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G Values (g + value)
Selected AbstractsPossible Pathways of CVD Processes Leading to III,V Semiconductors via a Two-Dimensional Growth,CHEMPHYSCHEM, Issue 4 2005Hans-Jörg Himmel Priv.-Doz. Abstract Quantum-chemical calculations were carried out to shed more light upon possible first intermediates formed during chemical vapour deposition (CVD) processes leading to III,V semiconductors. Information about possible structures of intermediates and about thermodynamic properties in dependence of the temperature were collected. Because some of the systems calculated herein contain a substantial number of atoms (up to 144), it is limited to intermediates on the way to solid AlN. According to our suggestion, the first intermediates are species with the overall formula AlNH2, Al2N2H4, Al3N3H6, Al5N5H8, Al7N7H, and Al9N9H, Al12N12H12. In further calculations the growth of a second layer, on top of the first layer, which is modelled by the Al12N12H12 cluster, is simulated. The Al12N12H12 "surface" offers two places for the attachment of an Al3N3H6 ring. At temperatures of 1000 K, the ,G value for this attachment is only slightly negative, which suggests that the molecules can move relatively freely on the surface. This might be of importance for a defect-free growth process. Up to four layers are built on top of the first Al12N12H12 layer leading to Al24N24H24 (two layers), Al36N36H36 (three layers) and Al48N48H48 (four layers). The structures are compared with that of solid AlN in its most stable Wurzit-type structure. [source] The Fe-only nitrogenase and the Mo nitrogenase from Rhodobacter capsulatusFEBS JOURNAL, Issue 6 2002A comparative study on the redox properties of the metal clusters present in the dinitrogenase components The dinitrogenase component proteins of the conventional Mo nitrogenase (MoFe protein) and of the alternative Fe-only nitrogenase (FeFe protein) were both isolated and purified from Rhodobacter capsulatus, redox-titrated according to the same procedures and subjected to an EPR spectroscopic comparison. In the course of an oxidative titration of the MoFe protein (Rc1Mo) three significant S = 1/2 EPR signals deriving from oxidized states of the P-cluster were detected: (1) a rhombic signal (g = 2.07, 1.96 and 1.83), which showed a bell-shaped redox curve with midpoint potentials (Em) of ,195 mV (appearance) and ,30 mV (disappearance), (2) an axial signal (g|| = 2.00, g, = 1.90) with almost identical redox properties and (3) a second rhombic signal (g = 2.03, 2.00, 1.90) at higher redox potentials (> 100 mV). While the ,low-potential' rhombic signal and the axial signal have been both attributed to the one-electron-oxidized P-cluster (P1+) present in two conformationally different proteins, the ,high-potential' rhombic signal has been suggested rather to derive from the P3+ state. Upon oxidation, the FeFe protein (Rc1Fe) exibited three significant S = 1/2 EPR signals as well. However, the Rc1Fe signals strongly deviated from the MoFe protein signals, suggesting that they cannot simply be assigned to different P-cluster states. (a) The most prominent feature is an unusually broad signal at g = 2.27 and 2.06, which proved to be fully reversible and to correlate with catalytic activity. The cluster giving rise to this signal appears to be involved in the transfer of two electrons. The midpoint potentials determined were: ,80 mV (appearance) and 70 mV (disappearance). (b) Under weakly acidic conditions (pH 6.4) a slightly altered EPR signal occurred. It was characterized by a shift of the g values to 2.22 and 2.05 and by the appearance of an additional negative absorption-shaped peak at g = 1.86. (c) A very narrow rhombic EPR signal at g = 2.00, 1.98 and 1.96 appeared at positive redox potentials (Em = 80 mV, intensity maximum at 160 mV). Another novel S = 1/2 signal at g = 1.96, 1.92 and 1.77 was observed on further, enzymatic reduction of the dithionite-reduced state of Rc1Fe with the dinitrogenase reductase component (Rc2Fe) of the same enzyme system (turnover conditions in the presence of N2 and ATP). When the Rc1Mo protein was treated analogously, neither this ,turnover signal' nor any other S = 1/2 signal were detectable. All Rc1Fe -specific EPR signals detected are discussed and tentatively assigned with special consideration of the reference spectra obtained from Rc1Mo preparations. [source] Configuration interaction calculation of electronic g tensors in transition metal complexes,INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3-4 2001Frank Neese Abstract An algorithm for the calculation of electronic g tensors from configuration interaction (CI) wave functions and its implementation for any ground-state spin S is described. The algorithm can be used together with either single- or multireference CI wave functions but assumes a spatially nondegenerate ground state. A one-electron approximation to the spin,orbit coupling (SOC) operator is used. Presently the implementation uses the INDO/S model of Zerner and coworkers. Results are presented for a representative collection of Cu(II) complexes with N-, O-, and S-containing ligands. The calculations reproduce the trends in the experimental data well and show no systematic errors. For the test set the g values are reproduced with a standard deviation of 0.021. The method has therefore a good cost,performance ratio and is expected to be helpful in chemical and biochemical applications. Further improvements are feasible and necessary and are briefly discussed. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 83: 104,114, 2001 [source] An empirical investigation of the g -index for 26 physicists in comparison with the h -index, the A -index, and the R -indexJOURNAL OF THE AMERICAN SOCIETY FOR INFORMATION SCIENCE AND TECHNOLOGY, Issue 9 2008Michael Schreiber J.E. Hirsch (2005) introduced the h -index to quantify an individual's scientific research output by the largest number h of a scientist's papers that received at least h citations. To take into account the highly skewed frequency distribution of citations, L. Egghe (2006a) proposed the g -index as an improvement of the h -index. I have worked out 26 practical cases of physicists from the Institute of Physics at Chemnitz University of Technology, and compare the h and g values in this study. It is demonstrated that the g -index discriminates better between different citation patterns. This also can be achieved by evaluating B.H. Jin's (2006) A -index, which reflects the average number of citations in the h -core, and interpreting it in conjunction with the h -index. h and A can be combined into the R -index to measure the h -core's citation intensity. I also have determined the A and R values for the 26 datasets. For a better comparison, I utilize interpolated indices. The correlations between the various indices as well as with the total number of papers and the highest citation counts are discussed. The largest Pearson correlation coefficient is found between g and R. Although the correlation between g and h is relatively strong, the arrangement of the datasets is significantly different depending on whether they are put into order according to the values of either h or g. [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] Single-crystal EPR studies of a perchlorate-bridged dimeric copper(II) complex with 2-[(4-methyl-pyridin-2-ylimino)-methyl]-phenolPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 10 2007R. Srinivasan Abstract A new dimeric perchlorate-bridged copper(II) complex with the Schiff's base ligand 2-[(4-methyl-pyridin-2-ylimino)-methyl]-phenol has been synthesized. The complex has been characterized using elemental analysis, ultraviolet,visible and infrared spectroscopy, conductivity, magnetic studies and EPR techniques. Single-crystal EPR studies indicate that zero-field splitting (D) is relatively small, and that the half-field transition is not observed due to its low intensity. An interesting observation in the frozen solution is that the low-field side of the zero-field splitting line splits into seven lines, due to interaction between the two copper ions. The magnetic susceptibility data indicate a weak ferromagnetic property and that the complex is a 1:2 electrolyte. Another set of EPR resonances corresponding to higher D and g values, compared to the system reported, are noticed, whose origin is not yet known. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Evolution of magnetic fields in stars across the upper main sequence: II.ASTRONOMISCHE NACHRICHTEN, Issue 6 2007Observed distribution of the magnetic field geometry Abstract We re-discuss the evolutionary state of upper main sequence magnetic stars using a sample of Ap and Bp stars with accurate Hipparcos parallaxes and definitely determined longitudinal magnetic fields. We confirm our previous results obtained from the study of Ap and Bp stars with accurate measurements of the mean magnetic field modulus and mean quadratic magnetic fields that magnetic stars of mass M < 3 M, are concentrated towards the centre of the main-sequence band. In contrast, stars with masses M > 3 M, seem to be concentrated closer to the ZAMS. The study of a few known members of nearby open clusters with accurate Hipparcos parallaxes confirms these conclusions. Stronger magnetic fields tend to be found in hotter, younger and more massive stars, as well as in stars with shorter rotation periods. The longest rotation periods are found only in stars which spent already more than 40% of their main sequence life, in the mass domain between 1.8 and 3 M, and with log g values ranging from 3.80 to 4.13. No evidence is found for any loss of angular momentum during the main-sequence life. The magnetic flux remains constant over the stellar life time on the main sequence. An excess of stars with large obliquities , is detected in both higher and lower mass stars. It is quite possible that the angle , becomes close to 0. in slower rotating stars of mass M > 3 M, too, analog to the behaviour of angles , in slowly rotating stars of M < 3 M,. The obliquity angle distribution as inferred from the distribution of r -values appears random at the time magnetic stars become observable on the H-R diagram. After quite a short time spent on the main sequence, the obliquity angle , tends to reach values close to either 90. or 0. for M < 3 M,. The evolution of the obliquity angle , seems to be somewhat different for low and high mass stars. While we find a strong hint for an increase of , with the elapsed time on the main sequence for stars with M > 3 M,, no similar trend is found for stars with M < 3 M,. However, the predominance of high values of , at advanced ages in these stars is notable. As the physics governing the processes taking place in magnetised atmospheres remains poorly understood, magnetic field properties have to be considered in the framework of dynamo or fossil field theories. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Muon Implantation of Metallocenes: FerroceneCHEMISTRY - A EUROPEAN JOURNAL, Issue 8 2007Upali Abstract Muon Spin Relaxation and Avoided Level Crossing (ALC) measurements of ferrocene are reported. The main features observed are five high field resonances in the ALC spectrum at about 3.26, 2.44, 2.04, 1.19 and 1.17,T, for the low-temperature phase at 18,K. The high-temperature phase at 295,K shows that only the last feature shifted down to about 0.49,T and a muon spin relaxation peak at about 0.106,T which approaches zero field when reaching the phase transition temperature of 164,K. A model involving three muoniated radicals, two with muonium addition to the cyclopentadienyl ring and the other to the metal atom, is postulated to rationalise these observations. A theoretical treatment involving spin-orbit coupling is found to be required to understand the Fe,Mu adduct, where an interesting interplay between the ferrocene ring dynamics and the spin-orbit coupling of the unpaired electron is shown to be important. The limiting temperature above which the full effect of spin-orbit interaction is observable in the ,SR spectra of ferrocene was estimated to be 584,K. Correlation time for the ring rotation dynamics of the Fe,Mu radical at this temperature is 3.2,ps. Estimated electron g values and the changes in zero-field splittings for this temperature range are also reported. [source] Low-Temperature EPR and Mössbauer Spectroscopy of Two Cytochromes with His,Met Axial Coordination Exhibiting HALS Signals,CHEMPHYSCHEM, Issue 6 2006Giorgio Zoppellaro Dr. Abstract C-type cytochromes with histidine,methionine (His,Met) iron coordination play important roles in electron-transfer reactions and in enzymes. Low-temperature electron paramagnetic resonance (EPR) spectra of low-spin ferric cytochromes c can be divided into two groups, depending on the spread of g values: the normal rhombic ones with small g anisotropy and gmax below 3.2, and those featuring large g anisotropy with gmax between 3.3 and 3.8, also denoted as highly axial low spin (HALS) species. Herein we present the detailed magnetic properties of cytochrome c553 from Bacillus pasteurii (gmax 3.36) and cytochrome c552 from Nitrosomonas europaea (gmax 3.34) over the pH range 6.2 to 8.2. Besides being structurally very similar, cytochrome c553 shows the presence of a minor rhombic species at pH 6.2 (6,%), whereas cytochrome c552 has about 25,% rhombic species over pH 7.5. The detailed Mössbauer analysis of cytochrome c552 confirms the presence of these two low-spin ferric species (HALS and rhombic) together with an 8,% ferrous form with parameters comparable to the horse cytochrome c. Both EPR and Mössbauer data of axial cytochromes c with His,Met iron coordination are consistent with an electronic (dxy)2 (dxz)2 (dyz)1 ground state, which is typical for Type I model hemes. [source] | |||||||||||||