron species with S = 1/2; apparently, the {RuNO}7 configuration involves a rather invariant and relatively covalent metal,NO interaction. DFT calculations were employed for [(NC)5Ru(NO)]3, to reproduce the EPR data, to evaluate the spin distribution (58% spin density on NO), and to reveal structural changes on reduction such as the Ru,N,O bending and Ru,NO bond lengthening. In addition, the possibility of staggered and eclipsed conformations is discussed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] ESR Imaging of Conduction Electrons in Organic ConductorsISRAEL JOURNAL OF CHEMISTRY, Issue 1 2008Elmar Dormann In organic conductors built from stacks of planar aromatic hydrocarbons the mobility of the conduction electrons perpendicular to the radical cation stacks is more than four orders of magnitude smaller than the respective along-the-stack value. Thus the macroscopic properties of real single crystals of such radical cation salts are strongly influenced by the distribution of inevitable defects. Therefore, spatial resolution is required for a meaningful interpretation of the ESR data. Various schemes for 2D and 3D ESR imaging have been realized in the radio frequency and microwave frequency range with a resolution down to 10 m,m. Spatial distribution of spin density, T1, T2, and spin diffusion coefficient D were recorded, often as a function of temperature. The conducting-chain length distribution was observed. Our first attempts for the imaging of the integral and the spatially distributed carrier motion caused by an applied electric current were successful. [source] Revisiting N -continuous density-functional theory: Chemical reactivity and "Atoms" in "Molecules"ISRAEL JOURNAL OF CHEMISTRY, Issue 3-4 2003Morrel H. Cohen We construct an internally-consistent density-functional theory valid for noninteger electron numbers N by precise definition of a density functional that is continuous in N. In this theory, charge transfer between the atoms of a heteronuclear diatomic molecule, which have been separated adiabatically to infinity, is avoided because the hardness for fractional occupation of a single HOMO spin-orbital is negative. This N -continuous density functional makes possible a variational theory of "atoms" in "molecules" that exactly decomposes the molecular electron density into a sum of contributions from its parts. The parts are treated as though isolated. That theory, in turn, gives a deep foundation to the chemical reactivity theory provided that the hardness of entities with vanishing spin density is positive, as argued to be the case here. This transition from negative to positive hardness closely parallels the transition from the Heitler-London to the Hund-Mulliken picture of molecular bonding. [source] Stability of carbon-centered radicals: Effect of functional groups on the energetics of addition of molecular oxygenJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 7 2009James S. Wright Abstract In this paper we examine a series of hydrocarbons with structural features which cause a weakening of the CH bond. We use theoretical calculations to explore whether the carbon-centered radicals R, which are created after breaking the bond can be stabilized enough so that they resist the addition of molecular oxygen, i.e. where the reaction R, + O2 , ROO, becomes energetically unfavorable. Calculations using a B3LYP-based method provide accurate bond dissociation enthalpies (BDEs) for RH and ROO, bonds, as well as Gibbs free energy changes for the addition reaction. The data show strong correlations between ROO, and RH BDEs for a wide variety of structures. They also show an equally strong correlation between the ROO, BDE and the unpaired spin density at the site of addition. Using these data we examine the major functional group categories proposed in several experimental studies, and assess their relative importance. Finally, we combine effects to try to optimize resistance to the addition of molecular oxygen, an important factor in designing carbon-based antioxidants. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009 [source] Orbital-orthogonality constraints and basis-set optimizationJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2006Fabio E. Penotti Abstract A new procedure is presented for introducing arbitrary orbital-orthogonality constraints in the variational optimization of otherwise nonorthogonal multiconfiguration electronic wave functions. It is based on suitable analytical changes to the expressions for the first and second derivatives of the electronic energy with respect to the independent variational parameters, and can be applied in the presence of symmetry constraints. It is tested using a second-derivative optimization procedure, the Optimized Basis Set,Generalized Multiconfiguration Spin-Coupled (OBS-GMCSC) approach, that can treat basis-function exponential parameters as variational parameters, to be optimized simultaneously with configuration, spin-coupling, and orbital coefficients. This enables rigorous optimization of basis-set exponential parameters even for fully orthogonal multiconfiguration wave functions. Test calculations are carried out, with optimized even-tempered basis sets, on Li2 and on the CH radical. For the latter, special attention is paid to the electronic spin density at the nuclei. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 762,772, 2006 [source] Spin densities in two-component relativistic density functional calculations: Noncollinear versus collinear approachJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 8 2002Christoph Van WüllenArticle first published online: 3 APR 200 Abstract With present day exchange-correlation functionals, accurate results in nonrelativistic open shell density functional calculations can only be obtained if one uses functionals that do not only depend on the electron density but also on the spin density. We consider the common case where such functionals are applied in relativistic density functional calculations. In scalar-relativistic calculations, the spin density can be defined conventionally, but if spin-orbit coupling is taken into account, spin is no longer a good quantum number and it is not clear what the "spin density" is. In many applications, a fixed quantization axis is used to define the spin density ("collinear approach"), but one can also use the length of the local spin magnetization vector without any reference to an external axis ("noncollinear approach"). These two possibilities are compared in this work both by formal analysis and numerical experiments. It is shown that the (nonrelativistic) exchange-correlation functional should be invariant with respect to rotations in spin space, and this only holds for the noncollinear approach. Total energies of open shell species are higher in the collinear approach because less exchange energy is assigned to a given Kohn-Sham reference function. More importantly, the collinear approach breaks rotational symmetry, that is, in molecular calculations one may find different energies for different orientations of the molecule. Data for the first ionization potentials of Tl, Pb, element 113, and element 114, and for the orientation dependence of the total energy of I and PbF indicate that the error introduced by the collinear approximation is ,0.1 eV for valence ionization potentials, but can be much larger if highly ionized open shell states are considered. Rotational invariance is broken by the same amount. This clearly indicates that the collinear approach should not be used, as the full treatment is easily implemented and does not introduce much more computational effort. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 779,785, 2002 [source] Water diffusion in a rat glioma during ganciclovir-thymidine kinase gene therapy-induced programmed cell death in vivo: Correlation with cell densityJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 4 2004Piia K. Valonen MSc Abstract Purpose To study the characteristics of diffusion magnetic resonance imaging (MRI) contrast in a rat brain BT4C glioma during progression of ganciclovir (GCV)-thymidine kinase gene therapy-induced programmed cell death (PCD) in vivo. Materials and Methods The trace of the diffusion tensor (Dav = 1/3Trace ), T2, and spin density were determined by MRI and the apparent diffusion coefficient (ADC) of water by diffusion nuclear MR (NMR) spectroscopy using largely varying b values and diffusion times (tD) at 4.7 T. Cell count and apoptotic cells were quantified by histological means. Results Decline in cell count was strongly associated with increase in both Dav and T2. Spin density ratio between tumor and contralateral parietal cortex increased with a very similar time course as Dav and T2, indicating net water gain into the eradicating tumor. Diffusion spectroscopy showed a nonmonoexponential signal decay at all tD values ranging from 14,192 msec. During PCD, the ADC of the component yielding fast diffusion coefficient (D1), as acquired with tD , 47 msec, increased with kinetics similar to those of Dav (tD = 4.8 msec). The fractional size of D1 increased by 10% to 15% throughout the entire tD range. Apparent water residence time of the slow diffusion component, D2, shortened from a value of 38.3 ± 1.7 msec on day 0 to 33.4 ± 0.5 msec by day 8. Conclusion The present results show that reduced cell density and increased water content, leading to altered water microenvironment, are associated with increased water diffusion coefficient in eradicating gliomas as a result of PCD. J. Magn. Reson. Imaging 2004;19:389,396. © 2004 Wiley-Liss, Inc. [source] THE APPLICATION OF ELECTRON SPIN RESONANCE AS A GUIDE TO THE MATURATION AND TYPING OF ORGANIC MATTER IN THE NORTH SEAJOURNAL OF PETROLEUM GEOLOGY, Issue 1 2003R. A. McTavish In early electron spin resonance (ESR) analysis of North Sea wells, maturation of organic matter (OM) was expressed in terms of maximum palaeotemperature (MPT) based on North American calibrations that did not consider the influences of kerogen composition or overpressure. In the North Sea, the MPTs were anomalous in overpressured sequences and relative to other indices of OM maturation such as vitrinite reflectance, so the ESR method was abandoned there in geochemical studies. However, early empirical study of North Sea ESR data indicated that, in relation to functions that linked temperature and pore pressure, some ESR parameters were predictable without reference to MPTs. In order to re-evaluate ESR parameters as indices of OM maturation, the physical factors (temperature and pressure) which affect OM maturation are related in the present paper to the ESR parameters "g" (spectral position) and Ng (spin density) at six well locations in the northern North Sea. A third ESR parameter, W (line width), is not an effective guide to maturation levels due to its complex relationship to the physical factors and kerogen types. However, cross-plots of W versus "g" and Ng appear to be as effective as pyrolysis for kerogen typing. Levels of maturation investigated in the North Sea wells range through the equivalent vitrinite reflectance values of about 0.50,1.50%. The values of "g" and Ng have been differentiated for kerogen type, but undifferentiated values of "g" have also been studied. Regression analysis has shown that there are linear relationships between the ESR parameters "g" and Ng, and the physical factors present-day temperature (To), "effective" temperature (Te), and differential pressure (Pd). Correlation coefficients for both "g" (undifferentiated and differentiated) and Ng (differentiated) relative to the physical factors are high; the highest values are for "g" and Ng relative to Te and Pd (r =,0.950 for "g" differentiated or undifferentiated, r = 0.944,0.976 for Ng differentiated, respectively). However, correlation coefficients were lower for "g" and Ng relative to To. More frequent high correlation coefficients and larger sample populations suggest that "g" (undifferentiated) is a more reliable index of OM maturation than Ng(differentiated). However, the estimation of levels of OM maturation is improved if both indices are used together. The ESR method appears to be effective both for estimating levels of OM maturation and for kerogen typing. It has a number of potential advantages over other geochemical methods: firstly, it is more sensitive for estimating OM maturation than most other methods; secondly, it can be used to analyze organic matter which is as old as Proterozoic; thirdly, it does not destroy the samples analyzed. [source] ESR/DFT study of bis-iminophosphorane cation radicalsMAGNETIC RESONANCE IN CHEMISTRY, Issue 12 2007Adil Matni Abstract Bis-iminophosphoranes containing various types of linkers between two R3PN moieties were electrochemically oxidized at controlled potential in situ in the electron spin resonance (ESR) cavity. For linkers constituted of phenylenes, conjugated phenylenes or merely a dicyanoethylenic bond, this oxidation led to well-resolved ESR spectra which were characterized by their g values and by their 1H, 14N and 31P isotropic hyperfine constants. These coupling constants agree with those calculated by DFT for the corresponding cation radicals. Experimental and theoretical results clearly indicate that in these species the unpaired electron is mostly delocalized on the bridge and on the nitrogen atoms while the spin density on the phosphorus atoms is particularly small. Cyclic voltammetry and ESR spectra show that the nature of the bridge between the two iminophosphoranes considerably influences the oxidation potential of the compound as well as the stability of the radical cation. Information about the conformation of the precursor containing two Ph3PN moieties separated by a C(CN)C(CN)group was obtained from its crystal structure. Copyright © 2007 John Wiley & Sons, Ltd. [source] Evidence from ESR studies for [Co(,-C2H4)3] produced at 77 K in a rotating cryostat,MAGNETIC RESONANCE IN CHEMISTRY, Issue 10 2006Lynda J. Hayton Abstract Co atoms were reacted with ethene at 77 K and the paramagnetic products studied by electron spin resonance (ESR) at X- and K-bands. The ESR spectra of the major product at both frequencies showed eight cobalt multiplets (ICo = 7/2) indicating a mono-cobalt complex. The spectra have orthorhombic g and cobalt hyperfine tensors and were simulated by the parameters; g1 = 2.284, g2 = 2.0027, g3 = 2.1527; A1 < , 25 MHz, A2 = , 109 MHz, A3 = , 198 MHz. Proton and 13C (1% natural abundance) hyperfine couplings were lower than the line widths (<2 MHz) indicating less than 0.5 spin transfer to the ethene ligands. We assigned the spectrum to a Jahn,Teller-distorted planar trigonal mono-cobalt tris-ethene [Co(,-C2H4)3] complex in C2v symmetry. The SOMO is either a 3dx2,y2 (2a1) orbital in a T-geometry or a 3dxy (b1) orbital in a Y-geometry but there is only a spin density, a2, of 0.30 in these d orbitals. The spin deficiency of 0.70 is attributed to two factors; spin transfer from the Co to ethene ,/,* orbitals and a 4p orbital contribution, b2, to the SOMO. Calculations of a2 and b2 have been made at three levels of spin transfer, ,. At , = 0.00a2 is 0.23 and b2 is 0.78, at , = 0.25a2 is 0.25 and b2 is 0.52 and at , = 0.50a2 is 0.28 and b2 is 0.23. The other possible assignment to a mono-cobalt bis-ethene complex [Co(,-C2H4)2] cannot be discounted from the ESR data alone but is considered unlikely on other grounds. The complex is stable up to ,220 K indicating a barrier to decomposition of ,50 kJ Mol,1 Copyright © 2006 John Wiley & Sons, Ltd. [source] The g -values and hyperfine coupling of amino acid radicals in proteins: comparison of experimental measurements with ab initio calculations,MAGNETIC RESONANCE IN CHEMISTRY, Issue S1 2005Sun Un Abstract Electron paramagnetic resonance (EPR) spectroscopy has been extensively used to identify and characterize protein-based redox active amino acid radicals based on their g -values and hyperfine couplings. To better understand how these parameters depend on the electronic structure and environment of the radical, the theoretical g -values and proton hyperfine tensors of three models corresponding to the tyrosyl, tryptophanyl and glycyl radicals were calculated using Gaussian 03. The g -values were determined using the B3LYP/6,31+G(D,P) combination of density functional and basis set, while the hyperfine tensors were determined using the B3LYP/EPR-III and PBE0/EPR-III combinations. Comparisons are made to measured values. It was found that by appropriately accounting for hydrogen bonds and the dielectric constant of the environment, good agreement could be achieved between the calculated and measured g -values. In all three cases, the g -anisotropy arose from significant spin density on a nitrogen or oxygen atom. The calculated hyperfine tensors for the three radicals did not differ significantly from previous calculations. In the case of the tyrosyl radical, it is shown for the first time that the para -position substituent that is opposite of the CO group can break the symmetry of the phenyl ring, leading to different hyperfine tensors for the two large ortho proton couplings. For the tyrosyl and tryptophanyl models, the calculated hyperfine couplings to hydrogen-bonding protons were in very good agreement with measured values for both the tyrosyl and tryptophanyl models. Copyright © 2005 John Wiley & Sons, Ltd. [source] Solid-state 1H , 19F/19F , 1H CP/MAS NMR study of poly(vinylidene fluoride)MAGNETIC RESONANCE IN CHEMISTRY, Issue 2 2002Shinji Ando Abstract Solid-state 1H , 19F and 19F , 1H cross-polarization magic angle spinning (CP/MAS) NMR spectra have been investigated for a semicrystalline fluoropolymer, namely poly(vinylidene fluoride) (PVDF). The 1H , 19F CP/MAS spectra can be fitted by five Lorentzian functions, and the amorphous peaks were selectively observed by the DIVAM CP pulse sequences. Solid-state spin-lock experiments showed significant differences in T1,F and T1,H between the crystalline and amorphous domains, and the effective time constants, THF* and T1,*, which were estimated from the 1H , 19F CP curves, also clarify the difference in the strengths of dipolar interactions. Heteronuclear dipolar oscillation behaviour is observed in both standard CP and 1H , 19F inversion recovery CP (IRCP) experiments. The inverse 19F , 1H CP-MAS and 1H , 19F CP-drain MAS experiments gave complementary information to the standard 1H , 19F CP/MAS spectra in a manner reported in our previous papers for other fluoropolymers. The value of NF/NH (where N is a spin density) estimated from the CP-drain curve is within experimental error equal to unity, which is consistent with the chemical structure. Copyright © 2001 John Wiley & Sons, Ltd. [source] Diffusion measurements free of motion artifacts using intermolecular dipole-dipole interactionsMAGNETIC RESONANCE IN MEDICINE, Issue 1 2004Scott D. Kennedy Abstract Diffusion encoding, or diffusion weighting, is commonly achieved by applying a pair of balanced pulsed-field gradients during spin evolution. An alternative way to obtain diffusion measurements is to select dipolar correlation distances using the distant dipolar field (DDF) in systems with abundant spin density, such as water in tissues. Diffusion weighting using this effect is unique in that the refocusing "gradient" is carried within the sample, and thus the macroscopic motion of the sample is not expected to interfere with signal formation. The experiments presented here demonstrate that in moving phantoms, the phase shift of the signal due to linear motion is minimal in diffusion-weighted (DW) DDF measurements, and that motion artifacts in images of moving phantoms and the abdomen of live mice are small compared to standard pulsed-field-gradient methods. The technique may facilitate the use of DWI in typically motion-prone regions such as the abdomen, lungs, and heart. Magn Reson Med 52:1,6, 2004. © 2004 Wiley-Liss, Inc. [source] Defects and structure of µc-SiOx:H deposited by PECVDPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3-4 2010Lihong Xiao Abstract Electronic transport and paramagnetic defects detected by Electron Spin Resonance (ESR) in both intrinsic and Nanostructured antiferromagnetic spin glass in doped Ge near the insulator,metal transitionPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2008A. G. Zabrodskii Abstract It is shown that, on the insulator side of the insulator , metal transition, the single spin density exponentially disappears as T , 0. Such spins are bound into pairs to give an antiferromagnetic (AFM) phase. Upon an increase in temperature the AFM phase is destroyed, the single-spin density, and, as a result, ESR absorption signal becomes stronger. The temperature dependencies of the densities of the pairs and single spins are typical of a chaotic distribution of neutral donors. In this case, there is no Neel temperature. In low degree of compensation, the crystal lattice of Ge with the AFM phase is actually a nanostructured system characterized by anisotropic internal stresses that are the strongest along one of the [110] directions. These stresses give rise to anisotropy of the g-factor which is responsible for experimentally observed splitting of the ESR line. The compensating impurities destroy the AFM phase and reduce this splitting. Local stresses are present in this case, too, but now they appear because of the Coulomb interaction of oppositely charged impurities and have no preferred orientation. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Spin densities in parabolic quantum wires with Rashba spin-orbit interactionPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2006Sigurdur I. Erlingsson Abstract Using canonical transformations we diagonalize approximately the Hamiltonian of a gaussian wire with Rashba spin-orbit interaction. This proceedure allows us to obtain the energy dispersion relations and the wavefunctions with good accuracy, even in systems with relatively strong Rashba coupling. With these eigenstates one can calculate the non-equilibrium spin densities induced by applying bias voltages across the sample. We focus on the z -component of the spin density, which is related to the spin Hall effect. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Antiferromagnetic phase in doped semiconductors near the insulator,metal phase transitionPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2006A. I. Veinger Abstract We show for n -type semiconductors, using Ge:As and 4H -SiC:N as examples, that the spin density measured by electron spin resonance (ESR) falls sharply near the insulator,metal (IM) transition. Two reasons might be responsible for this phenomenon: potential fluctuations with concentration of the electrons in local minima or local antiferromagnetic spin pairing ("spin glass") in the insulating state just below the IM transition. In the investigated semiconductors manifestation of the latter is observed as a change from Curie to a Curie,Weiss behaviour with negative constant ,. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] X-ray constrained unrestricted Hartree,Fock and Douglas,Kroll,Hess wavefunctionsACTA CRYSTALLOGRAPHICA SECTION A, Issue 1 2010Martin Hudák The extension of the X-ray constrained (XC) wavefunction approach to open-shell systems using the unrestricted Hartree,Fock formalism is reported. The XC method is also extended to include relativistic effects using the scalar second-order Douglas,Kroll,Hess approach. The relativistic effects on the charge and spin density on two model compounds containing the copper and iron atom are reported. The size of the relativistic effects is investigated in real and reciprocal space; in addition, picture-change effects are investigated and discussed for the isolated Cu atom. It is found that the relativistic terms lead to changes in the densities that are much smaller than those from the X-ray constraint. Nevertheless, the use of the relativistic corrections in the ab initio model always leads to an improvement in the agreement statistics. An interesting result of the unrestricted XC technique is the possibility of obtaining experimentally derived spin densities from X-ray data. [source] Ring Opening of the Cyclobutane in a Thymine Dimer Radical AnionCHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2007Chryssostomos Chatgilialoglu Dr. Abstract The reactions of hydrated electrons (eaq,) with thymine dimer 2 and thymidine have been investigated by radiolytic methods coupled with product studies, and addressed computationally by means of BB1K-HMDFT calculations. Pulse radiolysis revealed that one-electron reduction of the thymine dimer 2 affords the radical anion of thymidine (5) with t1/2<35,ns. Indeed, the theoretical study suggests that radical anion 3, in which the spin density and charge distribution are located in both thymine rings, undergoes a fast partially ionic splitting of the cyclobutane with a half-life of a few ps. This model fits with the in vivo observation of thymine dimer repair in DNA by photolyase. ,-Radiolysis of thymine dimer 2 demonstrates that the one-electron reduction and the subsequent cleavage of the cyclobutane ring does not proceed by means of a radical chain mechanism, that is, in this model reaction the T,. is unable to transfer an electron to the thymine dimer 2. [source]
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