An EPR study on cytosine irradiated

In this study, paramagnetic centers over the cytosine were formed by photolysis then these centers were investigated using EPR method. EPR signals were not recorded from non‐irradiated the cytosine, but irradiated polycrystalline exhibited complex EPR spectra. For obtaining of cytosine polycrystalline, novel crystallization method was performed on powder cytosine. Effective crystallization conditions were achieved by adjustment of the concentration of the metal ions, chemical solutions, NaCl, KCl, glacial asetic acid, nitric oxide, percloric acid, glutamic acid, and pH of buffer. Cytosine (C₄H₅N₂O) polycrystalline obtained were irradiated with ⁶⁰Co – rays at room temperature for 24 and 72 h. At the sample irradiated for 72 h, the paramagnetic centers were determined between 120 and 450 K by X‐band EPR spectrometer. The spectra were found to be dependent slightly on temperature. Two cation radicals were determined in the structure and these were called Radicals I and II. The g and hyperfine constants were found to be a_H2a = 61 G, a_N2 = 9.39 G, a_N1 = 7.15 G, and g₁=2.0026 for the Radical I; a_H3 = 10.57 G, a_H1 = 3 G, a_N3 = 6.72 G, a_N1 = 5.36 G for, and g₂=2.0034 the Radical II. Copyright © 2010 John Wiley & Sons, Ltd.     

ESR spectroscopics study of radicals formed from methoxycarbonylcholine picrate hemihydrate

Abstract

The electron spin resonance of γ-irradiated single crystals of methoxycarbonylcholine picrate hemihydrate, C₇H₁₆NO₃ ⁺ · C₆H₂N₃O₇ ^? · ½ H₂O has been observed and analyzed for different orientations of the crystal in the magnetic field. The crystals have been investigated between 70 and 350 K. The spectra were found to be temperature independent and the radiation damage centers are attributed to – ?[Obar]OCH₃ and –CH₂CH₂O? radicals. The g and hyperfine coupling constants were found to be almost isotropic with an average, g = 2.0060, a _H1 = 4.4G for –CH₂CH₂O?, g = 2.0050, a _H2 = 3.5G for –CH₂CH₂O? and g = 2.0045, a _H = 3.5 G for –?[Obar]OCH₃. These values indicate a long-range coupling between the unpaired electron and H protons.     

A Variable Temperature X- and W-Band EPR Study of Fe-Doped SiCN Ceramics Annealed at 1000, 1100, and 1285 °C: Dangling Bonds,Ferromagnetism and Superparamagnetism

Polymer-derived SiCN ceramics, annealed (also referred to as pyrolyzed) at 1000, 1100, and 1285 °C, and doped with Fe(III) acetylacetonate, are investigated by electron paramagnetic resonance (EPR) from 4 to 120 K at X-band (9.425 GHz). In addition, the SiCN ceramic, annealed at 1100 °C, was studied by EPR at 300 K at W-band (93.96 GHz). There was observed a significant increase in EPR linewidth due to dangling bonds (g = 2.001) below 20 K at X-band. The low-field X-band FMR line (g ≈ 12) indicated the presence of ferromagnetic Fe₅Si₃ crystallites. There were found two EPR lines due to carbon-related dangling bonds: (1) those present as defects on the surface of the free-carbon phase (as sp² carbon-related dangling bonds with g = 2.0011) and (2) those present within the bulk of carbon phase (as sp³ carbon-related dangling bonds with g = 2.0033). On the other hand, the intense low-field EPR signal observed at X-band was not observed at W-band. As well, there was observed splitting of the single broad EPR signal observed at g = 2.05 at X-band into two signals at W-band at g = 1.99 and g = 2.06, due to two different Fe-containing superparamagnetic nanocrystallites. Two new EPR signals, not observed at X-band, were observed at W-band, namely at g = 2.28 and g = 3.00, which are also due to g_∥ of these superparamagnetic nanocrystallites.     

Effect of irradiation on ethyl 2‐amino‐4,5,6,7‐tetrahydrobenzo[b]thiophene‐3‐carboxylate: an ESR study

Ethyl 2‐amino‐4,5,6,7‐tetrahydrobenzo[b]thiophene‐3‐carboxylate [C₁₁H₁₅NO₂S] was synthesized by the Gewald method. Its single crystals were grown from an alcohol/ethyl acetate solution at 15 °C and characterized using IR and ¹H‐NMR. These single crystals were irradiated for 72 h at 298 K by a ⁶⁰Co gamma source with a dose speed of 0.864 kGy/h. After irradiation, electron spin resonance (ESR) measurements were carried out to study radiation‐induced radicals in the temperature range from 120 to 450 K. Additionally, for the single crystal, ESR angular dependencies were measured in the xy, xz and yz planes of the substance. This irradiated single crystal was analyzed based on the ESR spectra. Analysis of the spectra revealed that the radical was formed by a C–H bond fission at the carbon end of the substance. It was also observed that the color of the sample changed after irradiation. The hyperfine and g parameters were determined from the experimental spectra. It was inferred from these results that the hyperfine parameters and g value exhibited anisotropic behavior. The average values of these parameters were calculated as follows: g = 2.0088, A_H1=H2 = 20.70 G, A_H3=H4 = 10.80 G, A_Ha = 4.59 G, A_Hb = 3.24 G and, A_N = 6.10 G. Copyright © 2011 John Wiley & Sons, Ltd.     

Theoretical studies of the local structures and electron paramagnetic resonance parameters for Cu2+ center in Zn(C3H3O4)2(H2O)2 single crystal

The electron paramagnetic resonance (EPR) parameters (g factors g_xx, g_yy, g_zz and hyperfine structure constants A_xx, A_yy, A_zz) are interpreted by taking account of the admixture of d-orbitals in the ground state wave function of the Cu²⁺ ion in a Zn(C₃H₃O₄)₂(H₂O)₂ (DABMZ) single crystal. Based on the calculation, local structural parameters of the impurity Cu²⁺ center were obtained (i.e. R_a≈1.92 Å, R_b≈1.96 Å, R_c≈1.99 Å). The theoretical EPR parameters based on the above Cu²⁺?O^2? bond lengths in the DABMZ crystal show good agreement with the observed values and some improvements have been made as compared with those in the previous studies.     

E.S.R. and ENDOR of an α-chloro radical in X-irradiated 5-chlorodeoxyuridine single crystals

The most prominent radical formed from X-irradiation of the nucleic acid constituent analogue 5-chlorodeoxyuridine at room temperature is shown to be an α-chloro radical formed by hydrogen addition to C₆. The E.S.R. analysis of the ³⁵Cl hyperfine interaction combined with theoretical simulation of the spin hamiltonian yields tensor components a_xx =46·98 MHz, a_yy =-10·98 MHz and a_zz =-17·01 MHz with a quadrupole coupling constant of eqQ=72 MHz. The principal g-tensor values are g_xx =2·0012, g_yy =2·00862 and g_zz =2·00687. Three additional hyperfine interactions in the radical are observed combining E.S.R. and ENDOR spectroscopy. Besides the two nearly equivalent β-protons on C₆ with principal values of 103·39 MHz and 110·12 MHz, there is hyperfine interaction with the ¹⁴N nucleus of nitrogen N₃ (a_xx =9·81 MHz, a_yy =a_zz ? 0 MHz) and with the proton of the hydrogen bonded to N₃. The latter interaction has tensor components of 2·65 MHz, -10·80 MHz and -8·09 MHz as obtained from ENDOR data. The chlorine hyperfine coupling parameters are related to those observed in other α-chloro radicals. The mechanism of the formation of the radical in 5-chlorodeoxyuridine is discussed briefly.     

EPR of Cu2+ Impurities in Cytosine Hydrochloride: A Case of Superhyperfine Structure

Electron paramagnetic resonance spectra of Cu²⁺ impurities in cytosine hydrochloride single crystals are observed at liquid nitrogen temperature. Two magnetically equivalent sites for Cu²⁺ have been observed. The parameters of ⁶³Cu obtained with the fitting of spectra to rhombic symmetry spin Hamiltonian are: g _x  = 2.047 ± 0.002, g _y  = 2.187 ± 0.002, g _z  = 2.390 ± 0.002, A _x  = (86 ± 3) × 10^?4 cm^?1, A _y  = (87 ± 3) × 10^?4 cm^?1, and A _z  = (138 ± 3) × 10^?4 cm^?1. The observed bands in optical spectra of the single crystal recorded at room temperature are assigned to various d–d and charge-transfer transitions. Using both EPR and optical data, the nature of bonding of metal ion with different ligands is discussed.     

EPR and Optical Absorption Study of Cr3+-Doped Dipotassium Tetrachloropalladate

X-band electron paramagnetic resonance (EPR) study of Cr³⁺-doped dipotassium tetrachloropalladate single crystal is done at liquid nitrogen temperature. EPR spectrum shows two sites. The spin-Hamiltonian parameters have been evaluated by employing hyperfine resonance lines observed in EPR spectra for different orientations of crystal in externally applied magnetic field. The values of spin-Hamiltonian and zero-field splitting (ZFS) parameters of Cr³⁺ ion-doped DTP for site I are: g _x  = 2.096 ± 0.002, g _y  = 2.167 ± 0.002, g _z  = 2.220 ± 0.002, D = (89 ± 2) × 10^?4 cm^?1, E = (16 ± 2) × 10^?4 cm^?1. EPR study indicates that Cr³⁺ ion enters the host lattice substitutionally replacing K⁺ ion and local site symmetry reduces to orthorhombic. Optical absorption spectra are recorded at room temperature. From the optical absorption study, the Racah parameters (B = 521 cm^?1, C = 2,861 cm^?1), cubic crystal field splitting parameter (Dq = 1,851 cm^?1) and nephelauxetic parameters (h = 2.06, k = 0.21) are determined. These parameters together with EPR data are used to discuss the nature of bonding in the crystal.     

EPR and Optical Absorption Study of Cu2+-Doped Diammonium Hexaaqua Magnesium Sulphate Single Crystals

Electron paramagnetic resonance (EPR) study of Cu²⁺ ions doped in diammonium hexaaqua magnesium sulphate single crystal over the temperature range of 4.2–320 K is reported. Copper enters the lattice substitutionally and is trapped at two magnetically equivalent sites. The spin Hamiltonian parameters are evaluated at 320, 300, 77, and 4.2 K. The angular variations of the resonance lines in three mutually perpendicular planes ab, bc* and c*a are used to determine principal g and A values. The observed spectra are fitted to a spin Hamiltonian of rhombic symmetry with parameters of Cu²⁺ at 77 and 4.2 K: g _xx  = 2.089, g _yy  = 2.112, g _zz  = 2.437 (±0.002) and A _xx  = 38, A _yy  = 14, A _zz  = 110 (±2) × 10^?4 cm^?1. The ground state wave function of Cu²⁺ ion in this lattice is determined. The g-factor anisotropy is calculated and compared with the experimental value. The optical absorption spectra of the crystal are also recorded at room temperature. With the help of assigned bands the crystal-field parameters (Dq, Ds and Dt) are evaluated. By correlating the optical and EPR data, the nature of bonding in the complex is discussed. The temperature dependence of the g values is explained to conclude the occurrence of both static and dynamic Jahn–Teller effects over the temperature range of investigation.     

A New Paramagnetic Nitrogen Center in Natural Titanium-Containing Diamonds

A new type of nitrogen-related center in natural diamond labeled NU1 is identified as a 〈100〉 split interstitial configuration by means of electron paramagnetic resonance (EPR) and photoluminescence (PL) techniques. In PL this center is seen as an electron-vibration system with a zero-phonon line at 485 nm in the crystals containing S1/OK1 and 440.3 nm/N3 centers. NU1 EPR spectra are described by a spin-Hamiltonian with parameters: S = ½, I = 1 and A _xx (N) = 22.5 G, A _yy (N) = 19.5 G, A _zz (N) = 20.55 G, g _xx  = 2.0043, g _yy  = 2.0032, g _zz  = 2.0020. Directions of A _zz and g _zz coincide and correspond to [001]. The directions of A _xx and A _yy coincide with those of g _xx and g _yy and correspond to [110] and [?110], respectively. Analysis of the phonon structure of the NU1 center suggested that titanium can be the second atom together with nitrogen in the structure of a split interstitial.     

EPR, Optical Absorption and Superposition Model Study of Fe3+-Doped Ammonium Dihydrogen Phosphate Single Crystals

X-band electron paramagnetic resonance (EPR) studies are carried out on Fe³⁺ ions doped in ammonium dihydrogen phosphate (ADP) single crystals at room temperature. The crystal field and spin Hamiltonian parameters are evaluated from the resonance lines obtained at different angular rotations. The obtained values of spin Hamiltonian and zero-field parameters of the Fe³⁺ ion in ADP are: g = 1.994 ± 0.002, |D| = (220 ± 5) × 10^?4 cm^?1 and a = (640 ± 5) × 10^?4 cm^?1. On the basis of EPR data, the site symmetry of the Fe³⁺ ion in the crystal is discussed. The Fe³⁺ ion enters the lattice substitutionally replacing the NH₄ ⁺ sites. The optical absorption of the crystal is also studied at room temperature in the wavelength range of 195–925 nm. The energy values of different orbital levels are calculated. The observed bands are assigned as transitions from the ⁶ A _1g (S) ground state to various excited quartet levels of the Fe³⁺ ion in a cubic crystalline field. From the observed band positions, Racah interelectronic repulsion parameters (B and C), cubic crystal field splitting parameter (D _q ) and Trees correction are calculated. There values are: B = 970, C = 1,923, D _q  = 1,380 cm^?1 and α = 90 cm^?1, respectively. On the basis of EPR and optical data, the nature of bonding in the crystal is discussed. The zero-field splitting (ZFS) parameters are also determined theoretically using B _kq parameters estimated from the superposition model. The values of ZFS parameters thus obtained are |D| = (213 ± 5) × 10^?4 cm^?1 and |E| = (21 ± 5) × 10^?4 cm^?1.     

Optical and EPR Properties of Mn- and Eu-Activated LaMgAl11O19 Phosphor

LaMgAl₁₁O₁₉ phosphor doped with Eu and Mn ions has been prepared by the urea combustion route. The as-prepared phosphor was studied using X-ray diffraction, electron paramagnetic resonance (EPR), diffuse reflectance and photoluminescence studies. The EPR spectra of LaMgAl₁₁O₁₉:Eu, Mn phosphor exhibit signals with the effective g values at g = 1.98, 4.29 and 7.23. The resonance signals at g = 1.98 and 4.29 were attributed to Mn²⁺ ions in tetrahedral and rhombic environment, respectively. The resonance signal at g = 7.23 was attributed to Eu²⁺ ions. The optical spectrum of this phosphor exhibits an intense band in the visible region and this band has been attributed to spin-allowed ⁵E_g → ⁵T_2g transition of Mn³⁺ ions. Upon excitation at 324 nm, the material displays emission in the blue, green and red spectral region.     

Electron paramagnetic resonance of NH 3 + radical in potassium sulphate

Room temperature EPR spectra of (NH₄)₂SO₄ doped K₂SO₄ monocrystals irradiated with x-rays show the presence of NH₃ ⁺ radicals. The EPR parameters areg ‖=2.0037 andg ⊥ = 2.0068;¹⁴NA _XX=13.75;A _YY=24.5;A _ZZ=25.5 gauss;¹HA _XX=A _YY=22 andA _ZZ=25 gauss. From the¹⁴N and¹H coupling constants it has been inferred that at room temperature the planar NH₃ ⁺ radical undergoes rotation about theC ₃ axis which corroborates with the equivalence of the protons, but the radical itself is in an asymmetric crystal field environment. The 77K spectra indicate a considerable reduction in the motion of the radical with the free motion almost completely stopped. Part of Ph.D. work of the second author     

EPR and dielectric relaxation of Fe3+ in KTaO3

EPR and the method of dielectric losses have been used to investigate Fe³⁺ centers of axial and orthorhombic symmetry in KTaO₃ single crystals. The EPR spectrum of orthorhombic-symmetry Fe³⁺ obtained in the 8-mm wavelength range at T=77 K is described by the spin-Hamiltonian with parameters g _x=1.98, g _y=2.01, g _z=2.00, D=0.43 cm⁻¹, and E=5.87×10^t-2 cm⁻¹. From the dielectric measurement data we have obtained the following parameters of the relaxation of the orthorhombic Fe³⁺ centers in KTaO₃: characteristic relaxation frequency τ ₀ ⁻¹ =2.33×10¹² Hz and activation energy E _a=0.044 eV. A model of the orthorhombic Fe³⁺ center in KTaO₃ is discussed within the framework of the kinetic parameters obtained. Fiz. Tverd. Tela (St. Petersburg) 39, 861–864 (May 1997)     

EPR and Optical Absorption Study of VO2+-Doped Lithium Hydroxylammonium Sulphate

ABSTRACT

Single-crystal and powder EPR studies of VO²⁺-doped lithium hydroxylammonium sulphate (LiNH₃OHSO₄) were carried out at room temperature. The results indicate the presence of two magnetically inequivalent VO²⁺ sites. The VO²⁺ ion takes up a substitutional position in the host lattice. The angular variation of EPR spectra in three mutually perpendicular planes were used to determine the spin Hamiltonian parameters, and the values obtained were the following: For Site 1, g_x = 2.0249 ± 0.0002, g_y = 1.9698 ± 0.0002, g_z = 1.9552 ± 0.0002, A_x = (51 ± 2) × 10^?4 cm^?1, A_y = (93 ± 2) × 10^?4 cm^?1, and A_z = (165 ± 2) × 10^?4 cm^?1; and for Site 2, g_x = 2.0267 ± 0.0002, g_y = 1.9743 ± 0.0002, g_z = 1.9213 ± 0.0002, A_x = (40 ± 2) × 10^?4 cm^?1, A_y = (80 ± 2) × 10^?4 cm^?1, and A_z = (155 ± 2) × 10^?4 cm^?1. The optical absorption spectrum recorded at room temperature shows four bands. From the optical and EPR data, various molecular coefficients are evaluated, and the nature of bonding in the crystal is discussed.     

Variable Temperature Single Crystal EPR Studies of Cu(II) in Dipotassium Diaquabis(Malonato-<Emphasis Type="Italic">κ</Emphasis><Superscript>2</Superscript><Emphasis Type="Italic">O,O</Emphasis>′) Nickelate Dihydrate: An Example of Contaminated Ground State

Single crystal X-band electron paramagnetic resonance (EPR) studies on divalent copper ions embedded in dipotassium diaquabis(malonato-κ ² O,O′) nickelate dihydrate have been performed at 300, 123 and 77 K to understand the nature of Jahn–Teller distortion in the paramagnetic host lattice. The angular variation of the EPR spectra reveals the presence of two sites, with one site not showing hyperfine resolution even at 77 K. The spin-Hamiltonian parameters of this six-coordinated Cu(II) ion, evaluated from EPR spectra at various temperatures, are:

• 300 K: g ₁₁ = 2.125, g ₂₂ = 2.118, g ₃₃ = 2.290, no copper hyperfine resolution
• 123 K: g ₁₁ = 2.229, g ₂₂ = 2.113, g ₃₃ = 2.319 and A ₁₁ = 5.02, A ₂₂ = 3.82, A ₃₃ = 6.87 mT
• 77 K: g ₁₁ = 2.224, g ₂₂ = 2.114, g ₃₃ = 2.324 and A ₁₁ = 5.32, A ₂₂ = 3.90, A ₃₃ = 7.06 mT

respectively. The low value observed for A ₃₃ at 123 and 77 K has been explained by assuming a ground state \({\text{d}}_{{x^{2} - y^{2} }}\) wave function for Cu(II) ions, contaminated with the excited state \({\text{d}}_{{z^{2} }}\). From the temperature dependence of the EPR spectra, the Cu(II) ions can be considered as a static Jahn–Teller system, with contaminated ground state. The admixture coefficients and bonding parameters have also been calculated by combining EPR and optical data. The EPR spectrum of powder sample confirms single crystal data.

     

EPR of C60 thermal/photochemical reactions with polystyrene and polymethyl methacrylate

Thermal and photochemical free radical reaction products of C₆₀ with polymethyl methacrylate (PMMA) and polystyrene (PS) in orthodichlorobenzene solution were detected by EPR (electron paramagnetic resonance). Thermal radicals (<100°C) of C₆₀/PMMA and C₆₀/PS samples gave single line first-derivative EPR spectra withg=2.0029. Ultraviolet photolysis of a C₆₀/PMMA solid phase sample gave two radical species; whereas, photolysis of a C_60/PS solid phase sample gave only one free radical. EPR signals were also recorded for UV and thermal C₆₀ reaction with free radical initiator, azobis(isobutyronitrile).     

Application of a Stopped-Flow EPR Method for the Detection of Short-Lived Flavonoid Semiquinone Radicals Produced by Oxidation Using <Superscript>15</Superscript>N-Labeled Nitrosodisulfonate Radical (Fremy’s Salt)

A stopped-flow-electron paramagnetic resonance (EPR) method was applied for the detection of short-lived radicals of flavonoids bearing a catechol moiety as the B-ring, such as flavonols (quercetin, fisetin, and rutin), flavanones (eriodictyol and taxifolin), flavanols (catechin and epicatechin), and flavone (luteolin). ¹⁵N-labeled sodium salt of nitrosodisulfonate (¹⁵NDS) was employed to obtain the highly resolved EPR hyperfine structure (hfs) of flavonoid-derived semiquinone radicals under stoichiometrically regulated reaction conditions in aqueous media (pH 10). The EPR hfs of these flavonoids radicals, except catechin and epicatechin, were recorded. Based on the g value and the proton hyperfine coupling constants (hfcc), these flavonoid-derived radicals were assigned to be semiquinone radicals of the catechol moiety (B-ring). For example, the observed EPR hyperfine structure (hfs) of the luteolin radical (Lut^?·) was composed of four sets of doublet splitting, which could be ascribed to the three protons of the B-ring (a^2′?=?0.136, a^5′?=?0.102, and a^6′?=?0.272 mT) and a vinyl proton of the C-ring (a³?=?0.099 mT). In addition, the characteristically small doublet splitting resolved for the fisetin anion radical (Fis^?·, 0.028 mT) was assigned to the aromatic proton at the C₅ carbon of the A-ring, indicating that the unpaired electron of the radials was partially delocalized onto the A-ring through the π bonds involved in the vinyl-carbonyl moiety of the C-ring. The hfcc of the methine protons at the C₂ carbon of taxifolin and eriodictyol-derived radicals (Tax^?· and Eri^?·) was, respectively, evaluated to be 0.102 and 0.230 mT. The assignment of the proton hfcc of flavonoid-derived semiquinone radicals will be discussed in relation with the molecular structure of the C-ring.     

EPR study of Fe3+ and Mn2+ in CeO2 and ThO2

Attempts were made to grow CeO₂ and ThO₂ single crystals doped with transition metal ions. Only Fe³⁺ and Mn²⁺ could be detected by the EPR technique. The EPR spectrum of Fe³⁺ in CeO₂ exhibits the well-known fine structure in cubic fields. The parameters areg=2.0044(1) anda=15.6(1)·10^?4 cm^?1. The hyperfine constantA for⁵⁷Fe in hexahedral coordination was found to be 8.9(1)·10^?4 cm^?1. The EPR spectrum of Mn²⁺ in CeO₂ reveals two cubic Mn²⁺ centers. The parameters for center 1 areg=1.9999(1) andA=86.9(1)·10^?4 cm^?1 and for center 2g=1.9984(1) andA=87.0(1)·10^?4 cm^?1. Heating the Mn doped CeO₂ samples in hydrogen, the Mn²⁺ centers transform from cubic into trigonal centers with approximate values ofg=1.9988(2),A=84.5(6)·10^?4 cm^?1 andD=203(1)·10^?4 cm^?1. The two observed Mn²⁺ centers in ThO₂ exhibita priori axial symmetry with approximate values ofg=2.0006(2),A=88.9(4)·10^?4 cm^?1 andD=33(3)·10^?4 cm^?1.