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| 2. |
- Vestad, T.A., et al.
(författare)
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EPR dosimetric properties of formates
- 2003
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Ingår i: Applied Radiation and Isotopes. - 0969-8043 .- 1872-9800. ; 59:2-3, s. 181-188
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Tidskriftsartikel (refereegranskat)abstract
- As a part of a program to develop an electron paramagnetic resonance (EPR) dosimeter suited for clinical use (doses in the cGy range), polycrystalline samples of lithium formate monohydrate (HCO2Li·H 2O), magnesium formate dihydrate (C2H2O 4Mg·2H2O), and calcium formate (C2H 2O4Ca) have been examined. L-Alanine was included for comparison and reference. Samples were irradiated with 60Co ?-rays and 60-220kV X-rays. The dosimeter response was assessed using the peak-to-peak amplitude of the first-derivative EPR spectrum. Dose-response curves for the 60Co ?-irradiated samples were constructed, and the dependences of the response on the photon energy, microwave power, and modulation amplitude were studied. Stability of the irradiation products upon storage (signal fading) was also investigated. Lithium formate monohydrate is by far the best candidate of the tested formates, suitable for measuring doses down to approximately 0.1Gy. Lithium formate monohydrate is more sensitive than alanine by a factor of 5.6-6.8 in the tested photon energy range, it exhibits no zero-dose signal and shows a linear dose response in the dose range from 0.2 to 1000Gy. Its EPR signal was found unchanged in shape and intensity 1 week after irradiation to 10Gy. Various less favorable properties rendered the other formates generally unsuitable, although calcium formate exhibits some interesting EPR dosimetric properties. © 2003 Elsevier Ltd. All rights reserved.
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| 3. |
- Lund, Eva, et al.
(författare)
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Formates and dithionates : sensitive EPR-dosimeter materials for radiation therapy
- 2005
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Ingår i: Applied Radiation and Isotopes. - : Elsevier BV. - 0969-8043 .- 1872-9800. ; 62:2, s. 317-324
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Tidskriftsartikel (refereegranskat)abstract
- Polycrystalline, formates and dithionates are promising materials for EPR dosimetry, as large yields of radiation induced stable radicals are formed with a linear dose response. Rapid spin relaxation rates were detected in many of the substances, indicating that a high microwave power can be applied during EPR acquisition in order to improve sensitivity. Different techniques used to further improve the sensitivity, such as the replacement of Li-7 with Li-6 or exchange of protons with deuterons in the corresponding crystalline matrices and metal ion doping are discussed. It is concluded that formates and dithionates may be up to 10 times as sensitive as L-alpha-alanine. (C) 2004 Elsevier Ltd. All rights reserved.
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| 4. |
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| 5. |
- Eid, O.I., et al.
(författare)
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Allyl type radical formation in X-irradiated glutarimide crystals studied by ENDOR and ENDOR-induced EPR
- 2004
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 6:13, s. 3604-3610
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Tidskriftsartikel (refereegranskat)abstract
- Glutarimide single crystals X-irradiated at room temperature were reinvestigated at 150 K with the purpose to obtain information about possible ring opening and other fragmentation processes involving free radicals in this compound, previously only observed to take place in aqueous solutions. In previous work, using EPR,ENDOR and EIE spectroscopy, two H-abstraction radicals present in a 3:1 relative ratio in the irradiated crystals were identified. In the present work the detection of a third radical species, III, is reported. The g- and hyperfine coupling tensors for all three radicals at 150 K were obtained. Based on simulations of the EPR spectra the relative abundance of the three radicals was estimated to be 60, 25 and 15% for radicals I, II and III, respectively. Radical III is proposed to be of the allyl radical type -CH=CH-1CH- formed formally by a concerted H2 elimination from C2 and C3 of radical I and/or from C3 and C4 of radical II. This proposal and structure is at variance with observations from aqueous solution studies of succinimide, where the CO-CH2 bond was susceptible for rupture. However, the assignment is consistent with density functional theory (DFT) calculations, predicting equivalent hyperfine interactions to the two a-type protons at C2 and C4 in excellent. agreement with the experimentally determined hyperfine coupling tensors. EIE results confirmed that both couplings originate from the same radical species. Possible mechanisms for the formation of radical III are discussed.
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| 6. |
- Hosseini, A., et al.
(författare)
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EPR and ENDOR studies of single crystals of 2-oxazolidinone X-irradiated at 295 K
- 2002
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 4:24, s. 6086-6091
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Tidskriftsartikel (refereegranskat)abstract
- When single crystals of the 5-membered heterocyclic ring structure 2-oxazolidinone (C3O2NH5) are irradiated at room temperature, the major radical formed (R1) decays during a period of a few hours, leaving a broad, unstructured EPR spectrum not amenable to analysis. Cooling the crystals to about 140 K immediately after irradiation at room temperature allows analysis of the R1 radical by EPR and ENDOR. The radical is formed by a net H-abstraction from one of the two methylene groups of the molecule. A full EPR and ENDOR analysis of four proton interactions (one a-coupling, and three ß-couplings) together with ESR evidence for a small nitrogen hyperfine interaction allowed for a precise identification of R1. The results show that the carbon-centered radical R1 is puckered at the radical center. Using DFT calculations together with the experimental EPR and ENDOR results, the torsion angle of the C1-H bond with respect to the N-C1-C2 plane is estimated to be 13-15° (bending angle 0˜ 7°). The DFT calculations reproduced the carbon-bonded proton hyperfine coupling constants satisfactorily but failed to reproduce the experimental results for the nitrogen and nitrogen-bonded proton hyperfine interactions. © The Owner Societies 2002.
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| 7. |
- Itagaki, Y, et al.
(författare)
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ENDOR study of N-14 hyperfine and quadrupole couplings of N2D4 center dot+ formed in deuterated Li(N2H5)SO4 single crystal
- 2002
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 106:11, s. 2617-2622
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Tidskriftsartikel (refereegranskat)abstract
- X-irradiated Li(N2D5)SO4 single crystals were investigated using EPR and ENDOR spectroscopy. The N-14-ENDOR spectra of the deuterated hydrazine radical cation N2D4.+ were clearly observed at 240 K. N-14 hyperfine (life) and nuclear quadrupole (nqc) tensors of the N2D4.+ cation were determined from angular variation ENDOR measurements in the three orthogonal planes of the crystal. The life tensor obtained for the two equivalent N-14 atoms is very close to that reported in a N2H5HC2O4 single crystal, and the present results support the previous conclusion that the cation has a planar structure at 240 K. The N-14 nqc tensor was estimated using semiempirical methods and also calculated from the field gradients in the LiHzS crystal as evaluated by density functional theory methods. The general agreement with experimental observations further supported the suggested geometrical structure of the N2H4.+ radical. The H-1 and N-14-ENDOR enhancements observed may be due to weakened dipolar interactions between N-14-D-2 and H-1-D-2 leading to increased T-ln relaxations of the N-14 and residual H-1 nuclei of the N2D4.+ cation (and/or other isotopomers) in the deuterated crystals as compared to those in the Li(N2H5)SO4 crystals.
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| 8. |
- Itagaki, Y., et al.
(författare)
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Structure of N2H4.+ formed in X-irradiated Li(N2H5)SO4 single crystals
- 2000
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 2:1, s. 37-42
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Tidskriftsartikel (refereegranskat)abstract
- X-irradiated Li(N2H5)SO4 single crystals were investigated using ESR and ENDOR spectroscopy at several temperatures. The hydrazine radical cation N2H4.+ was selectively produced by irradiation at room temperature. From the analysis of the orientation dependent ENDOR spectra, the 1H-hfc tensors of the cation radical were precisely obtained and the radical structure was supported by theoretical calculations. It is suggested that the cation radical has a planar p* structure D(2h) (2B(2g)) in the crystal down to 230 K. By using the evaluated 1H-hf tensor the powder ESR line shape was successfully simulated. Concomitant with the radical formation, the N-N bond of N2H4.+ is suggested to reorient so as to optimize hydrogen bond interactions. 1H-ENDOR line splitting for the N2H4.+ radical was observed at temperatures below 230 K. Apparently this splitting is due to a reversible structural change where one of the NH2 moleties in N2H4.+ becomes slightly bent out of the molecular plane, whereas the other one remains planar. This deformation evidently arises from interactions between the cation radical, adjacent H2N-NH3+ molecules and the SO4-LiO4 framework. Interacting N2H5+···N2H4.+···N2H5+ molecules along the c-axis are proposed to explain the deformation mechanism.
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| 9. |
- Sagstuen, E, et al.
(författare)
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Weakly coupled proton interactions in the malonic acid radical : Single crystal ENDOR analysis and EPR simulation at microwave saturation
- 2000
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Ingår i: Journal of Physical Chemistry A. - 1089-5639 .- 1520-5215. ; 104:27, s. 6362-6371
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Tidskriftsartikel (refereegranskat)abstract
- The alpha-proton hyperfine coupling observed by electron paramagnetic resonance (EPR) spectroscopy on the radical (CH)-C-.(COOH)(2) in irradiated crystals of malonic acid, CH2(COOH)(2), has served as a standard against which hundreds of observations of similar couplings have been held and scaled. The major doublet of the malonic acid radical is accompanied by less intense "forbidden" (f) alpha-proton transitions and "spin-flip" (s) transitions due to weakly interacting protons. Both s and f transition lines exhibit microwave power saturation behaviors different from that of the major doublet. At high microwave power, the prominence of these s and f lines may be misinterpreted as originating from different radical species. Computer simulations could help distinguish between the different cases, but no computer simulation programs taking into account the microwave power saturation case are commonly available. On the basis of classical line-shape theory, an algorithm describing the microwave power dependence of an EPR line shape has been developed and implemented in an existing simulation program. To test this new program, malonic acid was selected because of the simplicity of its EPR spectra. However, sufficiently detailed information about the hyperfine coupling parameters for a satisfactory simulation of the room-temperature data (including s and f lines) was not available in the literature. Therefore, a detailed room-temperature EPR/ENDOR study on a single crystal of malonic acid was performed. In addition to the major cc-proton coupling, seven weaker proton interactions have been characterized and partly identified. Simulations under nonsaturating conditions reproduce very well all features of the experimental EPR spectra. Simulations under saturating conditions similarly reproduce the power-dependent EPR spectra and yield information about the relaxation behavior of the radical system, which is amenable to verification using other spin-resonance methods.
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| 10. |
- Sanderud, A, et al.
(författare)
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EPR and ENDOR studies of deuteron hyperfine and quadrupole coupling in center dot CD(COOD)(2) : Experimental and theoretical estimates of electric field gradients from an alpha-carbon
- 2000
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 104:27, s. 6372-6379
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Tidskriftsartikel (refereegranskat)abstract
- In single crystals of malonic acid grown from heavy water, the methylene protons have been partially exchanged with deuterons. Upon X irradiation at room temperature, the (CD)-C-.(COOD)(2) radical is formed in an amount comparable to the (CH)-C-.(COOD): radical species. In the present work, EPR and ENDOR analyses of the a-deuteron hyperfine coupling (hfc) and nuclear quadrupolar coupling (nqc) tensors at room temperature have been performed. The hyperfine coupling tensor is, when scaled with the differences in the nuclear g-factor, almost identical to the a-proton coupling of the (CH)-C-.(COOH)(2) radical at room temperature. The quadrupolar coupling tensor was found to be virtually coaxial with the hyperfine coupling tensor. The quadrupolar coupling constant is 149.8 +/- 1 kHz, and the asymmetry factor eta = 0.092 +/- 0.020. It is known that, at room temperature, the malonic acid radical exhibits thermal motion between two potential energy minima separated by about +/-12 degrees. Assuming that the observed hfe and nqc tensors are the result of thermal avenging between these two conformations of the radical, a simple two-site jump model was used to estimate the rigid-limit tensors. The most significant result obtained was for the nyc tensor, for which the calculations resulted in a quadrupolar coupling constant of 160 kHz and an asymmetry factor eta = 0.026. These values are fairly close to the nqc parameters for the methylene deuterons in malonic acid at low temperature. The quadrupolar coupling tensor has been theoretically modeled using Slater orbitals and formal electronic populations, as well as electron populations obtained from RHF/CI INDO-type calculations. The simple model to compute the electric field gradient at the alpha-deuteron caused by the charge distribution at the sp(2)-hybridized alpha-carbon was found to be as successful as more advanced methods. Furthermore, density functional theoretical (DFT) calculations for both the malonic acid radical and the native malonic acid molecule have been performed. Field gradients calculated by the DFT method significantly overestimate the quadrupolar tensors for both the alpha-deuteron of the radical and the methylene deuterons of the malonic acid molecule. Calculations using electron populations from the RHF/CI INDO calculations show that contributions to the quadrupolar coupling tensor from electrons and nuclei beyond the nearest-neighbor atom of the Jeuteron are significant.
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