Hyperfine Interaction Constants Research Articles

Magnetoresistance for organic semiconductors: Small molecule, oligomer, conjugated polymer, and non-conjugated polymer

Organic semiconductor (OSC) devices have been shown to have a large magnetoresistance (MR) response at room temperature for relatively small-applied magnetic fields of 0.1–100 milli-Tesla (mT). This large MR is not limited to one class of organics, but is seen in small molecules, oligomers, conjugated polymers, and non-conjugated polymers. In this paper, data is presented on the MR effect for the poly(phenylene vinylene) (PPV) derivative “Super Yellow,” for poly(vinylenecarbazole) (PVK), for alpha-sexithiophene (α-6T), and for tris(8-hydroxyquinoline) aluminum (Alq 3). The data is analyzed in the context of the Magnetoresistance by the Interconversion of Singlets and Triplets (MIST) model. The MR data of Alq 3 for magnetic fields of less than 1 mT are fitted to a polynomial expansion, and an estimate for the hyperfine interaction constant, which is consistent with values for small molecules, is extracted from the fitting parameters. Curve fits at fields in the 100 mT range are also presented and they show that there exist two kinds of magnetic field behavior, inverse square root, and inverse even orders. Furthermore, the scaling factor at this range is one order of magnitude larger than that found in the 3 mT range.

Hyperfine structures, isotope shifts, and transition rates of C II, N III, and O IV from relativistic configuration interaction calculations

Abstract Energy levels, specific mass shift parameters, hyperfine interaction constants, Lande g J factors, and transition probabilities between computed levels are reported for C II, N III, and O IV. Results include levels belonging to 2 s 2 2 p, 2 s 2 p 2 , 2 p 3 , 2 s 2 3 s, 2 s 2 3 p, 2 s 2 3 d, 2 s 2 p 3 s and, in the case of C II, the 2 s 2 4 s and 2 s 2 4 p configurations. Wavefunctions were determined using the multiconfiguration Dirac–Hartree–Fock method and account for valence, core–valence, and core–core correlation effects.

EPR spectra of the trimethyl-1,4-benzoquinone anion-radical: Results of calculations by the density functional method and their comparison with experiment

Constants of the isotropic hyperfine interaction of an unpaired electron with the hydrogen nuclei of the trimethyl-1,4-benzoquinone anion-radical were calculated using the density functional method. The EPR spectrum of the radical was simulated on the basis of these calculations, which were in good agreement with the experimental EPR spectrum of trimethyl-1,4-benzosemiquinone dissolved in dimethylsulfoxide.

NLTE analysis of Co i /Co ii lines in spectra of cool stars with new laboratory hyperfine splitting constants

We investigate the statistical equilibrium of Co in the atmospheres of cool stars, and the influence of NLTE and HFS (hyperfine splitting) on the formation of Co lines and abundances. Significant departures from LTE level populations are found for Co I, also number densities of excited states in Co II differ from LTE at low metallicity. The NLTE abundance of Co in solar photosphere is 4.95 +/- 0.04 dex, which is in agreement with that in C I meteorites within the combined uncertainties. The spectral lines of Co I were calculated using the results of recent measurements of hyperfine interaction constants by UV Fourier transform spectrometry. For Co II, the first laboratory measurements of hyperfine structure splitting A and B factors were performed. A differential abundance analysis of Co is carried out for 18 stars in the metallicity range -3.12 < [Fe/H] < 0. The abundances are derived by method of spectrum synthesis. At low [Fe/H], NLTE abundance corrections for Co I lines are as large as +0.6 >... +0.8 dex. Thus, LTE abundances of Co in metal-poor stars are severely underestimated. The stellar NLTE abundances determined from the single UV line of Co II are lower by ~0.5-0.6 dex. The discrepancy might be attributed to possible blends that have not been accounted for in the solar Co II line and its erroneous oscillator strength. The increasing [Co/Fe] trend in metal-poor stars, as calculated from the Co I lines under NLTE, can be explained if Co is overproduced relative to Fe in massive stars. The models of galactic chemical evolution are wholly inadequate to describe this trend suggesting that the problem is in SN yields.

Radical anions of 3,3′-bi(2-R-5,5-dimethyl-4-oxopyrrolinylidene) 1,1′-dioxides in MeCN-H2O solvent mixtures

Radical anions of cyclic dinitrones, 3,3′-bi(2-R-5,5-dimethyl-4-oxopyrrolinylidene) 1,1′-dioxides (R = Me, Ph, But, CF3) have been studied by ESR in combination with quantum chemical calculations of geometrical parameters and isotropic hyperfine interaction constants by the DFT/PBE methods in the 3z basis set and UB3LYP in the 6–31+G* basis set with allowance for the solvation in the framework of continual PCM model. The radical anions were generated electrochemically by reduction of their neutral precursors in MeCN-H2O solvent mixtures and found to possess enough stability for measurement of their ESR spectra in a wide range of the mixture compositions. Radical anions of compounds of nitrone series were generated and characterized in MeCN-H2O mixtures and water (for the dinitrone with R = Me) for the first time. Differences in the solvent depending behavior of the nitrogen hyperfine interaction constants in the series of radical anions with various substituents R are explained by competition of transsolvation and structural effects.

A study of the hyperfine structure of Ta I lines based on Fourier transform spectra and laser-induced fluorescence

The hyperfine structure of several spectral lines of the tantalum atom has been investigated by laser excitation in the spectral range 4180–7000 Å. We report the discovery of 14 new energy levels of even parity and 21 new energy levels of odd parity. For the new levels, besides their wave number, their total angular momentum quantum number J, their parity, as well as their magnetic dipole and electric quadrupole hyperfine interaction constants A and B were determined. We classified altogether 73 lines by means of laser spectroscopy in a hollow cathode discharge, 52 of them by laser excitation and 21 via fluorescence decay. Additionally, 120 lines, observed in high-resolution Fourier transform spectra, were classified by using their wave numbers and their characteristic hyperfine patterns.

Trityl-based EPR probe with enhanced sensitivity to oxygen

An asymmetric derivative of the triarylmethyl radical, TAM-H, containing one aldehyde and two carboxyl groups, was synthesized. The electron paramagnetic resonance (EPR) spectrum of TAM-H is characterized by a doublet of narrow lines with a linewidth of 105 mG under anoxic conditions and hyperfine interaction constant of 245 mG. The partial overlap of the components of the doublet results in enhanced sensitivity of the spectral amplitudes ratio to oxygen compared with oxygen-induced linewidth broadening of a single line. Application of the TAM-H probe allows for EPR measurements in an extended range of oxygen pressures from atmospheric to 1 mm Hg, whereas the EPR spectrum linewidth of the popular TAM-based oxygen sensor Oxo63 is practically insensitive to oxygen partial pressures below 20 mm Hg. Enhanced sensitivity of the TAM-H probe relative to Oxo63 was demonstrated in the detection of oxygen consumption by Met-1 cancer cells. The TAM-H probe allowed prolonged measurements of oxygen depletion during the hypoxia stage and down to true anoxia (≤ 1.5 mm Hg).

Direct Detection and Characterization of Chloride in the Active Site of the Low-pH Form of Sulfite Oxidase Using Electron Spin Echo Envelope Modulation Spectroscopy, Isotopic Labeling, and Density Functional Theory Calculations

Electron spin echo envelope modulation (ESEEM) investigations were carried out on samples of the low-pH (lpH) form of vertebrate sulfite oxidase (SO) prepared with (35)Cl- and (37)Cl-enriched buffers, as well as with buffer containing the natural abundance of Cl isotopes. The isotope-related changes observed in the ESEEM spectra provide direct and unequivocal evidence that Cl(-) is located in close proximity to the Mo(V) center of lpH SO. The measured isotropic hyperfine interaction constant of about 4 MHz ((35)Cl) suggests that the Cl(-) ion is either weakly coordinated to Mo(V) at its otherwise vacant axial position, trans to the oxo ligand, or is hydrogen-bonded to the equatorial exchangeable OH ligand. Scalar relativistic all-electron density functional theory (DFT) calculations of the hyperfine and nuclear quadrupole interaction parameters, along with steric and energetic arguments, strongly support the possibility that Cl(-) is hydrogen-bonded to the equatorial OH ligand rather than being directly coordinated to the Mo(V).

Peroxidase Activity Of Respiratory Proteins. The Role Of Protein Bound Free Radicals

Peroxidase activity of respiratory haem proteins is associated with formation of two active species on the protein when it reacts with peroxides: the oxoferryl state of the haem and a protein bound free radical. Both the free radical and the oxoferryl haem are capable of oxidising a range of substrates. General principles of formation of these two species and of their further reactions will be considered. Experimental confirmation of the proposed view involves a complex analysis of diverse pathways by which electrons can be passed between different parts of the protein. Associated with this electron transfer, the process of free radical character transfer around the protein as well as between different protein molecules can be monitored by the EPR spectroscopy. If a protein contains tyrosine residues (which is often but not always the case), the chance of observing a tyrosyl radical is high. EPR data will be presented on formation, transformation and transfer of protein bound radicals in different respiratory proteins. A method will be described that allows very accurate determination of the tyrosyl radical parameters. By using the method, it is possible to determine the three principal g-values of the radical solely from an X-band EPR spectrum. (And if a high field EPR spectrum of the radical is available, and the g-values are measured directly, the other radical parameters, e.g. the hyperfine interaction constants, can be determined much more accurately.) The radicals parameters extracted from experimental EPR spectra are then compared with either some structural features of the tyrosines (phenoxyl ring rotation angle) or with the values of the parameters obtained from the DFT calculation. This allows identification of the tyrosine in the protein responsible for the observed radical.

Exchangeable oxygens in the vicinity of the molybdenum center of the high-pH form of sulfite oxidase and sulfite dehydrogenase.

The electron spin echo envelope modulation (ESEEM) investigation of the high-pH (hpH) form of sulfite oxidase (SO) and sulfite dehydrogenase (SDH) prepared in buffer enriched with H(2)(17)O reveals the presence of three types of exchangeable oxygen atoms at the molybdenum center. Two of these oxygen atoms belong to the equatorial OH ligand and the axial oxo ligand, and are characterized by (17)O hyperfine interaction (hfi) constants of about 37 MHz and 6 MHz, respectively. The third oxygen has an isotropic hfi constant of 3-4 MHz and likely belongs to a hydroxyl moiety hydrogen-bonded to the equatorial OH ligand. This exchangeable oxygen atom is not observed in the ESEEM spectra of the Y236F mutant of SDH, where the active site tyrosine has been replaced by phenylalanine.

Shallow acceptor impurities in diamond-like semiconductors studied by polarized negative muons

Abstract The results of μ SR study of the behavior of shallow acceptor centers (AC) in diamond, silicon, and germanium are presented. It was found that the muonic atom, which imitates acceptor in semiconductors, is formed in the paramagnetic state in silicon and germanium at low temperatures whereas in diamond it is formed in the diamagnetic state. The hyperfine interaction constant for the aluminium AC in silicon was estimated for the first time. It was shown that the main contribution to the relaxation of the magnetic moment of the AC is due to interaction of the AC with the phonon via the Roman process in non-degenerated silicon at temperatures ⩽ 50 K . In the case of germanium, there is an experimental evidence for a change in the contribution of different phonon processes to the relaxation of the AC at temperature ∼ 10 K . The hole capture rate by the ionized boron acceptor in diamond was determined.

Evidence of Cd1−xMnxS nanocrystal growth in a glass matrix by the fusion method

In this work, CdS nanocrystals (NCs) doped with Mn were synthesized in a glass matrix by fusion. The as-grown Cd1−xMnxS NCs were investigated by optical absorption, atomic force microscopy, and electron paramagnetic resonance (EPR). The incorporation of Mn2+ ions in CdS NCs was confirmed by an absorption transition blueshift with increasing Mn concentration x. EPR spectra demonstrated the existence of two distinct Mn2+ ion locations: one incorporated in the core and the other near the surface of the Cd1−xMnxS NCs. The hyperfine interaction constants used to simulate the EPR spectra were A=7.6 and 8.2 mT, respectively. The synthesis of high quality Cd1−xMnxS NCs may allow the control of optical and magnetic properties.

Microwave spectroscopy of the PBr radical in the XΣ−3 state

The microwave spectrum of the PBr radical in the X (3)Sigma(-) ground electronic state has been observed by a source modulated spectrometer. The PBr radical was generated in a free space cell by an acdc glow discharge in a mixture of PBr(3) with He andor H(2). A spectrum with three spin components for each of the two isotopomers, P(79)Br and P(81)Br, was observed. The spectrum showed hyperfine splitting caused by interactions due to both bromine and phosphorus nuclei. The molecular constants including the magnetic hyperfine and nuclear quadrupole hyperfine interaction constants were determined by analyzing the observed spectrum. The spin density of the unpaired electrons was estimated from the observed hyperfine coupling constants to be 85.4% and 16.3% on the phosphorus and bromine atoms, respectively.

Superhyperfine Structure of EPR Spectra in LiLuF4:U3+ and LiYF4:Yb3+ Single Crystals

Electron paramagnetic resonance (EPR) spectra of doped paramagnetic crystals LiLuF4:U3+ and LiYF4:Yb3+ have been investigated at a frequency of about 9.42 GHz in the temperature range of 10–20 K. The U3+ ion spectrum is characterized by g-factors g ∥ = 1.228 and g ⊥ = 2.516, and contains the hyperfine structure due to the 235U isotope with nuclear spin I = 7/2 and natural abundance of 0.71%. The observed hyperfine interaction constants are A ∥ = 81 G and A ⊥ = 83.8 G. Moreover, the spectrum reveals the well-resolved superhyperfine structure (SHFS) due to two groups of four fluorine ions forming the nearest surrounding of the U3+ ion. This SHFS contains up to nine components with the spacing between components being about 12.7 G. The SHFS is observed also in the EPR spectrum of the LiYF4:Yb3+ crystal; up to 17 components with spacing of about 3.7 G may be traced. Some parameters of the effective Hamiltonian of the SHF interaction are estimated, the contribution of covalent bonding of f-electrons with ligands into these parameters is discussed.

HFSZEEMAN—A program for computing weak and intermediate field fine and hyperfine structure Zeeman splittings from MCDHF wave functions

Given electronic wave functions generated by the grasp2K relativistic atomic structure package, this program calculates diagonal magnetic dipole A J and electric quadrupole B J hyperfine interaction constants and Landé g J factors. In addition the program computes diagonal and off-diagonal reduced hyperfine and Zeeman matrix elements and constructs the total interaction matrix for an atom in an external magnetic field. By diagonalizing the interaction matrix and plotting eigenvalues as functions of the magnetic field, Zeeman splittings of hyperfine levels are obtained. The method is applicable in the weak and intermediate field regions and yields results that are useful when analyzing spectra from e.g. EBIT sources and magnetic stars. The program can also be used in the field free limit to calculate mixing coefficients that determine rates of hyperfine induced transitions. For atoms with zero nuclear spin I the program computes splittings of the fine-structure levels. Program summary Program title: HFSZEEMAN Catalogue identifier: ADZS_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADZS_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 1528 No. of bytes in distributed program, including test data, etc.: 294 664 Distribution format: tar.gz Programming language:Fortran, Matlab Computer: IBM-compatible PC, unix workstation Operating system: Unix, Linux Classification: 11.6 Subprograms used: Cat Id: ADZL_v1_0; Title: grasp2K v1.0; Reference: CPC 177 (2007) 597 Nature of problem: Prediction of weak and intermediate field Zeeman splittings of fine- and hyperfine structure levels using multiconfiguration Dirac–Hartree–Fock wave functions. Solution method: The electronic wave function for a state labeled ΓJM is expanded in terms of jj-coupled configuration state functions | Γ J M 〉 = ∑ γ c γ | γ J M 〉 . In this representation the reduced matrix elements used to construct the interaction matrix can be computed as sums over one-particle radial integrals. By diagonalizing the interaction matrix and plotting eigenvalue as functions of the magnetic field, Zeeman splittings of fine- and hyperfine structure levels are obtained. Restrictions: The complexity of the cases that can be handled is entirely determined by the grasp2K package [P. Jönsson, H. Xe, C. Froese Fischer, I.P. Grant, Comput. Phys. Commun. 177 (2007) 597] used for the generation of the electronic wave functions. Running time: CPU time required to execute test cases: a few seconds.

Use of spin labels to study membrane proteins by high-frequency electron nuclear double resonance spectroscopy

The applicability of spin labels to study membrane proteins by high-frequency electron nuclear double resonance spectroscopy is demonstrated. With the use of bacteriorhodopsin embedded in a lipid membrane as an example, the spectra of protons of neighboring amino acids are recorded, electric field gradients at the membrane surface are detected, and the constant of hyperfine interaction with the chlorine nucleus at the site of ion trapping is measured.

Spectral properties of In II from MCDHF calculations

We report extensive relativistic multiconfiguration Dirac–Hartree–Fock calculations of oscillator strengths and hyperfine structures for a large number of electric dipole transitions in In II. Results for the 5s21S0–5s5p3Po0 hyperfine induced transition are also presented. Core polarization is accounted for by means of explicit CI. To describe spin–polarization effects configuration state functions obtained by single excitations from all core-shells are included in the expansions. The computed oscillator strength for the 5s21S0–5s5p3Po1 intercombination transition is in good agreement with laser spectroscopy measurements of In+ ions in a radio-frequency trap. The calculated magnetic dipole hyperfine interaction constants agree very well with experimental constants derived from Fourier transform spectra. The problem with off-diagonal interactions affecting the hyperfine structures in closely spaced fine-structure levels is discussed.

Dechlorination of diphenyl chlorophosphate with tin and sodium 3,6-di- tert-butyl- ortho-semiquinones

Dechlorination reactions of diphenyl chlorophosphate (PhO) 2P(O)Cl with 3,6-di- tert-butyl- ortho-semiquinone complexes of triphenyl tin, SQSn(Ph) 3 and sodium, SQNa have been investigated by electron spin resonance (ESR) both in solutions at 300 K and in solid phase at 77 K using mechanochemical activation. Paramagnetic 3,6-di- tert-butyl- ortho-semiquinone ligand (SQ) was used as spin probe to monitor changes in the Sn coordination sphere during the dechlorination reaction of (PhO) 2P(O)Cl and consecutive formation of tin chloride derivatives SQSnCl(Ph) 2, SQSnCl 2Ph, and SQSnCl 3. Their structure was revealed based on the analysis of hyperfine interaction constants due to 1H nuclei in the 4,5-positions of the aromatic ring of the SQ ligand, hyperfine interaction constants due to 35Cl and 37Cl nuclei of chlorine atoms in the Sn coordination sphere, and hyperfine interaction constants due to 117Sn and 119Sn nuclei of the central metal ion. Interaction of SQSnCl 3 with (PhO) 2P(O)Cl leads to the formation of a meta-stable radical–anion complex [SQSnCl 4] − P +(O)(OPh) 2, which transforms to SQSnCl 3⋯P(O)R 3, a stable adduct of SQSnCl 3 with dechlorinated phosphate of a general formula P(O)R 3. Analysis of solution dechlorination reaction products suggests the formation of diphenyl-phosphoryl radical (PhO) 2P (O), which was not observed in solutions. Dechlorination of (PhO) 2P(O)Cl with sodium 3,6-di- tert-butyl- ortho-semiquinone SQNa can proceed in solid phase in liquid nitrogen at 77 K via mechanochemical activation using a ball mill. ESR analysis of the cryo-mechanochemical reaction showed the formation of the adduct of (PhO) 2P (O) radical with 3,6-di- tert-butyl- ortho-quinone.

Interplay of methyl-group internal rotation and fine and hyperfine interaction in a free radical: Fourier transform microwave spectroscopy of the acetyl radical

The fine and hyperfine structure in a free radical containing a methyl group is examined by paying special attention to the effects of the methyl group internal rotation on the fine and hyperfine interactions. The effects are taken into account by expanding pertinent constants describing couplings among angular momenta of electron and nuclear spins, internal rotation and overall rotation in terms of the internal rotation angle. The three-fold symmetry of the methyl internal rotation potential is lifted to some extent by fine and hyperfine interactions. The formulation thus developed is applied to the analysis of Fourier transform microwave spectra of the acetyl radical, and the potential barrier to internal rotation [139.958 (18) cm−1] and fine and hyperfine interaction constants are determined precisely.

Paramagnetic Properties of the Halide‐Bound Derivatives of Oxidised Tyrosinase Investigated by 1H NMR Spectroscopy

The (1)H NMR relaxation characteristics of the histidines in the oxidised type-3 copper site of tyrosinase (Ty(met)) from the bacterium Streptomyces antibioticus in the halide-bound forms (Ty(met)X with X = F(-), Cl(-), Br(-)) have been determined and analysed. The (1)H NMR spectra of the Ty(met)X species display remarkably sharp, well-resolved, paramagnetically shifted (1)H signals, which originate from the protons of the six His residues coordinated to the two Cu(II) ions in the type-3 centre. From the temperature-dependence of the (1)H paramagnetic shifts the following values for the exchange-coupling parameter -2J were determined: 260 (Ty(met)F), 200 (Ty(met)Cl) and 162 cm(-1) (Ty(met)Br). The (1)H T(1) relaxation is dipolar in origin and correlates with the Cu--H distances. Electronic relaxation times tau(S) derived from the (1)H T(1) data amount to about 10(-11) s and follow the order Ty(met)F>Ty(met)Cl>Ty(met)Br. They are two orders of magnitude shorter than the tau(S) values reported for mononuclear copper systems, in accordance with the sharpness of the (1)H signals. The results corroborate the Cu(2) bridging mode of the halide ions. On the basis of the measured hyperfine interaction constants for the ligand histidine nuclei, it is concluded that 70-80 % of the spin density in the excited triplet state resides on the two copper ions and the bridging atoms.