Isotropic Hyperfine Interaction Constants Research Articles

Rotational Dynamics of Nitroxide Biradical in Room-Temperature Ionic Liquids Measured by Quantitative Simulation of EPR Spectra.

Temperature dependences of electron paramagnetic resonance (EPR) spectra of an imidazoline nitroxide biradical spin probe in a series of room-temperature ionic liquids in the temperature range 124-390 K have been quantitatively simulated. The unusual asymmetric EPR spectrum shape previously observed in these systems [Kokorin et al., Appl. Magn. Res. 48 (2016) 287] is shown to originate from anisotropic rotational diffusion of the probe molecule. All experimental spectra were quantitatively reproduced in simulation using a unified set of geometrical and magnetic parameters of the spin probe, which were found to be fully consistent with the biradical geometry obtained from density functional theory calculations. Temperature dependences of rotation diffusion coefficient of the probe characterize the molecular mobility of the ionic liquid, whereas the temperature dependences of the spin-exchange integral J and of the isotropic hyperfine interaction constant, aN, are shown to reflect the intramolecular conformation motions of the biradical probe.

Identification of an isolated arsenic antisite defect in GaAsBi

Optically detected magnetic resonance and photoluminescence spectroscopy are employed to study grown-in defects in GaAs0.985Bi0.015 epilayers grown by molecular beam epitaxy. The dominant paramagnetic defect is identified as an isolated arsenic antisite, AsGa, with an electron g-factor of 2.03 ± 0.01 and an isotropic hyperfine interaction constant A = (900 ± 20) × 10−4 cm−1. The defect is found to be preferably incorporated during the growth at the lowest growth temperature of 270 °C, but its formation can be suppressed upon increasing growth temperature to 315 °C. The AsGa concentration is also reduced after post-growth rapid thermal annealing at 600 °C.

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.

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).

Solvation Dependences of Isotropic Hyperfine Interaction Constants and out-of-Plane Distortions of ortho-Substituted Nitrobenzene Radical Anions

For a number of ortho-substituted nitrobenzene radical anions (RAs) generated in DMF and its binary mixtures with water, it is shown that for radical anions with a substituent of minor effective volume in one ortho position to the nitro group, the dependences of the isotropic hyperfine interaction (ihfi) constants on the mole fraction of water are S-like and dictated by the medium composition and the concerted out-of-plane rotational and pyramidal distortions of the nitro group of the radical anion. The S-like shape of the solvation dependences of the ihfi constants is dictated by the dominant rotational distortions of the nitro group. For most radical anions with two ortho substituents or with one ortho substituent with a large effective volume, the S-like dependences are not observed, and the values of the nitrogen ihfi constants depend on the dominant pyramidal distortion of the nitro group. For the 2-tert-butylnitrobenzene radical anion in water, the nitrogen ihfi constant is aN=25.62 G, which is typical of nitroaliphatic radical anions. This effect is explained based on the pyramidal structure of the nitro group in the case of its large rotation angles.

Out-of-Plane Distortions in Fluorinated Nitrobenzene Radical Anions

According to the data of UHF/INDO calculations of the model conformations of fluorinated nitrobenzene radical anions, rotation of the nitro group relative to the plane of the benzene ring is accompanied by a pyramidal distortion of the group, which is of pseudo-Jahn–Teller nature. The degree of structural distortions depends on the position of the fluorine atoms in the benzene ring and on the solvent, increasing from DMF to DMF–water mixtures. The values of isotropic hyperfine interaction constants are interpreted in the series of fluorinated nitrobenzene radical anions, and the effects of water content in binary mixtures of solvents are discussed.

Indo study of the angular dependences of isotropic hyperfese interaction constants in the model conformations of the nitrobenzene radical anion

The UHF/INDO calculations of the model conformations of the nitrobenzene radical onion show that rotation of the nitro group relative to the plane of the benzene ring is accompanied by a pyramidal distortion of the group caused by the pseudo-Jahn-Teller effect (vibronic interaction between the ground n and totally symmetric lowest excited σ states). The angular dependences of the14N,13C,1H, and17O Isotropic hyperfine interaction constants are analyzed. Experimental ESR data are interpreted for the radical anions of nitrobenzene derivatives with ortho-alkyl groups.

EPR and Optical Spectra of Cobalt in SrLaAlO4

The paramagnetic resonance and optical spectra of Co 2+ ions in SrLaAlO 4 single crystals are reported. The EPR spectrum shows one electronic transition consisting of eight hyperfine lines with anisotropic g-factor and nearly isotropic hyperfine interaction constant. A tetragonal distortion of the octahedral crystal field acting on Co 2+ ions has been found. The optical spectrum consists of intensive lines corresponding to transition ΔS = 0 and weak intercombination lines. In the weak-field coupling scheme satisfactory agreement is obtained between the observed and calculated spectrum with Dq = 1030 cm -1 and Racah parameter B = 810 cm -1 . Both the optical and EPR data indicate a covalent bonding of Co 2+ to the nearest oxygen atoms.

EPR and Optical Spectra of Cobalt in SrLaAlO4

The paramagnetic resonance and optical spectra of Co2+ ions in SrLaAlO4 single crystals are reported. The EPR spectrum shows one electronic transition consisting of eight hyperfine lines with anisotropic g-factor and nearly isotropic hyperfine interaction constant. A tetragonal distortion of the octahedral crystal field acting on Co2+ ions has been found. The optical spectrum consists of intensive lines corresponding to transition ΔS = 0 and weak intercombination lines. In the weak-field coupling scheme satisfactory agreement is obtained between the observed and calculated spectrum with Dq = 1030 cm—1 and Racah parameter B= 810 cm—1. Both the optical and EPR data indicate a covalent bonding of Co2+ to the nearest oxygen atoms.

ChemInform Abstract: Anion Radicals of para‐Substituted Nitrobenzenes with CFH2, CHF2, and CF3 Groups and the Angle Function Parameters of β‐Fluorine Isotropic Hyperfine Interaction Constants.

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Methyl group tunneling rotation in the lowest nπ* state of toluquinone. An optically detected ENDOR, LAC and CR study

In this paper we report and discuss the effects of methyl group tunneling rotation on the methyl proton ENDOR, LAC and CR spectra in the lowest triplet state of toluquinone at 1.8 K. From a detailed analysis of the ENDOR spectra in the lowest rotational state (A) we obtain for the methyl protons the following isotropic hyperfine interaction constants: A 1 = A 2 = 34.4 MHz and A 3 = −53.7 MHz. The tunneling frequency (3 J) is calculated to be 2.9 GHz. The most likely equilibrium configuration of the torsional oscillator is found to be the one whereby the molecular plane is a mirror plane and the out-of-plane methyl protons point towards the closest oxygen atom. We also show that cross-relaxation between |A> and |E> states of translational equivalent toluquinone molecules is responsible for the observed sidebands in the level-anti-crossing spectra of toluquinone.

Dependence of the isotropic hyperfine interaction constant in molecules of nitroxide radicals on the dielectric properties of the solvent

Dependence of the isotropic hyperfine interaction constant in molecules of nitroxide radicals on the dielectric properties of the solvent

Calculation of the isotropic hyperfine-interaction constants in the cyclopentyl radical

Calculation of the isotropic hyperfine-interaction constants in the cyclopentyl radical

NMR Studies of a Series of Phenyl Substituted Ferric N-Ethyl-N-Phenyldithiocarbamates

The proton NMR results of a series of phenyl substituted tris(N-ethyl-N-phenyldithiocarbamato)-iron(III) complexes have been successfully interpreted assuming the iron atom is in an intermediate crystal field of octahedral symmetry. The substituent in the phenyl ring has very little effect on the energy separation of the 6A1 and 2T2 electronic states. However, the substituent substantially influences the methylene proton isotropic hyperfine interaction constants in the 6A1 state. These results have been interpreted as arising from changes in the π character of the N-phenyl bond due to the degree of hindered rotation with the aromatic substitution.

A theoretical study of the 14N and 17O N.M.R. shifts in lanthanide complexes

The experimental 14N and 17O n.m.r, results in a series of lanthanide complexes are successfully interpreted from a second-order perturbation treatment of the calculation of (S2), where bonding effects and spin-orbit coupling mixing are incorporated. The isotropic hyperfine interaction constants are shown to be negative for 14N and positive for 17O but both independent of the particular lanthanide ion. We also confirm that the 4f orbitals are not involved in direct bonding with the ligands.

Nuclear Magnetic Relaxation of 19F in Transition-Metal Complexes. II. Complexes with Mn(II), Co(II), Fe(II), and Ni(II)

The effect of Mn(II), Fe(II), Co(II), and Ni(II) on T1 and T2 of aqueous 19F has been measured from pH 0–6 at 25°C as a function of fluoride concentration and magnetic field, similar to previous work with Cu(II) [J. Chem. Phys. 48, 4445 (1968)]. Equilibrium constants for the first coordination complex MF+ were obtained from the data and ranged from 1 to 5; there was no evidence of higher complexes. The principal results were the association and dissociation rates of the complex. The latter is found to be of the form kd = τm−1 = ka + kb(H+) + kx(F−) + ky(HF) + kz(HF2−), where τm is the lifetime of the complex. These rates are given in the following table: Mn(II)Fe(II)Co(II)Ni(II)logka5.95.85.03.3logkb8.78.88.45.8logkx6.2···5.23.8logky7.1······4.7logkz6.9···6.3···The electron relaxation time T1e was >1 × 10−8 sec for Mn(II) and <5 × 10−12 sec for the other three metals, similar to results found for the aquo complex. The isotropic hyperfine interaction constants Ai for FeF+ and CoF+ were 8.7 × 108 and 2.1 × 108 rad/sec. The temperature dependence of T1 and T2 for Ni19F+ was similar to that of proton relaxation in the aquo complex.

THE EPR SPECTRUM OF PF4 IN IRRADIATED NH4PF6

Second-order splittings predicted recently by Fessenden were observed in the EPR spectrum of NH/sub 4/PF/sub 6/ irradiated at 295 un. Concent 85% K with Co/sup 60/ gamma rays. The dominant paramagnetic fragment apparently is PF/sub 4/, but at least two other fragments appear. Isotropic hyperfine interaction constants and g values of the fragments are given. (D.C.W.)

The effect of alkali-metal ion association on the electronic spectra of the mononegative ions of naphthalene and anthracene

The energies of the absorption bands of mononegative ions of naphthalene and anthracene in tetrahydrofuran solution in the presence of alkali-metal cations show slight shifts when Li+ or K+ is substituted for Na+. This is interpreted as an ion-association effect, and it appears that for all three metal ions the M+R- ion-pairs are practically undissociated in solutions of concentration ∼ 10-3 M at 300°k. A simple perturbation theory is given and is found to fit the observed trends of individual bands reasonably well. The absence of hyperfine structure from interaction with the K nucleus in the electron spin resonance spectra of tetrahydrofuran solutions of potassium naphthalenide found in the recent work of Atherton and Weissman is attributed to the rather small isotropic hyperfine interaction constant for 39K.