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- Komaguchi, K., et al.
(författare)
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ESR and theoretical studies of trimer radical cations of coronene
- 2006
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Ingår i: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy. - : Elsevier BV. - 1386-1425 .- 1873-3557. ; 63:1, s. 76-84
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Tidskriftsartikel (refereegranskat)abstract
- Highly resolved ESR spectra of monomer, dimer and trimer radical cations of coronene (C24H12) were observed at room temperature for a solution of 1,1,1,3,3,3-hexafluoro-2-propan-2-ol (HFP) containing thallium(III) trifluoroacetate as oxidant. The spectra consisting of multiple lines with isotropic 1H-hyperfine splitting (hfs) constants of 0. 0766 mT (24H) and 0.013 mT (6H) were attributable to a mixture of the dimer with the trimer radical cations, (C24H12)2+ and (C24H12)3+. For (C24H12)2+, the 1H-hfs constant agreed well with the reported value, 0.077 mT. However, for (C 24H12)3+, the values were significantly different from the reported ones, 0.117 mT (12H) and 0.020 mT (24H), by Ohya Nishiguchi et al. [H. Ohya-Nishiguchi, H. Ide, N. Hirota, Chem. Phys. Lett. 66 (1979) 581], but rather similar to those reported by Willigen et al. [H. van Willigen, E. De Boer, J.T. Cooper, W.F. Forbes, J. Chem . Phys. 49 (1968) 1190]. In conflict with Willigen's report, however, no ESR line broadening which has been ascribed to a low stationary concentration of (C 24H12)3+ was detected. Based on ab initio MO calculations for benzene as a compact model of C24H 12, the structure of (C24H12)3+ was investigated in terms of the observed 1H-hfs constants. A staggered sandwich C2v structure was suggested being at the "global" minimum for the benzene trimer cation. In the structure, the unpaired electron spin is predominantly localized to the central ring, which is qualitatively in agreement with the previous ESR results of (C 24H12)3+ by Ohya-Nishiguchi et al. In addition, as a "local" minimum, the benzene trimer was indicated to have a slipped sandwich Cs structure, which is less stable by ca. 19 kJ mol-1 than the "global" minimum. In this structure, the unpaired electron spin was nearly equally distributed on both the central and one of the two side C24H12 molecules. The observed 1H-hfs constants were possibly attributable to the (C 24H12)3+ cation with the analogous slipped sandwich Cs structure.
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| 2. |
- Yahiro, H., et al.
(författare)
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Copper-phthalocyanine encapsulated into zeolite-Y with high Si/Al : An EPR study
- 2005
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 415:1-3, s. 126-130
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Tidskriftsartikel (refereegranskat)abstract
- Copper (II) phthalocyanine (CuPc) molecules encapsulated into zeolite-Y with Si/Al ratios of 2.7 and 410 were prepared by an in situ synthesis and characterized by UV-Vis and electron paramagnetic resonance (EPR) spectroscopies. Resolved Cu-hyperfine and N-superhyperfine structures were observed in the EPR spectrum of CuPc encapsulated into zeolite-Y with a high Si/Al ratio. UV-Vis and EPR studies as well as theoretical calculations suggest that the encapsulated CuPc molecule was distorted in zeolite-Y with keeping of the square-planar symmetry around the center copper (II) ion. © 2005 Elsevier B.V. All rights reserved.
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| 3. |
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| 4. |
- Komaguchi, K, et al.
(författare)
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An ESR and ENDOR study of irradiated 6Li-formate
- 2007
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Ingår i: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy. - : Elsevier BV. - 1386-1425 .- 1873-3557. ; 66:3, s. 754-760
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Tidskriftsartikel (refereegranskat)abstract
- Lithium formate (6LiOOCH·H2O), 95% 6Li enrichment, combined with an exchange of crystallization water with D2O was investigated. The ESR spectrum of the radiation induced free radicals stable at room temperature consists of a singlet with a narrow line width, 0.92 mT. 6Li has smaller magnetic moment and nuclear spin, which resulted in the narrower line width accompanied with an increase in peak amplitude. In comparison with lithium formate with natural isotopic composition, 6Li (7.5%, I = 1) and 7Li (92.5%, I = 3/2), the sensitivity was increased by a factor of two. With optimised spectrometer settings 6Li formate had seven times higher sensitivity compared to alanine. Therefore this material is proposed as a dosimeter material in a dose range down to 0.1 Gy. The g and the 13C-hyperfine (hf) tensors of the CO2- radical anion, major paramagnetic products, were evaluated to be g = (2.0037, 1.9975, 2.0017), and A(13C) = (465.5, 447.5, 581.3) MHz for polycrystalline samples at room temperature. Furthermore, the 1H-hf and 6Li-hf tensors observed for the surroundings of CO2- by ENDOR technique were in fairly good agreement with DFT calculations. The CO2- radicals are found to be so stable that the formate is applicable to the ESR dosimetry, because of fully relaxing in a fully relaxed geometrical structure of the CO2- component and remaining tight binding with the surroundings after the H atom detachment from HCO2-. © 2006 Elsevier B.V. All rights reserved.
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| 5. |
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| 6. |
- Liu, W., et al.
(författare)
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Structure and dynamics of [(CH3)3N-CH2]+· radical generated in ?- irradiated Al-offretite
- 2000
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 2:11, s. 2515-2519
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Tidskriftsartikel (refereegranskat)abstract
- An EPR study was carried out to investigate the structure and motional dynamics of the [(CH3)3N-CH2]+· radical formed in a ?-irradiated Al- offretite. Strongly temperature-dependent EPR spectra were observed in the temperature range between 4 and 300 K. The observed 1H hyperfine (hf) splitting was a1 = 0.46 mT for three magnetically equivalent hydrogens, one of each methyl group at 110 K and a1 = 0.15 mT for three of each methyl group at 300 K, the hf splittings due to two equivalent CH2 hydrogens and the central 14N were a2 = 2.24 and a3 = 0.35 mT, respectively, independent of the temperature. This result suggests that rotation of the methyl group around the N-CH3 bond is responsible for the temperature dependent line shapes. Assuming a three-site jump model in which three methyl protons interchange their positions with each other, the EPR line shapes were successfully analyzed by a simulation method using the jump rate as a variable parameter in the temperature range from 140 to 270 K. The rate was found to increase from 7.0 x 106 s-1 (140 K) to 4.1 x 108 s-1 (270 K) with temperature. From an Arrhenius plot of the rate constants, an activation energy of 8.1 kJ mol-1 was evaluated for the methyl group rotation of [(CH3)3N-CH2]+· in Al-offretite.
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