Isotropic Hyperfine Splitting Research Articles

Characterization of Cuban and Brazilian natural zeolites by photoacoustic spectroscopy and electron paramagnetic resonance.

This report describes the photoacoustic and electron paramagnetic resonance investigations of Brazilian and Cuban zeolites. Photoacoustic optical absorption measurements indicate the presence of iron (Fe3+) ions with their respective transition bands for both zeolites. Two species of manganese (Mn2+ and Mn3+) were identified in the Cuban sample and the electronic transitions assigned. Iron and manganese ions were confirmed through nonradiative relaxation (τ) and characteristic diffusion (τβ) times evaluation, whose values were found to be τBRA = 5.40 ms, τCUB = 4.60 ms, τβBRA = 387 μs and τβCUB = 305 μs. Crystal field (Dq-BRA/Dq-CUB = 1048 cm-1/945 cm-1) plus Racah (B-BRA/B-CUB = 457 cm-1/813 cm-1 and C-BRA/C-CUB = 3655 cm-1/2496 cm-1) parameters were assessed as well. Paramagnetic resonance corroborated Fe3+ ions present in the Brazilian zeolite occupying sites showing axial and/or rhombic symmetry distortions. For the Cuban sample, results reveal the characteristic hyperfine sextet lines of Mn2+ overlapping the Fe3+ line. Values of Landé factor and isotropic hyperfine splitting constant were found to be 2.0 and 9.7 mT, respectively. This tells us that the Mn2+ lies in octahedral symmetry probably replacing calcium ions and point towards an ionic bonding character of the Mn2+ and its surroundings.

Peculiarities of Spin Exchange in Nitroxide Biradicals Containing Two para-Phenylene Groups in the Bridge: EPR Investigation and DFT Calculations

Two nitroxide biradicals of similar composition: R5-C≡C-Ph-Ph-C≡C-R5 (B3a) and R6-C≡C-Ph-Ph-C≡C-R6 (B3b), where Ph = p-C6H4, and R6 is 2,6,6-tetramethyl-5,6-dihydropyridin-1(2H)-yloxyl, and R5 is 2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-1-yloxyl nitroxide rings, have been studied by electron paramagnetic resonance (EPR) spectroscopy. Variations of the intramolecular electron spin exchange in the biradicals, dissolved in toluene, as a function of temperature were characterized by changes in the isotropic 14N hyperfine splitting (hfs) constant a, values of the exchange integral |J|, and compared with the data obtained by density functional theory calculations (DFT). Thermodynamic parameters of the conformational rearrangements were calculated. Geometries of nitroxide biradicals in PES local minima and transition states in the triplet state were calculated at UDFT/B3LYP level with split-valence basis set cc-PVTZ. Probable differences in biradicals behavior are discussed.

The benzene radical anion: A computationally demanding prototype for aromatic anions

The benzene radical anion is studied with ab initio coupled-cluster theory in large basis sets. Unlike the usual assumption, we find that, at the level of theory investigated, the minimum energy geometry is non-planar with tetrahedral distortion at two opposite carbon atoms. The anion is well known for its instability to auto-ionization which poses computational challenges to determine its properties. Despite the importance of the benzene radical anion, the considerable attention it has received in the literature so far has failed to address the details of its structure and shape-resonance character at a high level of theory. Here, we examine the dynamic Jahn-Teller effect and its impact on the anion potential energy surface. We find that a minimum energy geometry of C2 symmetry is located below one D2h stationary point on a C2h pseudo-rotation surface. The applicability of standard wave function methods to an unbound anion is assessed with the stabilization method. The isotropic hyperfine splitting constants (Aiso) are computed and compared to data obtained from experimental electron spin resonance experiments. Satisfactory agreement with experiment is obtained with coupled-cluster theory and large basis sets such as cc-pCVQZ.

Functionalised Mn(VI)-nanoparticles: an advanced high-valent magnetic catalyst.

We discover MnVI-nanoparticles (NPs) bearing functional groups, high oxidation state, strong electron affinity, unique redox and paramagnetic nature, which opens up a new avenue to catalysis, magnetism and material application. However, its synthesis is challenging and remains unexplored because of associated serious difficulties. A simple benign synthetic strategy is devised to fabricate the high-valent NPs using mild reducing agent bromide, which transformed MnVII to valuable MnVI-species. The EELS-imaging of individual elements, ESI-MS, XPS and other techniques established its composition as Br(Me3SiO)MnVIO2. It revealed significantly improved magnetic moment (SQUID) with isotropic hyperfine splitting of six line spectrum (EPR). The high-oxidation state and incorporated-ligands of the metals present on the active surface of the NPs led to development of a general catalytic process for oxidative heterodifunctionalisation to C-C triple bond towards formation of a new O-C/N-C/S-C and C-C coupling cum cyclisation to biologically important flavones and their aza- and marcapto-analogues, and valuable enaloxy synthons.

The Structure and EPR Behavior of Short Nitroxide Biradicals Containing Sulfur Atom in the Bridge

Two short nitroxide biradicals of similar composition: S(OR6)2 (1) and O=S(OR6)2 (2), where OR6 is 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine, have been studied by electron paramagnetic resonance spectroscopy, and X-ray structural analysis. Variations of the intramolecular electron spin exchange in the biradicals, dissolved in toluene and ethanol, as a function of temperature were characterized by changes in the isotropic 14N hyperfine splitting constant a, values of the exchange integral \( \left| J \right|, \) and compared with the X-ray structural data. Thermodynamic parameters of the conformational rearrangements were calculated. Geometry optimization and spin density distribution calculations of biradicals 1 and 2 were carried out on the DFT/UB3LYP/cc-pVdz and DFT/ROPBE/N07D levels of theory. Structural rigidity and probable differences in biradicals behavior are discussed.

EPR Spectroscopy of Mercury-Organic Compounds with Nitroxide Radicals

Intramolecular electron spin exchange as a function of temperature, solvent viscosity and polarity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy for two nitroxide biradicals containing mercury-organic groups in the bridge connecting two 1-oxyl-2,2,6,6-tetramethylpiperidine-3,4-ene-nitroxide rings, R. The temperature dependence of the isotropic hyperfine splitting (hfs) constant a N and the exchange integral value |J/a| of the biradicals were measured from EPR spectra and subsequently analyzed comparing to a N and a Hg hfs constants of ClHgR radical dissolved in the same solvents. In all cases, the interaction of solvent molecules (SM) with >N–O fragments of nitroxide rings led to a slight decrease in a values with increasing temperature. The |J/a| value varied slightly with temperature T changes. The changes of |J/a| are much less comparing to those with variation of the solvent polarity. The interaction between SM and Hg atoms inside the bridge is observed and discussed.

An EPR Study of Defects in Neutron-Irradiated Cubic SiC Crystals

EPR spectroscopy has been used to characterize neutron-irradiated cubic SiC samples after thermal annealing in the 200-1100OC temperature range. Three new paramagnetic defects named Ky6, Ky7 and Ky8 have been revealed. Based on the present results, these defects have been tentatively attributed to the negatively charged carbon vacancy-carbon antisite pair, negatively charged divacancy and neutral carbon <100> split interstitial, respectively. Furthermore, the finding of practically isotropic hyperfine splitting for EPR lines of the T6 center confirms its assignment as a carbon vacancy-interstitial pair.

Characterization of the Microenvironments of Alkylamidoamine-N-oxide Surfactant Aggregates by the EPR Spin Labeling Method

Spin labeling EPR methods were applied to characterize the local microviscosity and micropolarity of micellar aggregates formed by 3-(alkanoylamino)ethyldimethylamine-N-oxides (alkyl = nonyl, undecyl and tridecyl) in an aqueous environment in the temperature range 297–337 K. Four doxylstearic derivatives: 5-doxylstearate methyl ester (5-DSE), 16-doxylstearate methyl ester (16-DSE), 5-doxylstearic acid (5-DSA) and 16-doxylstearic acid (16-DSA) were used as spin probes to monitor mainly the rotational correlation time τ R and the isotropic hyperfine splitting constant A N. In the case of anisotropic spectra, the order parameter S was also determined. The selected probes were found to be sensitive to the microenvironments whereby it was possible to characterize in detail local changes in composition and micelle formation.

Behavior of Short Nitroxide Biradical in Room Temperature Ionic Liquids

The intramolecular electron spin exchange in short nitroxide biradical O=S(OR6)2 (I), where OR6 is 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine, dissolved in the room temperature ionic liquids (RTILs) 1-octyl-3-methylimidazolium hexafluorophosphate (omimPF6), 1-octyl-3-methylimidazolium tetrafluoroborate (omimBF4), 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4), and 1-ethyl-3-methylimidazolium tetrafluoroborate (emimBF4) has been studied by electron paramagnetic resonance (EPR) spectroscopy as a function of temperature. Temperature variations of the isotropic nitrogen hyperfine splitting constant a were measured from EPR spectra. Thermodynamic parameters of the conformational rearrangements were calculated and compared with literature data. These intramolecular movements in rather rigid short-chain biradical I dissolved in four different RTILs are described well by the Debye–Stokes–Einstein law. Unrestricted density-functional-theory calculations of the geometry and electronic structure of the biradical were carried out using the ORCA program package, and showed that the O=S< group is available for the interaction with anions and cations of RTIL. The possible mechanism of such conformational transitions in biradical I in RTIL is discussed.

Hydrogen hyperfine splitting constants for phenoxyl radicals by DFT methods: regression analysis unravels hydrogen bonding effects

DFT calculations using the B3LYP functional, medium-sized basis sets and empirical scaling of the results provide quantitative estimates of the hydrogen isotropic hyperfine splitting constants (hscs) in 2,6-di-alkyl phenoxyl radicals (1-11). Literature hscs for phenoxyl (12), 4-methylphenoxyl (13) and 4-methoxyphenoxyl (14) radicals, on the other hand, are poorly predicted by using this method. This different behaviour is explained considering that experimental hscs of 12-14 are influenced by H-bonds formed between phenoxyls and their parent phenols, usually present in large amounts in solution as radical precursors. This was confirmed experimentally by measuring the EPR spectra of 12-14 in the presence of increasing amounts of their parent phenols, and by calculating the hscs in the case of the formation of 1 : 1 and 1 : 2 complexes between these radicals and phenol. Relevance of these results to the choice of reference hscs as benchmarks for theoretical calculations and to kinetic and thermochemical determinations on unhindered phenoxyl radicals is discussed.

Features of Spin Exchange in Short-Chain Nitroxide Biradicals in Ionic Liquids

The intramolecular electron spin exchange has been studied by electron paramagnetic resonance (EPR) spectroscopy in the room temperature ionic liquids (RTIL) 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) and 1-octyl-3-methylimidazolium tetrafluoroborate (omimBF4) for three nitroxide biradicals with analogous structures of the connecting bridge between two >N–O· centers as a function of temperature. Temperature variations of the isotropic nitrogen hyperfine splitting constant a, and exchange integral values |J/a| were determined from EPR spectra and analyzed. Thermodynamic parameters of the conformational rearrangements were obtained. The spin exchange in rather rigid short-chain biradicals dissolved in omimBF4 and bmimPF6 was compared with that in toluene solutions. Interesting features of the spin exchange in biradicals in RTIL were observed and explained as a result of the specific intramolecular conformational transitions. Examples when rather rigid biradical molecules become flexible under an influence of RTIL are reported.

Features of spin exchange in nitroxide biradicals in the ionic liquid bmimPF6

The intramolecular electron spin exchange has been studied by electron paramagnetic resonance (EPR) spectroscopy in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) for various nitroxide biradicals as a function of temperature and the nature of the connecting bridge between two >NO· centers. Temperature variations of the isotropic nitrogen hyperfine splitting constant a and exchange integral values |J/a| were measured from EPR spectra and analyzed. Thermodynamic parameters of the conformational rearrangements were obtained. The spin exchange in rigid and flexible biradicals dissolved in the ionic liquid bmimPF6 was compared with that in toluene solutions. Interesting features of the spin exchange in biradicals in ionic liquid were observed and explained as a result of the specific intramolecular conformational transitions. The first example of a rather rigid biradical molecule becoming flexible under the influence of an ionic liquid is reported.

Application of electron paramagnetic resonance spectroscopy for validation of the novel (AN+DN) solvent polarity scale.

Based on solvation studies of polymers, the sum (1:1) of the electron acceptor (AN) and electron donor (DN) values of solvents has been proposed as an alternative polarity scale. To test this, the electron paramagnetic resonance isotropic hyperfine splitting constant, a parameter known to be dependent on the polarity/proticity of the medium, was correlated with the (AN+DN) term using three paramagnetic probes. The linear regression coefficient calculated for 15 different solvents was approximately 0.9, quite similar to those of other well-known polarity parameters, attesting to the validity of the (AN+DN) term as a novel “two-parameter” solvent polarity scale.

Application of Electron Paramagnetic Resonance Spectroscopy for Validation of the Novel (AN+DN) Solvent Polarity Scale

Based on solvation studies of polymers, the sum (1:1) of the electron acceptor (AN) and electron donor (DN) values of solvents has been proposed as an alternative polarity scale. To test this, the electron paramagnetic resonance isotropic hyperfine splitting constant, a parameter known to be dependent on the polarity/proticity of the medium, was correlated with the (AN+DN) term using three paramagnetic probes. The linear regression coefficient calculated for 15 different solvents was approximately 0.9, quite similar to those of other well-known polarity parameters, attesting to the validity of the (AN+DN) term as a novel “two-parameter” solvent polarity scale.

Ga self‐interstitials in GaN investigated by ab‐initio calculations of the electronic g ‐tensor

AbstractIn GaN, an exceptionally important role is played by Ga‐interstitials being mobile at room temperature. Despite rather large formation energy, they have been observed in irradiated wurtzite‐GaN: at least two similar, but clearly distict ODEPR‐signals L5 and L6/L6* have been identified (via exceptionally large, nearly isotropic hyperfine splittings of about 4 GHz) as interstitial $ {\rm Ga}^{2+}_{\rm i} $ in two diffent lattice configurations. However, judging from experimental data and total energy calculations alone, the exact microscopic configuration remains unclear. In this theoretical work, the situation is elucidated by ab‐initio calculating the complete set of EPR parameters, hyperfine splittings as well as g ‐tensors, for the stable structural configurations of the Ga‐interstitial using a gauge‐including projector augmented plane wave (GI‐PAW) approach. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Determining the Geometry and Magnetic Parameters of Fluorinated Radicals by Simulation of Powder ESR Spectra and DFT Calculations: The Case of the Radical RCF2CF2• in Nafion Perfluorinated Ionomers

The ESR spectrum of the chain-end radical RCF2CF2* detected in Nafion perfluorinated membranes exposed to the photo-Fenton reagent was accurately simulated by an automatic fitting procedure, using as input the hyperfine coupling tensors of the two F alpha and two F beta nuclei as well as the corresponding directions of the principal values from density functional theory (DFT) calculations. An accurate fit was obtained only for different orientations of the hyperfine coupling tensors for the two F alpha nuclei, indicating a nonplanar structure about the C alpha radical center. The fitted isotropic hyperfine splittings for the two F beta nuclei in the Nafion radical, 24.9 and 27.5 G, are significantly larger than those for the chain-end radical in Teflon (15 G), implying different radical conformations in the two systems. The excellent fit indicated that the geometry and electronic structure of free radicals can be obtained not only from single-crystal ESR spectroscopy, but also, in certain cases, from powder spectra, by combination with data from DFT calculations. The optimized structures obtained by DFT calculations for the CF3CF2CF2CF2* or CF3OCF2CF2* radicals as models provided additional support for the pyramidal structure determined from the spectral fit. Comparison and analysis of calculated and fitted values for the hyperfine splittings of the two F beta nuclei suggested that the radical detected by ESR in Nafion is ROCF2CF2*, which originates from attack of oxygen radicals on the Nafion side chain. The combination of spectrum fitting and DFT is considered important in terms of understanding the hyperfine splittings from 19F nuclei and the different conformations of fluorinated chain-end-type radicals RCF2CF2* in different systems, and also for elucidating the mechanism of Nafion fragmentation when exposed to oxygen radicals in fuel cell conditions.

Structures and cis-to-trans Photoisomerization of Hexafluoro-1,3-butadiene Radical Cation: Electron Spin Resonance and Computational Studies

The s-cis and s-trans isomer radical cations of hexafluoro-1,3-butadiene (s-cis-HFBD+ and s-trans-HFBD+) were generated by a gamma-irradiated solid solution of the neutral HFBD molecule in solid matrix at 77 K and observed by means of electron spin resonance (ESR) and electronic spectroscopies. In comparing the experimental isotropic and anisotropic 19F hyperfine splittings with the computational ones by the DFT B3LYP and MP2 methods, the generated s-cis-HFBD+ and s-trans-HFBD+ radical cations were confirmed to be 2A2 and 2Bg electronic ground states in C2v and C2h symmetries, respectively. The present spectroscopic study revealed that the relative abundance of s-cis-HFBD+ to s-trans-HFBD+ was 4.0 immediately after being formed by gamma-irradiation, and subsequently most s-cis-HFBD+ was isomerized to s-trans-HFBD+ by visible-light illumination with 500-600 nm wavelength. The process of nonplanar HFBD ionizing to form stable planar s-cis- and s-trans-HFBD+ and the reaction mechanism of the cis-to-trans photoisomerization were discussed by (MS-)CASPT2//CASSCF calculated vertical excitation energies (Tv) and torsional potential energy curves (TPECs) of HFBD and HFBD+.

ESEEM Studies of Peptide Nitrogen Hyperfine Coupling in Tyrosyl Radicals and Model Peptides

Tyrosyl radicals are important in long-range electron transfer in several enzymes, but the protein environmental factors that control midpoint potential and electron transfer rate are not well understood. To develop a more detailed understanding of the effect of protein sequence, we have performed 14N and 15N electron spin echo envelope modulation (ESEEM) measurements on tyrosyl radical, generated either in polycrystalline tyrosinate or in its 15N-labeled isotopomer, by UV photolysis. 14N-ESEEM was also performed on tyrosyl radical generated in tyrosine-containing pentapeptide samples. Simulation of the 14N- and 15N-tyrosyl radical ESEEM measurements yielded no significant isotropic hyperfine splitting to the amine or amide nitrogen; the amplitude of the anisotropic, nitrogen hyperfine coupling (0.21 MHz) was consistent with a dipole-dipole distance of 3.0 A. Density functional theory was used to calculate the isotropic and anisotropic hyperfine couplings to the amino nitrogen in four different tyrosyl radical conformers. Comparison with the simulated data suggested that the lowest energy radical conformer, generated in tyrosine at pH 11, has a 76 degrees Calpha-Cbeta-C1'-C2' ring and a -73 degrees C-Calpha-Cbeta-C1' backbone dihedral angle. In addition, the magnitude, orientation, and asymmetry of the nuclear quadrupole coupling tensor were derived from analysis of the tyrosyl radical 14N-ESEEM. The simulations showed differences in the coupling and orientation of the nuclear quadrupole tensor, when the tyrosinate and pentapeptide samples were compared. These results suggest sequence- or conformation-induced changes in the ionic character of the NH bond in different tyrosine-containing peptides.

Structures of Tetrafluorocyclopropene, Hexafluorocyclobutene, Octafluorocyclopentene and Related Perfluoroalkene Radical Anions Revealed by Electron Spin Resonance Spectroscopic and Computational Studies

Isotropic and anisotropic ESR spectra were observed for the radical anions of hexafluorocyclobutene (c-C(4)F(6)(-)), octafluorocyclopentene (c-C(5)F(8)(-)) and perfluoro-2-butene (CF(3)CF=CFCF(3)(-)) in gamma-irradiated plastically crystalline neopentane, tetramethylsilane (TMS) and TMS-d(12) matrices, or the rigid 2-methyltetrahydrofuran (MTHF) matrix. The isotropic spectra of c-C(4)F(6)(-) and c-C(5)F(8)(-) are characterized by three different sets of pairs of (19)F nuclei with the isotropic hyperfine (hf) splittings of 15.2 (2F), 6.5 (2F), 1.1 (2F) mT for c-C(4)F(6)(-) and 14.7 (2F), 7.4 (2F), 1.0 (2F) mT for c-C(5)F(8)(-). By comparison with the results of ab initio quantum chemical computations, the large triplet (19)F hf splittings of ca. 15 mT are assigned to the two fluorines attached to the C=C bond. The UHF, B3LYP and MP2 computations predict that the geometrical structures of the perfluoroalkenes are strongly distorted by one-electron reduction to form their radical anions; c-C(3)F(4)(-): C(2) symmetry ((2)A state) <-- C(2)(v) ((1)A(1)), c-C(4)F(6)(-): C(1) ((2)A) <-- C(2)(v) ((1)A(1)) and c-C(5)F(8)(-): C(1) ((2)A) <-- C(s) ((1)A'). The structural distortion arises from a mixing of the pi* and higher-lying sigma* orbitals at the C=C carbons similar to that previously found for CF(2)=CF(2)(-) with a C(2)(h) distortion. The isotropic (19)F hf splittings computed with the B3LYP method with 6-311+G(2df,p) basis set for the geometry optimized by the UHF and/or MP2 methods are within 6% error of the experimental values. The experimental anisotropic spectra of c-C(4)F(6)(-), c-C(5)F(8)(-) and CF(2)=CF(2)(-) were satisfactorily reproduced by the ESR spectral simulation method using the computed hf principal values and orientation of (19)F nuclei. In addition, the electronic excitation energies and oscillator strengths for the CF(2)=CF(2)(-), c-C(3)F(4)(-), c-C(4)F(6)(-) and c-C(5)F(8)(-) radical anions were computed for the first time by TD-DFT methods.

Transmembrane Organization of Yeast Syntaxin-Analogue Sso1p

Membrane fusion in secretory pathways is thought to be mediated by SNAREs. It is proposed that membrane fusion transits through hemifusion, a condition in which the outer leaflets of the bilayers are mixed, but the inner leaflets are not. Hemifusion then proceeds to the fusion pore that connects the two internal contents. It is believed that the transmembrane domains (TMDs) of the fusion proteins play an essential role in the transition from hemifusion to the fusion pore. In this work, the structure, dynamics, and membrane topology of the TMD of Sso1p, a target membrane (t-) SNARE involved in the trafficking from Golgi to plasma membrane in yeast, was investigated using site-directed spin labeling and EPR spectroscopy. The EPR analysis of spin-labeled mutants showed that the TMD of Sso1p is a well-defined membrane spanning alpha-helix. The results also indicate that there is an equilibrium between the monomers and the oligomers. The oligomerization is mainly mediated through the interaction at the N-terminal half of the TMD, whereas the C-terminal half is free of the tertiary interaction. Additionally, the isotropic hyperfine splitting values were examined for nitroxide-scanning mutants, and it was found that the hyperfine splitting values show a V-shaped profile across the bilayer. Thus, hyperfine splitting may be used as an additional parameter to measure bilayer immersion depths of nitroxide.