Electron Paramagnetic Resonance Absorption Spectra Research Articles

Studies in the field of polyethylene—IV.: Influence of γ-radiation on the structure and molecular weight of the AS polyethylene

A study was made of effects of 60Co γ-radiation on the average mol. wt. and the structure of AS polyethelene obtained with amyl-sodium catalyst according to the process of C. D. Nenizescu and co-workers. Examination was made of the viscosimetric characteristics, and the i.r. and electron spin resonance (ESR) absorption spectra for samples irradiated at room temperature and in the presence of air. The spectra were recorded immediately after irradiation and after various lengths of storage at different temperatures. The ionizing radiation causes an oxidative degradation of AS polyethylene, shown by the decrease of the average mol. wt., and the appearance of characteristic i.r. absorption bands for oxygen-containing groups. The ESR spectra indicate the existence of allyl, alkyl and polyenyl radicals. During storage of the irradiated AS polyethylene especially at high temperature (80°), a slight increase of the oxidative degradation process can be observed.

Electron reactions in gamma-irradiated thymidine and cystamine.

ALTHOUGH the technique of electron spin resonance (ESR) has been useful in the identification of some of the secondary radical species formed in γ-irradiated DNA and proteins, difficulties have been experienced in the interpretation of the spectra of the primary radiolytic products. Two of these secondary species are the thymine radical (with an additional H-atom on C-6 of the thymine ring) in DNA (refs. 1–3) and the radical R—CH2—S in cystine-containing proteins4–6. Both have been postulated to be the result of electron reactions7–10. We have irradiated two model compounds, thymidine and cystamine, in frozen glasses of either ethanediol–water (1:1) or 6 M NaOH and, by comparing the ESR and optical absorption spectra, we have identified the anions produced.

Optical and Electron Spin Resonance Absorption Spectra of Rh in NaCl

Optical and Electron Spin Resonance Absorption Spectra of Rh in NaCl

Electron Spin Resonance Absorption Spectra of Hole-Trap Centers Associated with Phosphorous in Irradiated Single Crystal Calcite

An irradiation-produced defect center associated with phosphorous impurities and corresponding to S = 12, I = 12 has been observed in natural single crystal calcite. Its electron spin resonance absorption spectrum consists of six doublets each exhibiting orthorhombic symmetry with spin-Hamiltonian parameters given by gξξ = 2.0024, gηη = 2.0382, gζζ = 2.0090, Aξξ = − 17.25 MHz, Aηη = 14.27 MHz, and Aζζ = 17.33 MHz, where the coordinates ξ, η, and ζ are canted with respect to the calcite crystal coordinates. In addition to providing the nuclear magnetic moment of 31P, ENDOR data were used to determine the signs of the hyperfine splitting tensor components which in turn yield unambiguous values for the Fermi contact term and the traceless dipole–dipole interaction. On the basis of the small contact term in comparison to the dipole–dipole interaction strength and upon g-value comparisons, it is proposed that the defect center is a CO3− molecule ion in close association with a PO23− molecule ion, probably mutual charge compensators. The ENDOR spectrum of the PO42− molecule ion in calcite was reinvestigated, and the resulting data were used to provide relative signs of the hyperfine structure tensor components. From an analysis of the latter, it is concluded that the unpaired electron spin is localized primarily in an oxygen nonbonding orbital.

Electron Spin Resonance Absorption Spectrum of the AsO22− Molecule–Ion in γ-Irradiated Single-Crystal Calcite

An electron spin resonance absorption spectrum corresponding to S = 12, I = 32, and exhibiting orthorhombic symmetry has been observed in γ-irradiated single-crystal calcite. At 4.2°K, the spin-Hamiltonian parameters are gzz = 2.0150, gxx = 1.9991, gyy = 1.9910, Azz = −152 MHz, Axx = 614 MHz, Ayy = −173 MHz, (gnβn/h) = −7.24 × 10−4MHz/Oe, e2qQ = 16.2 MHz, and η ≃ 0. The paramagnetic defect associated with this spectrum is identified as the AsO22− molecule–ion which is located at carbonate ion sites with its plane coincident with that of the carbonate ions. From the hyperfine structure splittings and the g-tensor anisotropy, it is deduced that the unpaired electron has approximately 1% 4s, 96% 4p, and 3% 4d character and that it is almost completely localized on the arsenic atom.

Electron Spin Resonance Absorption Spectrum of Y3+-Stabilized CO33− Molecule–Ion in Single-Crystal Calcite

An electron spin resonance absorption spectrum of effective spin S = 12 exhibiting two hyperfine structure doublet patterns has been observed in naturally occurring single crystals of calcite. The nuclei responsible for these doublets are identified from electron–nucleus double resonance and relative intensity measurements to be 89Y and 13C. From the mean of the spectroscopic splitting tensor values as well as from the 13C hyperfine structure separations, it is deduced that this spectrum is due to the CO33 − molecule–ion which is charge stabilized by a neighboring Y3+ ion. Experimental data suggest the existence of two sets of six equivalent centers each having an orthorhombic g tensor which is diagonal in a coordinate system whose z axis is canted with respect to the crystal [111] direction. The 89Y and 13C hyperfine tensors exhibit orthorhombic and axial symmetries, respectively, with the z axis of each being parallel to the crystal [111] direction. From an analysis of the 89Y hyperfine structure spectral components, an estimate is obtained for the unpaired electron spin density transferred to the Y3 + ion. This taken together with the 89Y contact term provides an estimate to the hyperfine field of the Y2+ (4d1) ion. Over the temperature range 4.2°-450°K, the 89Y hyperfine splitting increases by about 5% and asymptotically reaches a maximum value at the upper temperature. Attempts to interpret this effect using existing theories on the variation of hyperfine structure with temperature have proven unsatisfactory.

Electron Spin Resonance Evidence for a Trapped Electron and its Reversible Reaction in Gamma-irradiated Acetonitrile-d3

ALTHOUGH the effect of ionizing radiation is known to produce solvated and trapped electrons in a variety of liquid and glassy systems, we are not aware of any reports on the radiation-induced formation of localized electrons in molecular crystals. Bennett, Mile and Thomas1 have observed electron spin resonance (ESR) and optical absorption spectra of trapped electrons in polycrystalline ice, but the method of preparation in this case was to deposit water vapour and alkali metals together on the rotating drum of a cryostat at 77° K. A very interesting feature of this work1 was the observation of hyperfine splitting in the ESR spectrum of the trapped electron as the temperature of a potassium–water sample was raised to 173° K. The seven hyperfine lines have been attributed to the interaction of the trapped electron writh six equivalent protons in an octahedral arrangement.

Motional Effects in the Electron Spin Resonance Absorption Spectrum of CO2– Molecule-Ions in Single-Crystal Calcite

The electron spin resonance absorption spectrum of CO2– molecule-ion in single-crystal calcite has been reinvestigated as a function of temperature. Between 77° and 250°K the spectrum exhibits orthorhombic symmetry and from the manner in which the widths of the spectral components behave, it is concluded that in this temperature range the CO2– molecule-ion hops from one to another of three equivalent configurations each being rotationally displaced from the other two by ± 2π / 3 radians in the (111) plane. Such hops involve the crossing of one of three equivalent potential wells whose depths are estimated to be 0.165 eV and whose widths are not more than 0.1 radians. At 250°K the spectrum associated with the (111) plane is wiped out. Above this temperature an axially symmetric spectrum appears whose parameters are simply related to those of the low-temperature spectrum, and from the manner in which the widths depend upon temperature, it is concluded that in this temperature region the CO2– molecule-ion undergoes hindered rotations about the [111] direction.

Unstable intermediates. Part XLIX. Solvated electrons: electron spin resonance and optical absorption spectra of solutions of the alkali metals in hexamethylphosphoramide

Dilute, blue solutions (<10–3M) of alkali metals in hexamethylphosphoramide are characterised by an intense, asymmetric absorption band in the 4000 cm.–1 region and a single e.s.r. absorption at g= 2·0021 with a width (ΔHms) of 0·035 gauss. More concentrated solutions (ca. 10–1M) have, in addition, an intense, broad band at 13,800 cm.–1, which is still present when the solutions have largely decomposed. Attempts to make the radical anion (Me2N)3PO– by standard procedures were unsuccessful. It is concluded that the blue solutions resemble metal–ammonia solutions, and the optical and e.s.r. absorption bands are assigned to solvated electrons.

Structure and reactivity of transition-metal complexes with polyatomic ligands. Part VI. Electron spin resonance and electronic absorption spectra of some chromium(I) nitrosyl complexes

E.s.r. spectra have been recorded for the ions [Cr(NO)(H2O)5]2+ and [Cr(NO)(NH3)5]2+ in dilute fluid and rigid solution. The g and A(53Cr) tensors have been calculation for both ions and the A(14N) tensor for the former. Superhyperfine structure was observed in fluid solutions of [Cr(NO)(NH3)5]2+ from Aiso(14N) for the NO, NH3(axial) and NH3(equat.) ligands were estimated. Isotropic coupling constants to 14N of the ammonia ligands are attributed to spin polarisation of σ-electrons. The electronic absorption spectra of these complexes, together with those of intermediates in the acid hydrolysis of [Cr(CN)5NO]3–, were measured and the results fitted to a generalised energy-level scheme.

Electromagnetic Properties of Hemoproteins: II. THE EFFECT OF PHYSICAL STATES ON ELECTRON PARAMAGNETIC RESONANCE PARAMETERS OF HEMOPROTEINS

Abstract Low temperature electron paramagnetic resonance absorption spectra of randomly oriented preparations of cytochrome c peroxidase, ferrimyoglobin, ferrimyoglobin-azide, and ferrimyoglobin-fluoride are measured in various physical states. In general, the line width of resonance absorptions increases in relation to the physical state of preparation in the following order: concentrated solutions l wet polycrystalline suspensions l lyophilized preparations l dried polycrystalline preparations. The broadening of resonance absorptions caused by dehydration can be reversed by rehydration and dissolution of the dried preparation. When cytochrome c peroxidase and ferrimyoglobin-azide are dehydrated, their spin types are reversibly fixed in the state at dehydration and become temperature-independent until these compounds are rehydrated. The intensity of the high spin resonance absorption of ferrimyoglobin decreases by a factor of 10 to 20 on dehydration. The initial intensity can be restored by rehydration and dissolution of the dehydrated ferrimyoglobin. Resonance spectra of dehydrated ferrimyoglobin exhibit hitherto unreported absorptions in the high field region. These absorptions may represent a compound or compounds which may be responsible for the low spin characteristics of lyophilized ferrimyoglobin detected by Mossbauer absorption spectroscopy.

Electron Spin Resonance Absorption Spectra of CO3− and CO33− Molecule—Ions in Irradiated Single-Crystal Calcite

Single crystals of x- and γ-irradiated calcite reveal a number of paramagnetic defect centers, two of which have been tentatively identified as the CO3− and CO33− molecule—ions. The ESR absorption spectrum of what is believed to be the CO3− molecule—ion is found to have symmetry about the calcite [111] direction with spin-Hamilton parameters given by g‖=2.0051, g⊥=2.0162, A‖=13.1 Oe, and A⊥=9.4 Oe. The corresponding parameters for what is believed to be the CO33− molecule—ion which also exhibits symmetry about the crystal [111] direction are given by g‖=2.0013, g⊥ =2.0031, A‖=171.22 Oe, and A⊥=111.33 Oe. The parameters of these two spectra are discussed and compared with those reported for the isoelectronic NO3 and NO32− species. Optical measurements reveal two absorption bands, one at 6500 Å and another at 4850 Å. The longer-wavelength band exhibits anisotropy and is found to have temperature-dependent decay characteristics which are similar to those of the CO3− molecule—ion ESR spectrum. An activation energy of 0.12 eV is obtained for this center. Thermal decay data suggest that the shorter-wavelength band is not associated with either of these paramagnetic species.

Studies on Cytochrome c Peroxidase: VII. ELECTRON PARAMAGNETIC RESONANCE ABSORPTIONS OF THE ENZYME AND COMPLEX ES IN DISSOLVED AND CRYSTALLINE FORMS

Abstract Electron paramagnetic resonance absorption spectra cytochrome c peroxidase at 77°K exhibit well defined signal at g = ∼6.0, 4.3 (weak), 2.7, 2.2, ∼2.0 (weak), and 1. On the formation of Complex ES, by adding hydroperoxide to the enzyme, these electron paramagnetic resonance signals of the enzyme almost completely disappear and replaced by an intense narrow signal of a free radical type at g = 2.00. The spin concentration of this free radical is estimated to be about 1 eq to enzyme hematin unit. It is speculated that one of the 2 oxidizing equivalents of Complex ES may te retained in the form of a stable and reversible free radical of aromatic amino acid residue of the enzyme protein and the other in the hematin prosthetic group of the enzyme.

Complex formation between transition metal ions and organic solvents: electron spin resonance investigations.

THE electron spin resonance (ESR) technique provides a powerful tool for investigating transition metal complex formation in non-aqueous solutions. In recent work we have shown1 that ESR and optical absorption spectra of different paramagnetic ions are changed when glycerol replaces water as a solvent. Both the variation in line width and the optical ‘red shift’ obtained suggest a change in crystal-field energy levels by spin-orbit mechanisms. Newer theories have linked octahedral–tetrahedral changes to these ligand-field effects on non-bonding d electrons2. Thus, it might be of interest to investigate the ESR spectra of the chlorides of Mn2+, Fe3+ and Cu2+ in different organic solvents including dimethylsulphoxide (DMSO), dimethylformamide, formamide, acetone, glycerol and ethanol. These solvents were chosen to provide a varying dielectric constant and base strength since such parameters are known to affect the equilibrium between the octahedral and tetrahedral states.

ESR Absorption of Cesium and Rubidium Solutions in Ethylamine

Electron spin resonance absorption spectra of cesium and rubidium solutions in ethylamine have been observed at temperatures between −29° and +23°C. These spectra are characterized by a sharp single-line resonance near the center of a hyperfine pattern. The lines of a hyperfine pattern vary in amplitude and breadth. The larger amplitude, narrower lines lie near the center of the spectrum. The hyperfine patterns are attributed to 85Rb and 133Cs nuclei interacting with an unpaired electron. The spin density at the 85Rb nucleus varies from 1.05±0.1×1024 cm3 at −25°C to 1.90±0.05×1024 cm3 at 23°C and at the 133Cs nucleus from 2.29±0.03×1024 cm3 at −29°C to 2.80±0.03×1024 cm3 at −10°C. The variations in line breadths are interpreted in terms of the structure of the metal monomer. The difference in g values between the single-line resonance and the hyperfine pattern is attributed to spin—orbit coupling in the monomer and structural implications are discussed. The single-line resonance is assigned to the solvent anion, i.e., the solvated electron.

Radical ions in the reactions of p-chloranil with triphenylphosphine and with triethyl phosphite : Electron spin resonance and ultraviolet absorption spectra

The colour and the ESR spectrum which develop during the reaction of triphenylphosphine with p-chloranil are due to a phenoxy-O-phosphonium radical cation and to a semiquinone radical anion. These are formed by oxidation of the phosphine, or of a phosphine-quinone complex, by quinone. The radical ions are not formed by oxidation of the final product of the reaction (a phenoxide-O-phosphonium dipolar ion) by p-chloranil in solution. The reaction of triethyl phosphite with p-chloranil produces analogous coloured species with unpaired electrons. The g-factors and the hyperfine constants (ΔH) for the radical cations from triphenylphosphine and triethyl phosphite, are, respectively: 2·0050, 2·42 gauss and 2·0051, 1·83 gauss.

Photoelectric Emission and Electrical Conductivity of the Cesium Complex with Pyrene Derivatives

Abstract On the basis o the charge transfer process of the complexes, the photoemission and the electronic conduction of the complexes between cesium and pyrene derivatives, tetranitropyrene (TNO2P), tetrachloropyrene (TClP), tetrabromopyrene (TBrP) and tetracyanopyrene (TCNP) have been measured. A strong increase in the quantum yield of photoemission, from 10−3–10−4 to 10−1–102, resulting from the formation of the complexes, has been found. The promotion may take place by means of an electron transfer from cesium metal to a partner molecule; [Cs+] [Aromatics−]. The assumption is supported by the findings concerning electron spin resonance absorption and charge-transfer spectra. The electrical resistivity of the complexes, ρ at 15°C of pyrene-Cs=2.1×104 Ωcm., (TClP)-Cs=8Ωcm., (TBrP)-Cs=1.6×101 Ωcm., (TNO2P)-Cs=2×106 Ωcm. and (TCNP)-Cs=6×105 Ωcm., is strongly affected by the contribution of the, dative-bond character of the complexes.

Electron Spin Resonance and Electronic Absorption Spectra of Rh II in ZnWO4

The electron spin resonance (ESR) and electronic absorption spectra of Rh II in a single crystal of ZnWO4 have been recorded. Excited state energies determined from the electronic spectrum are used in an attempt to interpret the measured ESR g values. The orientation of the Rh II magnetic axes relative to the crystalline axes has been measured, and from the magnitude of the hyperfine splitting for 103Rh(I=½), determined along each each axis, values of P=2γββN〈r—3〉 and κ are calculated.

Electron Spin Resonance of Amine Solutions of Alkali Metals

Nuclear hyperfine structure has been observed in the electron spin resonance (ESR) absorption spectra of potassium and lithium dissolved in some alkylated amines. The four-line spectrum displayed by the potassium solutions is attributed to 39K-electron contact interaction. This interaction increases with increasing temperature. The spin density at the potassium nucleus in ethylamine solution is 3.37±0.05×1023 cm—3 at —40°C and 1.21±0.05×1024 cm—3 at 50°C. The contact interaction also decreases when methylamine is added to an ethylamine solution. This behavior is interpreted in terms of a modified view of the monomer of the theory of Becker, Lindquist, and Alder. The monomer changes its structure as its environment changes. A statistical model which relates the experimental observations to these changes is constructed. The differences in the ESR absorption spectra observed for the three systems Na–NH3, K—EtNH2, and Li—EtNH2 are discussed, and in part, attributed to differences among the three systems in the rates of spin exchange, electron exchange, and solvent exchange. Order-of-magnitude calculations relating measured line breadths to the rates of these processes are carried out in certain limiting cases. The results of these calculations lend support to the idea that the monomer includes remote solvent molecules within its structure. The rate constant for spin exchange is shown to increase with decreasing temperature in some potassium—amine solutions. No evidence for the solvated electron is found in the ESR spectra of K dissolved in ethylamine, or in a range of mixtures of ethylamine with methylamine. An upper limit of 10—4 is placed on the dissociation constant of the monomer. For Li in ethylamine a composite spectrum is observed under some conditions. It consists of a nine-line hyperfine pattern superimposed on a single relatively sharp line with the same g value. This sharp line is attributed to the solvated electron. The nine-line pattern is attributed to hyperfine interaction between the unpaired electron and four equivalent 14N nuclei in the Li monomer. The spin density at each 14N is 1.4±0.1×1023 cm—3, independent of temperature. Similar spectra are observed for Li dissolved in two mixed amine solvents. The size of the monomer is discussed in terms of an expanded orbital approach. The K monomer is shown to vary between two and three times the size of atomic K. The trend toward lower percent atomic character with decreasing atomic number of the alkali metal is also discussed. Part of this trend may come about because the small atoms undergo a larger expansion in forming a monomer.

STUDIES ON THE DIBENZO-P-DIOXIN (DIPHENYLENE DIOXIDE) DERIVATIVES. XXXIX. ELECTRON SPIN RESONANCE ABSORPTION SPECTRA OF DIBENZO-P-DIOXIN DERIVATIVES IN ANTIMONY PENTACHLORIDE.

Dibenzo-p-dioxin (I) derivatives showed blue color in antimony pentachloride. These colored solutions gave electron spin resonance absorption spectra and the formation of the cation radicals such as Ia was confirmed.