Observed EPR Spectrum Research Articles

Comment on “Photochemical Activities of N‐Nitroso Carboxamides and Sulfoximides and Their Application to DNA Cleavage”

AbstractThis Correspondence reports on a structural and mechanistic misinterpretation of an observed EPR spectrum, a misleading presentation of DNA cleavage data, and the physicochemically unsupported assignment of nitric oxide as a major DNA cleaving agent in the article cited in the title.

EPR detection of sphalerite ZnO in mechanically treated ZnO+0.1C nanosystem

The EPR spectrum of Mn2+ ions observed in nano-powders of wurtzite ZnO phase (ZnOWt) with 0.1wt% of carbon nanoparticles subjected to mechanical processing (MP) for tMP>90min where different than the well-known EPR spectrum of Mn2+ in ZnOWt. Such spectrum was observed earlier and it was attributed to either a new phase or a highly disordered nanocrystalline ZnOWt. Spin–Hamiltonian (S–H) parameters and line width (ΔB) have been determined by computer simulation of observed EPR spectrum of Mn2+: g=2.0010±0.0004, |A|=(73.6 ±0.3)·10−4cm−1, |D|=(102 ±30)·10−4cm−1, ΔB≈0.6mT. The comparison of S–H parameters of the EPR spectra of Mn2+ ions in a new formed phase and in ZnOWt indicates that the parameter A of the hyperfine structure in a new phase is slightly smaller and that the D parameter is more than two times lower than in ZnOWt. The content of the newly formed phase (~1% to 2%) is significantly higher than the content of external impurity states in the initial ZnO sample. The observed EPR spectrum is attributed to the Mn2+ ions in the zinc blende (sphalerite) ZnO phase (ZnOZB), created on the surface of ZnOwt nanoparticles during mechanical processing.

Observation of splitting of EPR spectral lines without any concomitant splitting in energy levels.

Splitting of spectral lines is generally associated with corresponding splitting of appropriate energy levels. However, here we report our observation of the splittings of hyperfine lines in the time-resolved EPR spectrum of a stable free radical that participates in the quenching of an excited molecule. The observed splitting does not arise from any splitting of the energy levels of the radical, nor due to Torrey oscillations but is a culmination of a detailed interplay of photophysical and magnetic resonance dynamics of the quenching process. In particular, sequential quenching of an excited singlet and triplet states by the free radical, generation of opposite electron spin polarization, and time-dependent EPR line width evolving according to the Bloch equations contribute to the appearance of unusual lineshapes of the observed EPR spectrum. This effect is sufficiently general and should always be observable in the time-resolved EPR experiments on free radicals involved in photophysical quenching of excited molecules. We also point out that this effect cannot be seen in Fourier transform EPR spectroscopy.

O-Capped Heteroadamantyl-Substituted Hydrazines and Their Oxidation Products

The mixed valence bishydrazine radical cation 6(+), obtained by oxidation of 2,6-bi-(2'-oxa-6'-azaadamantane-6'-yl)-2,6-diazaadamantane-2,6-diyl (6) with silver or nitrosonium salts, has been prepared and studied. 6 is obtained along with lesser amounts of the trishydrazine, some of the tetrahydrazine, and apparently traces of the pentahydrazine upon reaction of deprotonated 2-oxa-6-azaadamantane with 2,6-dichloro-2,6-diazaadamantane. The EPR spectrum of 6(+) shows that its charge is localized on one hydrazine unit on the EPR time scale. It shows a Hush-type Robin-Day class II mixed valence band in its optical spectrum despite the fact that the nitrogen lone pairs are held in a perpendicular geometry that would lead to no electronic interaction between the nitrogen atoms that are separated by only four sigma bonds if its nitrogens were planar. The electron transfer distance that is estimated from the calculated dipole moment of 6(+) is the same as that obtained using the average distance between the electrons of the triplet state of the dication 6(2+), calculated from its dipolar EPR splitting, as a model for the electron transfer distance of 6(+), 3.7 A. Using Hush's Gaussian approximation for the optical spectrum with this electron transfer distance produces an estimate of the electronic coupling V(ab) through the saturated bridge of about 400 cm(-1), which is consistent with the observed EPR spectrum of 6(+). From the observed dipolar splitting, the trishydrazine diradical dication has its remote hydrazine units oxidized, although the monocation presumably forms at the central hydrazine unit, which lacks substitution by the more electron-withdrawing oxygen atoms.

Peculiarities of EPR spectra of methyl radicals in quench-condensed krypton films

Methyl radicals are trapped in the solid Kr film by simultaneous condensation of gaseous Kr and the products of a gas discharge in CH4-doped Kr on a low-temperature (4.2K) substrate located at the center of the microwave cavity. The observed EPR spectrum is a superposition of broad-line and narrow-line series. At a low resonance microwave power, the latter series consists of four hyperfine components with nearly equal intensities and shows small axial anisotropy of both the g and A tensors. At a sufficiently high power the two central narrow lines split, so that the narrow-line series takes on the appearance recorded elsewhere for CH3 in Ar at higher temperatures above 12K. Simultaneously, the intensity of the two central broad lines increases dramatically, while the outer components become saturated. A possible explanation is discussed.

Luminescence and EPR studies of Mn-activated SrAl 12O 19 phosphor prepared by facile combustion approach

Manganese-activated strontium hexa-aluminate (SrAl 12O 19) phosphor has been prepared at low temperature (500 °C) and in a very short time (<5 min) by urea combustion route. Powder X-ray diffraction pattern showed the presence of hexagonal SrAl 12O 19 phase. Scanning electron microscopy (SEM) indicated the presence of several particles with sizes of 200 nm. The luminescence of Mn 2+ activated SrAl 12O 19 exhibits a strong green emission peak around 515 nm from the synthesized phosphor particles under excitation (451 nm). The luminescence is assigned to a transition from the upper 4T 1→ 6A 1 ground state of Mn 2+ ions. EPR investigations also indicated the presence of Mn 2+ ions in the prepared material. From the observed EPR spectrum, the spin-Hamiltonian parameters have been evaluated. The magnitude of the hyperfine splitting ( A) constant indicates that there exists a moderately covalent bonding between Mn 2+ ions and the surrounding ligands. The variation of zero-field splitting parameter ( D) with temperature is measured and discussed. The mechanism involved in the generation of a green emission has been explained in detail.

EPR Study on Stable Magnesium Complexes of the Phenoxyl Radicals Derived from a Vitamin E Model and Its Deuterated Derivatives

Abstract The phenoxyl radical 1•, generated by the reaction of a vitamin E model, 2,2,5,7,8-pentamethylchroman-6-ol (1H), with 2,2-bis(4-tert-octylphenyl)-1-picrylhydrazyl (DOPPH•), was significantly stabilized by complex formation with Mg2+ in deaerated acetonitrile at 298 K. The assignments of the hyperfine coupling constants (hfc) obtained by computer simulations of the observed EPR spectrum of the Mg2+ complex of 1• (Mg2+–1•), were carried out using three deuterated isotopomers of 1•, i.e., 5-CD3-1•, 7-CD3-1•, and 8-CD3-1•, where a methyl group at the C5, C7, or C8 position is replaced by a CD3 group, respectively. The decreased spin density of the benzene ring in the Mg2+–1• complex indicates that delocalization of the unpaired electron in 1• into Mg2+ by complexation between Mg2+ and 1• results in the enhanced stability of 1• in the presence of Mg2+.

Ti(III) in synthetic pyrope: A single-crystal electron paramagnetic resonance study

Electron paramagnetic resonance measurements were undertaken on a synthetic Ti-bearing pyrope single crystal of approximate composition Mg3.01(2)Al1.93(2)Ti0.04(1)Si3.00(2)O12 in order to further (Geiger et al., 2000) characterize the oxidation state of titanium. The crystal was synthesized hydrothermally at high pressure and temperature and starting with Ti2O3. The angular dependence of the observed EPR spectrum can uniquely be interpreted as being due to an electron centre with spin 1/2 on a site of (3) over bar point symmetry. It is proposed, based on the composition of the pyrope and its known structure, that the EPR spectrum results from a small concentration of Ti(III) on the octahedral 16a site. Based on previously published optical absorption measurements and microprobe analysis, most of the octahedral Ti should be in the oxidation state (IV).

Alanine radicals, part 4: relative amounts of radical species in alanine dosimeters after exposure to 6-19 MeV electrons and 10 kV-15 MV photons.

The amino acid l-alpha-alanine can be used for high-precision dosimetry over a wide dose range, using EPR spectroscopy for monitoring radical concentrations. It is important, however, to understand the underlying composition of the observed EPR spectrum. In previous work, it was shown that the EPR signal from irradiated alanine consists of at least three different radical species, with the relative importance of each of these being almost independent of absorbed dose. However, it was not known whether the relative importance of each radical is independent of the radiation quality responsible for the EPR signal. In the present work, the relative contributions of the different radical species to the total EPR signal from alanine dosimeters irradiated with 6-19 MeV electrons and 10 kV-15 MV photons at a dose of 10 Gy were examined. By spectrum reconstruction using benchmark spectra generated from a simulation procedure, the relative amounts of the three different radical species were shown to be virtually independent of these radiation beam qualities.

Mixed alkali effect in borate glasses—EPR and optical absorption studies in xNa 2O–(30− x)K 2O–70B 2O 3 glasses doped with Mn 2+

The mixed alkali borate xNa 2O–(30− x)K 2O–70B 2O 3 (5≤ x≤25) glasses doped with 1 mol% of manganese ions were investigated using EPR and optical absorption techniques as a function of alkali content to look for ‘mixed alkali effect’ (MAE) on the spectral properties of the glasses. The EPR spectra of all the investigated samples exhibit resonance signals which are characteristic of the Mn 2+ ions. The resonance signal at g≅2.02 exhibits a six line hyperfine structure. In addition to this, a prominent peak with g≅4.64, with a shoulder around g≅4.05 and 2.98, was also observed. From the observed EPR spectrum, the spin-Hamiltonian parameters g and A have been evaluated. It is interesting to note that some of the EPR parameters do show MAE. It is found that the ionic character increases with x and reaches a maximum around x=20 and thereafter it decreases showing the MAE. The number of spins participating in resonance ( N) at g≅2.02 decreases with x and reaches a minimum around x=20 and thereafter it increases showing the MAE. It is also observed that the zero-field splitting parameter ( D) increases with x, reaches a maximum around x=15 and thereafter decreases showing the MAE. The optical absorption spectrum exhibits a broad band around ∼20,000 cm −1 which has been assigned to the transition 6A 1g(S)→ 4T 1g(G). From ultraviolet absorption edges, the optical bandgap energies and Urbach energies were evaluated. It is interesting to note that the Urbach energies for these glasses decrease with x and reach a minimum around x=15. The optical band gaps obtained in the present work lie in the range 3.28–3.40 eV for both the direct and indirect transitions. The physical parameters of all the glasses were also evaluated with respect to the composition.

EPR study of Cu2+ ions in the Pr0.5RE0.5Ba2Cu3O6 + compounds in tetragonal phase at low temperatures

Abstract Powdered samples of the Pr 0.5 RE 0.5 Ba 2 Cu 3 O 6 + x compounds, where RE  Yb, Ho, Tm, Er, and Y, were investigated by the EPR technique at low temperatures in the range 2.8–20 K. In the EPR spectra of these non-superconducting materials in tetragonal phase, resonance lines arising from the exchange interaction of coupled pairs of divalent copper ions and Cu 2+ in the tetragonal local crystal field symmetry were recorded. Apart from the usually observed ΔM s = 1 and ΔM s = 2 transitions for the CuCu system, rarely reported singlet-triplet transitions were detected as well. The exchange parameter J has roughly the same value of about 0.12 cm −1 for all investigated samples and the zero-field splitting parameter D is of the order of 0.013 cm −1 . It is suggested that formation of Cu(1)OCu(2) copper coupled pairs is responsible for the observed EPR spectrum.

Further EPR studies of molecular motions in polymer/plasticizer mixtures

AbstractMolecular motions in mixtures of the side chain polymer—poly(vinyl acetate) and dibuthyl phthalate were studied as a functionof polymer concentration and temperature using the technique of paramagnetic resonance (EPR). When the small spherical probe tempol (TPL) was used, we were able to approximate the observed EPR spectrum with a simulation using a single rotational correlation time τ. The peviously developed Grest–Cohen all‐temperature model matched the Arrhenius polts. The EPR spectra from a cigar‐shaped cholestane (COL) probe could not be adequately matched by single τ simulation when the polymer was at temperatures somewhat above the glass to rubber transition temperature (Tg). Points corresponding to these temperatures were left of the Arrhenius plot and a discontinuity was observed where the gap in the data occurred. As the concentration of plasticizer was increased, we found that the discontinuity became less steep, but the τ at which the gap occurs was always ≈ 10−8. The spectra observed at the temperature region of the gap were approximately 50–50 composites of experimental spectra observed at ± K. In both the TPL and COL cases, there was evidence of the existence of multiple correlation times. Preliminary studies of other polymers, both with and without side chains, also indicated the existence of the gap when COL is used as the probe. © 1993 John Wiley &amp; Sons, Inc.

Ethane cation-radical isomers and their interconversion pathways. Electron shift isomerism in cation radicals

An extensive computational exploration of the C2H6+˙ surface has been performed with electron correlation methods beyond MP2, up to the level of quadratic configuration interaction with single, double and triple substitutions, using the 6–311 G** basis set. Five ground-state species, two excited-state species and five transition states, for interconversions and internal rotations, are located. The results at the highest levels show the existence of three isomers, which in order of decreasing stability are: 2Ag(C2h)DB, 2A1g(D3d) and 2A″(Cs), where the most stable isomer has a diboranoid (DB) character. Two interconversion pathways are found to link the diboranoid 2Ag(C2h)DB isomer to the other two isomers. The lowest energy mechanism appears to be the one linking 2Ag(C2h)DB and 2A1g(D3d). Thus, the work identifies a low-energy mechanism which funnels the dynamics through the original point-group symmetry, away from the traditional Jahn–Teller pathway. Each event of 2A1g(D3d) formation is, in turn, followed by a faster reverse process back to 2Ag(C2h)DB which results in scrambling of the hydrogens in the bridging positions. These results offer the basis for an interpretation of the observed EPR spectrum (reference 8) in the low-as well as the high-temperature studies. A qualitative analysis of the origins of the various isomers and their interconversion pathways is presented. It is shown that a useful way of understanding the results is in terms of ‘electron-shift isomerism’ in which single electron-shifts amongst different fragments of the atomic skeleton generate both the C2H6+˙ isomers as well as their intervening transition structures.

Rhombic Fe3+ centers in KTaO3

AbstractThe EPR spectrum of rhombic Fe3+ paramagnetic centers in KTaO3 both nominally pure and with Li and Nb impurities is investigated. The absolute values and signs of the parameters of the spin‐Hamiltonian describing the observed EPR spectrum are determined on the basis of the angular and temperature dependences of the mentioned spectrum. The model of a rhombic iron center involving the Fe3+ ion substituted for a Ta5+ ion with a nearby interstitial bivalent positive ion in [110] direction for charge compensation is suggested.

Zur Elektronenstruktur metallorganischer Komplexe der f-Elemente : XXVII. Interpreation der optischen, magnetochemischen, ESR-und NMR-spektroskopischen Eigenschaften ausgewählter Neutralbasenaddukte, die sich vom Grundkörper Tris ( n5-cyclopentadienyl)-neodym(III) ableiten

The absorption spectrum of (Cp- d 5) 3Nd·THF- d 8 in hydrocarbon glasses and run as pellets was measured at room and low temperatures. The bands were assigned based on calculations assuming the crystal field parameters of the Nd complex were the same as for the previously analyzed Cp 3Pr·MeTHF. The parameters of an empirical Hamiltonian wee fitted to the energies of 96 levels to give an r.m.s. deviation of 26 cm −1. On the basis of the wave functions of the crystal field ground state obtained from these calculations, the observed EPR spectrum of Cp 3Nd 0.06La 0.94·THF could be explained. Making use of the calculated wave functions and eigenvalues the experimentally determined temperature dependence of μ 2 eff of powdered Cp 3Nd·THF and of an oriented single crystal of Cp 3Nd·Ncch 3 could be simulated. Assuming that the methyl protons of the γ-picoline ligand of Cp 3Nd·γ-pic and (MeCp) 3Nd· γ-pic, respectively, experience only an NMR shift of dipolar type, the paramagnetic anisotropy χ ∥-χ ⊥ was estimated.

Divalent europium as a paramagnetic probe to study strontium precipitates in monocrystalline sodium chloride

Precipitation of divalent strontium ions in NaCl has been investigated using Eu2+ as a paramagnetic probe. The results presented in this paper show that the annealing of the quenched samples at 200 °C produces the formation of the precipitated dihalide phase SrCl2 in the NaCl matrix. The EPR data established that the Eu2+ ions are located inside this precipitated structure and that they are substituting the host cation Sr2+ ions. However, the observed EPR spectrum shows a significant difference with respect to that reported for the same impurity ion in SrCl2 single crystals, i.e., the value for the cubic spin-Hamiltonian parameter b40 is larger and its magnitude seems to decrease as the particle size of the precipitate increases. On the other hand, annealing of the quenched samples at room temperature produces another precipitated phase. In this case, the EPR data reveal that the Eu2+ ions also occupy a cubic symmetry site in this precipitated structure. However, the value for the b40 parameter describing this cubic site is considerably smaller than the one measured for Eu2+ in SrCl2. Finally, an EPR spectrum, observed in the fast quenched samples, possessing axial symmetry and probably due to Sr2+–Eu2+-vacancy complexes is presented. Values for the spin-Hamiltonian parameters describing this new spectrum are also reported.

Crystal field splitting in Ti 3O 5 via EPR spectroscopy

Previous attempts to describe the crystal field environment of the Ti 3+ ion in the mixed valence phases of the titanium-oxygen system described by Ti nO 2n−1 have been hindered by the lack of single-crystal EPR data. Using g-shift results from single crystals of Ti 3O 5 in conjunction with LCAO calculations, we have shown that assignment of a D 2h symmetry with rhombic field splitting parameters of E 2 − E′ 0 = 3100cm −1; E 1 − E′ 0 = 6300cm −1; and E 3 − E′ 0 = 26,000cm −1 is consistent with available optical, x-ray and EPR data. In addition EPR spin-concentration measurements have been correlated with available electrical conductivity measurements to identify the site of the paramagnetic center responsible for the observed EPR spectrum.