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Hyperfine Coupling (hyperfine + coupling)
Terms modified by Hyperfine Coupling Selected AbstractsRadical annihilation of ,-ray-irradiated contact lens blanks made of a 2-hydroxyethyl methacrylate copolymer at elevated temperaturesJOURNAL OF APPLIED POLYMER SCIENCE, Issue 6 2010Young-Shang Lin Abstract The annihilation of the radicals in irradiated 2-hydroxyethyl methacrylate copolymer was analyzed by the use of electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra were deconvoluted into three radicals: a quartet (Ra), a triplet (Rb), and a broad singlet (Rc). Radical Ra was attributed to coupling with a methyl radical and/or a doublet or triplet with about the same hyperfine coupling due to a methylene radical. Radical Rb was due to a methylene radical produced by main-chain scission. Radical Rc was attributed to various free radicals without coupling to protons. By comparing the EPR spectra of radicals Ra, Rb, and Rc with the spectrum of a 2,2-diphenyl-1-picrylhydrazyl (DPPH) standard with a known spin number, we calculated the spin numbers of the radicals, which decreased with time in the temperature range 25,45°C, regardless of the irradiation dose. The annealing of Ra and Rb and the annealing of Rc at longer times followed second-order kinetics; these were different from the kinetics for the color formation and defect-controlled hardening of polymers. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] The oxygen vacancy in Ga2O3: a double resonance investigation,MAGNETIC RESONANCE IN CHEMISTRY, Issue S1 2005H. J. Kümmerer Abstract When produced under reducing conditions, ,-Ga2O3 is transformed into an n -type semiconductor with delocalized conduction electrons that exhibit a very strong electron spin resonance (ESR) and a strong hyperfine coupling to the gallium nuclei of the host lattice. We apply the Overhauser-shift technique to investigate single crystals of this compound. With extension to the high magnetic field of a W-band spectrometer, we were able to resolve all spectral lines that were recorded and to assign them to their corresponding electronic and nuclear states. This separate analysis was the basis to access additional sample characteristics: the hyperfine coupling that is actually averaged out in the ESR signal, as well as the nuclear relaxation rates could be analyzed. Systematic measurements by varying the microwave power revealed the Overhauser shift in thermal equilibrium. The signal could be tracked to very small microwave saturation parameters, at which the deviation from the usual linear relation between power and shift becomes evident and the shift clearly approaches a constant value. This value in equilibrium was determined directly from a fit to a sequence of measurements, whereas standard X-band experiments only provided indirect conclusions. The probability densities of the electrons at the nuclei in the two nonequivalent crystallographic positions,the lattice sites with octahedral and tetrahedral coordination,could also be determined directly. The enhanced resolution revealed an otherwise hidden substructure in the nuclear resonance signals. On the basis of a microscopic model, this structure could be used to probe the environment of the oxygen vacancy more precisely and to determine the extension of the electronic wave function of the donor electrons. Copyright © 2005 John Wiley & Sons, Ltd. [source] The g -values and hyperfine coupling of amino acid radicals in proteins: comparison of experimental measurements with ab initio calculations,MAGNETIC RESONANCE IN CHEMISTRY, Issue S1 2005Sun Un Abstract Electron paramagnetic resonance (EPR) spectroscopy has been extensively used to identify and characterize protein-based redox active amino acid radicals based on their g -values and hyperfine couplings. To better understand how these parameters depend on the electronic structure and environment of the radical, the theoretical g -values and proton hyperfine tensors of three models corresponding to the tyrosyl, tryptophanyl and glycyl radicals were calculated using Gaussian 03. The g -values were determined using the B3LYP/6,31+G(D,P) combination of density functional and basis set, while the hyperfine tensors were determined using the B3LYP/EPR-III and PBE0/EPR-III combinations. Comparisons are made to measured values. It was found that by appropriately accounting for hydrogen bonds and the dielectric constant of the environment, good agreement could be achieved between the calculated and measured g -values. In all three cases, the g -anisotropy arose from significant spin density on a nitrogen or oxygen atom. The calculated hyperfine tensors for the three radicals did not differ significantly from previous calculations. In the case of the tyrosyl radical, it is shown for the first time that the para -position substituent that is opposite of the CO group can break the symmetry of the phenyl ring, leading to different hyperfine tensors for the two large ortho proton couplings. For the tyrosyl and tryptophanyl models, the calculated hyperfine couplings to hydrogen-bonding protons were in very good agreement with measured values for both the tyrosyl and tryptophanyl models. Copyright © 2005 John Wiley & Sons, Ltd. [source] Radical Ions from 3,3,,,,3,,,,-Tris(butylsulfanyl)-2,2,:5,,2,:5,,2,,,,5,,,,2,,,,:5,,,,,2,,,,-sexithiophene: An Experimental and Theoretical Study of the p - and n -Doped OligomerCHEMPHYSCHEM, Issue 11 2003Angelo Alberti Dr. Abstract The 3,3,,,,3,,,,-tris(butylsulfanyl)-2,2,:5,,2,:5,,2,,,,5,,,,2,,,,:5,,,,,2,,,,-sexithiophene 1 was investigated through spectroscopic (NMR, EPR, UV/Vis-NIR), electrochemical, spectroelectrochemical and theoretical (DFT) studies. The charged species obtained upon its oxidation and reduction were characterised, showing that 1 can exist in at least five different oxidation states, that is, a neutral species, a radical cation, a dication, a radical anion, and a dianion. The long term stability of the radical cation 1+. was evidenced by the 1H NMR study in the presence of small quantities of trifluoroacetic acid (TFA). This approach allowed a comparison of the relative broadening of proton signals of 1, induced by the electron exchange process with traces of radical cation 1+., and the hfc (hyperfine coupling) constants obtained from the EPR study and DFT calculations. In the radical cation, all of the heterocyclic sulphur atoms are not significantly involved in the delocalisation of the unpaired electron, whereas the opposite holds for the radical anion. Time-dependent DFT calculations reproduced well the wavelengths of the optical transitions observed in the spectroelectrochemical experiments for all the five oxidation states and support the formation of the dianion 12,. [source] Evidence from ESR studies for [Co(,-C2H4)3] produced at 77 K in a rotating cryostat,MAGNETIC RESONANCE IN CHEMISTRY, Issue 10 2006Lynda J. Hayton Abstract Co atoms were reacted with ethene at 77 K and the paramagnetic products studied by electron spin resonance (ESR) at X- and K-bands. The ESR spectra of the major product at both frequencies showed eight cobalt multiplets (ICo = 7/2) indicating a mono-cobalt complex. The spectra have orthorhombic g and cobalt hyperfine tensors and were simulated by the parameters; g1 = 2.284, g2 = 2.0027, g3 = 2.1527; A1 < , 25 MHz, A2 = , 109 MHz, A3 = , 198 MHz. Proton and 13C (1% natural abundance) hyperfine couplings were lower than the line widths (<2 MHz) indicating less than 0.5 spin transfer to the ethene ligands. We assigned the spectrum to a Jahn,Teller-distorted planar trigonal mono-cobalt tris-ethene [Co(,-C2H4)3] complex in C2v symmetry. The SOMO is either a 3dx2,y2 (2a1) orbital in a T-geometry or a 3dxy (b1) orbital in a Y-geometry but there is only a spin density, a2, of 0.30 in these d orbitals. The spin deficiency of 0.70 is attributed to two factors; spin transfer from the Co to ethene ,/,* orbitals and a 4p orbital contribution, b2, to the SOMO. Calculations of a2 and b2 have been made at three levels of spin transfer, ,. At , = 0.00a2 is 0.23 and b2 is 0.78, at , = 0.25a2 is 0.25 and b2 is 0.52 and at , = 0.50a2 is 0.28 and b2 is 0.23. The other possible assignment to a mono-cobalt bis-ethene complex [Co(,-C2H4)2] cannot be discounted from the ESR data alone but is considered unlikely on other grounds. The complex is stable up to ,220 K indicating a barrier to decomposition of ,50 kJ Mol,1 Copyright © 2006 John Wiley & Sons, Ltd. [source] The g -values and hyperfine coupling of amino acid radicals in proteins: comparison of experimental measurements with ab initio calculations,MAGNETIC RESONANCE IN CHEMISTRY, Issue S1 2005Sun Un Abstract Electron paramagnetic resonance (EPR) spectroscopy has been extensively used to identify and characterize protein-based redox active amino acid radicals based on their g -values and hyperfine couplings. To better understand how these parameters depend on the electronic structure and environment of the radical, the theoretical g -values and proton hyperfine tensors of three models corresponding to the tyrosyl, tryptophanyl and glycyl radicals were calculated using Gaussian 03. The g -values were determined using the B3LYP/6,31+G(D,P) combination of density functional and basis set, while the hyperfine tensors were determined using the B3LYP/EPR-III and PBE0/EPR-III combinations. Comparisons are made to measured values. It was found that by appropriately accounting for hydrogen bonds and the dielectric constant of the environment, good agreement could be achieved between the calculated and measured g -values. In all three cases, the g -anisotropy arose from significant spin density on a nitrogen or oxygen atom. The calculated hyperfine tensors for the three radicals did not differ significantly from previous calculations. In the case of the tyrosyl radical, it is shown for the first time that the para -position substituent that is opposite of the CO group can break the symmetry of the phenyl ring, leading to different hyperfine tensors for the two large ortho proton couplings. For the tyrosyl and tryptophanyl models, the calculated hyperfine couplings to hydrogen-bonding protons were in very good agreement with measured values for both the tyrosyl and tryptophanyl models. Copyright © 2005 John Wiley & Sons, Ltd. [source] An EPR and ENDOR Investigation of a Series of Diazabutadiene,Group 13 ComplexesCHEMISTRY - A EUROPEAN JOURNAL, Issue 10 2005Robert J. Baker Dr. Abstract Paramagnetic diazabutadienegallium(II or III) complexes, [(Ar-DAB)2Ga] and [{(Ar-DAB.)GaX}2] (X=Br or I; Ar-DAB={N(Ar)C(H)}2, Ar=2,6-diisopropylphenyl), have been prepared by reactions of an anionic gallium N-heterocyclic carbene analogue, [K(tmeda)][:Ga(Ar-DAB)], with either "GaI" or [MoBr2(CO)2(PPh3)2]. A related InIII complex, [(Ar-DAB.)InCl2(thf)], has also been prepared. These compounds were characterised by X-ray crystallography and EPR/ENDOR spectroscopy. The EPR spectra of all metal(III) complexes incorporating the Ar-DAB ligand, [(Ar-DAB.)MX2(thf)n] (M=Al, Ga or In; X=Cl or I; n=0 or 1) and [(Ar-DAB)2Ga], confirmed that the unpaired spin density is primarily ligand centred, with weak hyperfine couplings to Al (a=2.85 G), Ga (a=17,25 G) or In (a=26.1 G) nuclei. Changing the N substituents of the diazabutadiene ligand to tert -butyl groups in the gallium complex, [(tBu-DAB.)GaI2] (tBu-DAB={N(tBu)C(H)}2), changes the unpaired electron spin distribution producing 1H and 14N couplings of 1.4 G and 8.62 G, while the aryl-substituted complex, [(Ar-DAB.)GaI2], produces couplings of about 5.0 G. These variations were also manifested in the gallium couplings, namely aGa ,1.4 G for [(tBu-DAB.)GaI2] and aGa ,25 G for [(Ar-DAB.)GaI2]. The EPR spectra of the gallium(II) and indium(II) diradical complexes, [{(Ar-DAB.)GaBr}2], [{(Ar-DAB.)GaI}2], [{(tBu-DAB.)GaI}2] and [{(Ar-DAB.)InCl}2], revealed doublet ground states, indicating that the GaGa and InIn bonds prevent dipole,dipole coupling of the two unpaired electrons. The EPR spectrum of the previously reported complex, [(Ar-BIAN.)GaI2] (Ar-BIAN=bis(2,6-diisopropylphenylimino)acenaphthene) is also described. The hyperfine tensors for the imine protons, and the aryl and tert -butyl protons were obtained by ENDOR spectroscopy. In [(Ar-DAB.)GaI2], gallium hyperfine and quadrupolar couplings were detected for the first time. [source] | ||||||||||