Single-crystal Electron Paramagnetic Resonance Spectra Research Articles

Electron paramagnetic resonance study of gamma-irradiated soduxin single crystal

Single crystals of soduxin were 60Co-gamma irradiated at room temperature. The irradiated single crystals were studied at 120 K using Electron Paramagnetic Resonance (EPR) spectroscopy. The investigation of EPR spectra of irradiated single crystals of soduxin showed the presence of two soduxin anion radicals. The g values of the radiation damage centers observed in soduxin single crystal and the hyperfine structure constants of the free electron with nearby protons were obtained. Highlights Soduxin single crystal was irradiated with gamma rays. Paramagnetic centers in soduxin single crystal were investigated. Two carbon-centered radicals with the same structure were detected. EPR parameters changed anisotropically.

Electron Paramagnetic Resonance Study of the Radiation Damage Centers in Menadione Single Crystal

The radiation damage centers in the 60Co- rays irradiated menadione (2-methyl-1,4-naphthoquinone; Vitamin K3; C11H8O2 ) single crystals were examined at 120 K by Electron Paramagnetic Resonance (EPR) spectroscopy. The presence of two radicals was detected in the compound. Both radicals are carbon centered radicals. Analysis of the EPR spectra of gamma-irradiated menadione single crystals along three different axes showed that the C(11)-H(19) bond of the compound was broken. It was determined that the unpaired electron located on the C(11) atom. The g values and the hyperfine structure constants of the radiation damage centers observed in menadione single crystal were obtained. Accuracy of experimental data was observed by simulation study.

Electron paramagnetic resonance study of the paramagnetic centers in gamma-irradiated oxalic acid dihydrate single crystal

The Electron Paramagnetic Resonance (EPR) spectra of the radicals produced by γ-irradiation of a single crystal of oxalic acid dihydrate (C2H6O6) have been reported. The EPR spectra of gamma-irradiated single crystals of oxalic acid dihydrate have been studied for different orientations of crystals in a magnetic field. As a result of studies carried out along three axes, it has been determined that there are two biradicals. The EPR spectra have been investigated at temperatures between 125 and 450 K. The spectra have been found to be temperature independent. The g values of the radiation damage centers observed in oxalic acid dihydrate acid single crystal and the hyperfine structure constants of the free electron with nearby protons have been obtained from the single crystal EPR analysis.

Electron paramagnetic resonance study of the paramagnetic center in gamma-irradiated tetrahydrophthalic anhydride single crystal

Electron Paramagnetic Resonance (EPR) analysis of gamma-irradiated tetrahydrophthalic anhydride (C8H8O3) single crystals was performed at 120 K. The single crystals were exposed to 60Co-gamma source at room temperature. The EPR spectra of gamma-irradiated single crystals of tetrahydrophthalic anhydride (THPA) were recorded at 10-degree intervals for different orientations of crystals in a magnetic field. Detailed examination of THPA single crystals by EPR spectroscopy method showed the presence of two carbon-centered THPA anion radicals. Both radicals were determined to have the same structure. The anion radicals observed in gamma-irradiated THPA single crystal were created by the scission of the carbon–oxygen double bond. The g values of the radiation damage centers observed in the THPA single crystal and the hyperfine structure constants of the free electron with nearby protons were obtained. Highlights Single crystals of tetrahydrophthalic anhydride were exposed to gamma radiation. Radiation damage centers in single crystals of tetrahydrophthalic anhydride were determined by EPR spectroscopy method. The presence of two anion radicals in single crystals of tetrahydrophthalic anhydride was determined. The formation mechanism of the paramagnetic centers in the tetrahydrophthalic anhydride is similar to that of succinic anhydride.

Electron paramagnetic resonance study of the radiation damage in phosphoryethanolamine single crystal

Gamma irradiated phosphoryethanolamine (C2H8NO4P) single crystals were investigated at 140 K using Electron Paramagnetic Resonance (EPR) technique. Phosphoryethanolamine single crystals have been irradiated with 60Co-γ rays, at room temperature. The electron paramagnetic resonance (EPR) spectra of gamma irradiated phosphoryethanolamine single crystals have been studied for different orientations of crystals in a magnetic field. The spectra have been found to be temperature independent. The structure of the radical produced by γ-irradiation of a single crystal of phosphoryethanolamine was discussed. The investigation of EPR spectra of gamma-irradiated single crystals of phosphoryethanolamine showed the presence of two radicals. One of the radicals is the H2PO4˙ neutral radical formed by the breakdown of the O(7)C(8) bond. The other is the HPO4−˙ anion radical formed by cleavage of the O(7)C(8) bond and the attachment of an electron to O(2) by breaking H(1) bound to O(2). The principal values of the hyperfine coupling tensor of the unpaired electron and the principal values of the g-tensor and direction cosines of the radiation damage centers have been determined. The results were found to be in good agreement with the existing literature.

Electron paramagnetic resonance study of radiation-induced paramagnetic centers in succinic anhydride single crystal

Succinic anhydride single crystals were exposed to 60Co-gamma irradiation at room temperature. The irradiated single crystals were investigated at 125 K by Electron Paramagnetic Resonance (EPR) Spectroscopy. The investigation of EPR spectra of irradiated single crystals of succinic anhydride showed the presence of two succinic anhydride anion radicals. The anion radicals observed in gamma-irradiated succinic anhydride single crystal were created by the scission of the carbon-oxygen double bond. The structure of EPR spectra demonstrated that the hyperfine splittings arise from the same radical species. The reduction of succinic anhydride was identified which is formed by the addition of an electron to oxygen of the CO bond. The g values, the hyperfine structure constants and direction cosines of the radiation damage centers observed in succinic anhydride single crystal were obtained.

EPR study of radiation damage in gamma-irradiated 3-nitroacetophenone single crystal

ABSTRACTThis paper presents the results of the electron paramagnetic resonance (EPR) study of the anion radical formed from 3-nitroacetophenone (C8H7NO3) (3NAP) single crystal, by gamma irradiation. The EPR spectra of gamma-irradiated single crystals of 3NAP have been recorded at 10-degree intervals for different orientations of crystals in a magnetic field, at room temperature. The EPR analysis of gamma-irradiated crystals of 3NAP has shown that the radiation damage center produced by gamma irradiation is the carbon-centered 3NAP anion radical. One-electron reduction of 3NAP results in general bond loosening. The single crystals have been investigated between 120 and 450 K. The spectra have been found to be temperature-dependent. The EPR parameters of the 3NAP anion radical have been evaluated.

Electron paramagnetic resonance study of gamma (γ)-irradiated methyl 4-methyl benzoate (C9H10O2)

ABSTRACTIn this study, gamma (γ)-irradiated methyl 4-methyl benzoate (C9H10O2) single crystals were investigated and the structure of the free radical occurring by γ-irradiation was determined by using Electron paramagnetic resonance (EPR) technique. C9H10O2 single crystals were produced by slow evaporation with distilled water solution at room temperature. These single crystals were irradiated by 60Co-gamma-ray source with a dose speed of 0.981 kGy/h at room temperature for 72 h. The EPR spectra of irradiated C9H10O2 single crystals were recorded between 125 and 400 K temperatures in steps of 10° intervals at different orientations in magnetic field with an X-band EPR spectrometer. The spectra were found to be slightly anisotropic and temperature independent; taking into consideration the chemical structure with experimental data we found only one free radical structure . The radical structure was simulated using the Bruker software WinEPR-Simfonia and the experimental g-value with anisotropic hyperfine coupling constants of radical were calculated as follows: g = 2.0124, = 26.51G, = 5.45G.

Electron paramagnetic resonance study of gamma-irradiated potassium hydroquinone monosulfonate single crystal

ABSTRACTGamma-irradiated potassium hydroquinone monosulfonate (PHM) single crystals were investigated between 125 and 300 K using the electron paramagnetic resonance (EPR) technique. Between 125 and 300 K no changes in the spectra have taken place. The spectra were found to be temperature independent. The investigation of EPR spectra of irradiated single crystals of PHM showed the presence of two PHM anion radicals. The g values of the radiation damage centers observed in the PHM single crystal and the hyperfine structure constants of the free electron with nearby protons were obtained.

Arsenic speciation in danburite (CaB2Si2O8): a synchrotron XAS and single-crystal EPR study

Gem-quality danburite containing 269 ppm As, from Charcas, San Luis Potosi, Mexico, has been investigated by synchrotron X-ray absorption spectroscopy (XAS) and single-crystal electron paramagnetic resonance (EPR) spectroscopy. Arsenic K -edge X-ray absorption near-edge-structure (XANES) spectra show that the dominant oxidation state is +3, and modeling of the extended X-ray absorption fine structure (EXAFS) spectra suggests that As 3+ mainly occupies the Si site. Single-crystal continuous-wave EPR spectra, measured before and after gamma-ray irradiation, reveal three arsenic-associated electron centers (I, II and III). Centers I and II have similar principal electron Zeeman g values and principal 75 As hyperfine constants [I: A 1 /h = 732.8(2) MHz, A 2 /h = −274.3(3) MHz and A 3 /h = −299.8(3) MHz; II: A 1 /h = 743.8(2) MHz, A 2 /h = −306.3(2) MHz and A 3 /h = −325.8(2) MHz]. These parameters suggest that Centers I and II are varieties of the [AsO 2 ] 2− radical, formed from electron trapping on substitutional As 3+ ions at the Si site. This model for the [AsO 2 ] 2− radical is further supported by 11 B (and 10 B) superhyperfine parameters determined from pulsed electron spin echo envelope modulation (ESEEM) spectroscopy. Center III is the [AsO 3 ] 2− radical on the basis of its characteristic 75 As hyperfine constants [A 1 /h = 2406.0(1) MHz, A 2 /h = 1903.4(1) MHz and A 3 /h = 1892.1(1) MHz]. The [AsO 3 ] 2− radical with its A 1 axis along the Si – O4 bond direction originated from electron trapping on a [AsO 4 ] 3− group after removal of the O4 atom during gamma-ray irradiation. Therefore, arsenic in the borosilicate danburite is present in both the +3 and +5 oxidation states and preferentially occupies the Si site. This study also highlights the advantages (and need) of multiple spectroscopic techniques for determining speciation of trace elements such as arsenic in minerals.

Synthesis, crystal structure, spectroscopic, fluorescent, thermal properties and EPR spectra of doped Cu2+ ions in [Cd(sac)2(H2O)2(meim)2] single crystal

The crystal structures of the six-coordinate complexes [Cd(sac)2(H2O)2(meim)2] (complex 1) formed by reaction of 4-methylimidazole(meim) with [Cd(sac)2(H2O)4]·2H2O (saccharinate=sac), was synthesized and characterized by elemental analysis, infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy, thermal analysis and X-ray single crystal diffraction. X-ray diffraction analysis revealed that complex 1 crystallized in the monoclinic crystal system with space group P21/c. The Cd(II) center was six-coordinated with four nitrogen atoms from two sac and two 4-meim ligands, two oxygen atoms from two aqua ligands. Spectral and thermal analysis data for complex 1 was in agreement with the crystal structures. In addition complex 1 displayed blue fluorescent emission in the solid state at room temperature. Single crystal EPR spectra at room temperature are resolved and have exhibited that two different Cu2+ complexes were located in different chemical environments which contained two magnetically nonequivalent Cu2+ sites. In low temperature EPR spectra down to 110°C did show no considerable change. At higher temperatures, however, both thermo gravimetric analyses (TGA) and EPR spectra showed detectable changes around 140°C; the causes and the mechanisms of changes are discussed.

Phase transitions and proton ordering in hemimorphite: new insights from single-crystal EPR experiments and DFT calculations

Single-crystal electron paramagnetic resonance spectra of gamma-ray-irradiated hemimorphite (Mapimi, Durango, Mexico) after storage at room temperature for 3 months, measured from 4 to 275 K, reveal a hydroperoxy radical HO2 derived from the water molecule in the channel. The EPR spectra of the HO2 radical confirm that hemimorphite undergoes two reversible phase transitions at ~98 and ~21 K and allow determinations of its spin Hamiltonian parameters, including superhyperfine coupling constants of two more-distant protons from the neighboring hydroxyl groups, at 110, 85, 40 and 7 K. These EPR results show that the HO2 radical changes in site symmetry from monoclinic to triclinic related to the ordering and rotation of its precursor water molecule in the channel at <98 K. The monoclinic structure of hemimorphite with completely ordered O–H systems at low temperature has been evaluated by both the EPR spectra of the HO2 radical at <21 K and periodic density functional theory calculations.

Single-crystal EPR study of three radiation-induced defects (Al–O2 3−, Ti3+ and W5+) in stishovite

Single-crystal electron paramagnetic resonance spectra of electron-irradiated stishovite, measured at temperatures from 3.5 to 294 K, reveal three S = 1/2 radiation-induced defects: an aluminum-associated oxygen hole center and two nd 1 centers (Ti3+ and W5+). The aluminum-associated oxygen hole center, characterized by an orthorhombic site symmetry, coaxial matrices of the electronic Zeeman g, nuclear hyperfine A(27Al) and nuclear quadrupole P(27Al), and the orientation of the g-minimum axis along an O–O direction and those of the unique A(27Al) and P(27Al) axes perpendicular to the O–O direction, is an Al–O2 3− center, with the unpaired electron equally distributed on two equatorial oxygen atoms of a substitutional Al3+ ion at the octahedral Si site. Fully optimized Al-doped structure, theoretical 27Al nuclear hyperfine and quadrupole coupling constants and directions, and 3D spin densities from periodic hybrid density functional theory calculations provide further support for this structural model. Spin Hamiltonian parameters of the Ti3+ and W5+ centers, which are confirmed by their diagnostic 47Ti, 49Ti and 183W hyperfine structures, arise from electron trapping on substitutional Ti4+ and W6+ ions at the octahedral Si site.

Single-crystal EPR and DFT study of a VIAl–O−–VIAl center in jeremejevite: electronic structure and 27Al hyperfine constants

Single-crystal electron paramagnetic resonance (EPR) spectra of a gem-quality jeremejevite, Al6B5O15(F, OH)3, from Cape Cross, Namibia, reveal an S = 1/2 hole center characterized by an 27Al hyperfine structure arising from interaction with two equivalent Al nuclei. Spin-Hamiltonian parameters obtained from single-crystal EPR spectra at 295 K are as follows: g 1 = 2.02899(1), g 2 = 2.02011(2), g 3 = 2.00595(1); A 1/g e β e = −0.881(1) mT, A 2/g e β e = −0.951(1) mT, and A 3/g e β e = −0.972(2) mT, with the orientations of the g 3- and A 3-axes almost coaxial and perpendicular to the Al–O–Al plane; and those of the g 1- and A 1-axes approximately along the Al–Al and Al–OH directions, respectively. These results suggest that this aluminum-associated hole center represents hole trapping on a hydroxyl oxygen atom linked to two equivalent octahedral Al3+ ions, after the removal of the proton (i.e., a VIAl–O−–VIAl center). Periodic ab initio UHF and DFT calculations confirmed the experimental 27Al hyperfine coupling constants and directions, supporting the proposed structural model. The VIAl–O−–VIAl center in jeremejevite undergoes the onset of thermal decay at 300 °C and is completely bleached at 525 °C. These data obtained from the VIAl–O−–VIAl center in jeremejevite provide new insights into analogous centers that have been documented in several other minerals.

NATURE OF HEAVY METALS IN HEMIMORPHITE: A CATION-EXCHANGE AND SINGLE-CRYSTAL EPR STUDY

Hemimorphite, a zeolite-like mineral commonly present in supergene non-sulfide Zn deposits and Zn mine tailings, is known to contain elevated levels of As, Cd, Cu and Pb. Cation-exchange experiments of a natural hemimorphite (Mapimi, Durango, Mexico) with 0.1 M CaCl 2 solution at 110 oC show that As and Cu are retained, whereas Cd and Pb are readily exchanged. The retention of As is consistent with previous single-crystal electron paramagnetic resonance (EPR) results that showed its occurrence as the substitutional As 5+ ion at the Si site. Single-crystal EPR spectra of hemimorphite from the M’Fouati Pb-Zn mine (Reneville, Congo), measured at 120 K and 295 K, show a Cu 2+ center. The best-fit spin Hamiltonian parameters g , A ( 63 Cu), P ( 63 Cu) and A ( 1 H) of this center at 120 K demonstrate that Cu 2+ resides at the tetrahedral Zn site, not in the channels as suggested by a previous powder EPR study. Cadmium and Pb in hemimorphite are probably present in the channels; this would account for their contrasting behavior from As and Cu during the cation-exchange experiments. However, single-crystal EPR spectra of ten samples of hemimorphite from various Pb-Zn deposits, including those taken after gamma-ray irradiation for doses up to 55 kGy, did not detect any paramagnetic Cd or Pb centers.

Mechanisms of Cr and H incorporation in stishovite determined by single-crystal EPR spectroscopy and DFT calculations

Synthetic stishovite before and after electron- and γ-ray irradiation has been investigated by single-crystal electron paramagnetic resonance (EPR) spectroscopy for the first time. Room-temperature single-crystal EPR spectra of as-is stishovite reveal two high-spin (S = 3/2) Cr 3+ centers: one with D 2 h symmetry and another of triclinic symmetry. Quantitatively determined spin Hamiltonian parameters, including matrices g , D , and A ( 53 Cr) and high-spin Zeeman term BS 3 , suggest that the D 2 h center represents a substitutional Cr 3+ ion at the Si site without an immediate charge compensator. The triclinic center, which is characterized by a well-resolved 1 H superhyperfine structure, also arises from a substitutional Cr 3+ ion at the Si site but has an H + charge compensator bonded to one of the four equatorial oxygen atoms. The magnitude and orientation of the 1 H superhyperfine structure yield the location of the H atom at (0.46, 0.12, 0). These structural models for Cr and H in stishovite also have been corroborated by periodic density functional theory (DFT) calculations using the Vienna ab initio simulation package (VASP), with 2 × 2 × 4 supercells, plane-wave basis sets and the projector augmented wave (PAW) potentials. In addition, 85 K EPR spectra of irradiated stishovite show that the two Cr 3+ centers are both converted to an S = 1/2 Cr 5+ center characterized by two 29 Si superhyperfine structures arising from interactions with two nearest and eight second-nearest Si atoms, respectively. The spin Hamiltonian parameters of this Cr 5+ center provide further support for the location of the two Cr 3+ centers at the Si site.

Correlation between Luminescence and EPR Spectroscopy as Evidence of Ytterbium Pair Formation in Li6Ln(BO3)3:Yb3+ (Ln=Gd, Y) Borate Single Crystals

Synthesized powders and grown single crystals of nominal compositions Li(6)Ln(BO(3))(3):Yb(3+) (Ln=Y, Gd) were investigated by means of powder and single-crystal X-ray diffraction (XRD), as well as optical near-IR spectroscopy in conjunction with electron paramagnetic resonance (EPR) spectroscopy. The appearance of two distinct zero-phonon lines suggests the existence of two kinds of Yb(3+) ions in the single crystals. The XRD results exclude the possibility of a phase transition occurring between room and low temperatures. EPR spectra of single crystals show the presence of both isolated ions and pairs of ytterbium ions substituted for Y(3+). A strong temperature dependence of the intensity of Yb-Yb pairs resonance lines coincides with temperature dependence of emission peak at 978 nm, confirming a common origin of the defect giving rise to these spectra. Calculated from EPR spectra, the distance between pairs of Yb(3+) is in good agreement with crystallographic ones: R=3.856 Å, R(cryst) =3.849 Å.

Spectroscopic Study of Cu(II) Ion Doped in Bis-Glycine Sodium Nitrate

ABSTRACT Single-crystal Electron Paramagnetic Resonance (EPR) and optical absorption studies were performed at room temperature on Cu(II)-ion–doped bis-glycine sodium nitrate, a semiorganic nonlinear optical material (NLO). Spin-Hamiltonian parameters, obtained from single-crystal EPR spectra, recorded in three mutually orthogonal planes, revealed the presence of two magnetically inequivalent Cu(II) sites. Molecular orbital parameters were also evaluated. The optical absorption spectrum at room temperature exhibits the lowering of octahedral symmetry to an axially elongated tetragonal of Cu(II) ion in NLO.

Thermal and EPR spectroscopic studies of novel coordination compound trans-bis(acesulfamato)tetraaquazinc(II) with Mn(II) impurity

The thermogravimetric analysis (TGA) and electron paramagnetic resonance (EPR) studies of powder and single crystals of bis(acesulfamato)tetraaquazinc(II), Zn(acs) 2(H 2O) 4, a novel coordination compound, are carried out. Previously synthesized bis(acesulfamato) tetraaquamanganese(II), Mn(acs) 2(H 2O) 4, is included into the host in trace amount as a paramagnetic probe for EPR analysis. Single crystal EPR spectra at room temperature are resolved and discussed. Low temperature EPR spectra down to 90 K do not show remarkable change. At higher temperatures, however, the TGA and EPR spectra show changes around 335 K and 395 K; the causes and the mechanisms of changes are discussed.

Electron paramagnetic resonance spectroscopy of Fe3+ ions in amethyst: thermodynamic potentials and magnetic susceptibility

Single-crystal and powder electron paramagnetic resonance (EPR) spectroscopic studies of natural amethyst quartz, before and after isochronal annealing between 573 and 1,173 K, have been made from 90 to 294 K. Single-crystal EPR spectra confirm the presence of two substitutional Fe3+ centers. Powder EPR spectra are characterized by two broad resonance signals at g = ~10.8 and 4.0 and a sharp signal at g = 2.002. The sharp signal is readily attributed to the well-established oxygen vacancy electron center E1′. However, the two broad signals do not correspond to any known Fe3+ centers in the quartz lattice, but are most likely attributable to Fe3+ clusters on surfaces. The absolute numbers of spins of the Fe3+ species at g = ~10.8 have been calculated from powder EPR spectra measured at temperatures from 90 to 294 K. These results have been used to extract thermodynamic potentials, including Gibbs energy of activation ΔG, activation energy Ea, entropy of activation ΔS and enthalpy of activation ΔH for the Fe3+ species in amethyst. In addition, magnetic susceptibilities (χ) have been calculated from EPR data at different temperatures. A linear relationship between magnetic susceptibility and temperature is consistent with the Curie–Weiss law. Knowledge about the stability and properties of Fe3+ species on the surfaces of quartz is important to better understanding of the reactivity, bioavailability and heath effects of iron in silica particles.