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Frequency-domain electron nuclear double resonance (ENDOR), two time-domain electron nuclear double resonance techniques, and electron spin echo envelope modulation spectroscopy are compared with respect to their merit in measurements of small hyperfine couplings to nuclei with intermediate gyromagnetic ratio such as 31P. The frequency-domain Mims ENDOR experiment is found to provide the most faithful line shapes. In the limit of long electron-nuclear distances of more than 0.5 nm, sensitivity of this experiment is optimized by matching the first interpulse delay to the transverse relaxation time of the electron spins. In the same limit, Mims ENDOR efficiency scales inversely with the sixth power of distance. Hyperfine splittings as small as 33 kHz can be detected, corresponding to an electron-31P distance of 1 nm. In systems, where a certain kind of nuclei is distributed in a plane, measurements of intermolecular hyperfine couplings can be analyzed in terms of a distance of closest approach of a paramagnetic center to that plane. By applying this technique to spin-labeled lipids in a fully hydrated lipid bilayer it is found that for a fraction of lipids, chain tilt angles can be 25 degrees larger than the mean tilt angle of the lipid chains. This model of all-trans hydrocarbon chains with a broad distribution of tilt angles is also consistent with orientation selection effects in high-field ENDOR spectra. 相似文献
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Daniella Morgos Ivan Geroy Richard G. Sevier Molly M. Gribb Kevin P. Ryan Herbert H. Hill 《International Journal for Ion Mobility Spectrometry》2010,13(1):1-7
An ion mobility spectrometer (IMS) probe system for real-time, subsurface soil-gas sampling applications is presented. The
system includes an IMS and supporting electronics encased in a 51 mm diameter stainless steel probe housing. The IMS was challenged
in the laboratory with 2,6-di-tert-butylpyridine (DtBP) and tetrachloroethylene (PCE) in zero air yielding reduced ion mobility
constants (Ko) values of 1.42 cm2/Vs (n = 3) and 1.79 ± 0.01 cm2/Vs (n = 3), respectively. A resolving power of 38 and 31 was obtained for DtBP and PCE, respectively. The system was deployed at
a PCE-contaminated site to demonstrate its performance under field conditions. PCE was detected in the vapor samples as evidenced
by peaks with a Ko value of 1.80 ± 0.01 cm2/Vs for two measurements that were taken 6 min apart. The presence of PCE at the contaminated site was confirmed by GC-MS
analysis of a gas sample at an EPA-certified laboratory, suggesting that this IMS system can be used to detect PCE under field
conditions. 相似文献
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Rotem Vardi Marcelino Bernardo Hans Thomann Karl G. Strohmaier David E.W. Vaughan Daniella Goldfarb 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1997,126(2):229-241
X-band (∼9.3 GHz) pulsed ENDOR measurements were carried out on57Fe-substituted sodalite (FeSOD) which contains only one type of Fe(III) (S=) located at a framework site. The ENDOR spectrum recorded atg= 2 shows three doublets corresponding to the sixMSmanifolds. The assignment of these signals was confirmed by hyperfine-selective and triple ENDOR experiments. The components of each of the doublets had different intensities, reflecting the different populations of the EPR energy levels at the measurement temperature, 1.8 K. ENDOR spectra were recorded at magnetic fields within the EPR powder pattern, and the field dependence observed showed an anisotropic behavior, unexpected from the isotropic character of the57Fe(III) hyperfine coupling. This dependence was attributed to the high-order effects of the zero-field splitting (ZFS) interaction on the ENDOR frequencies. Three different theoretical approaches were used to account for the dependence of the ENDOR spectrum on the ZFS interaction. The first involves the exact diagonalization of the total spin Hamiltonian, the second uses third-order perturbation approximations, and the third employs an effective nuclear Hamiltonian for each of theMSmanifolds. The simulations showed that the ENDOR signals of theMS= ±5/2 (ν±5/2) manifold are the least sensitive to the magnitude of the ZFS parameterDand are therefore the most appropriate for the determination ofaiso. It is shown that at X band andaisovalues of about 30 MHz, the perturbation approach is valid up toDvalues of 500 MHz if all three doublets are concerned. However, if only the ν±5/2doublet is considered, then this approach is valid forD< 1000 MHz. The third approach was found inappropriate foraisovalues of ∼30 MHz. Using the method of exact diagonalization together with orientation selectivity, the trends observed in the experimental spectra could be reproduced. The ENDOR spectra of the57Fe-substituted zeolites ZSM5, L, and mazzite showed broad and ill-defined peaks since the ZFS of Fe(III) in these zeolites is significantly larger than that of FeSOD. Because this broadening is a high-order effect, it can be significantly reduced at higher spectrometer frequencies. 相似文献
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Daniella Takács Ildikó Nagy Orsolya Egyed György Hajós 《Journal of organometallic chemistry》2010,695(24):2673-2678
Phenothiazinyldienes obtained from tetrazolopyridinium salts and phenothiazine derivatives were subjected to reduction by borane-dimethyl sulfide in THF. Structure elucidation of the products revealed that one of the olefinic bond underwent reduction and, furthermore, borane addition at the double bond took place to yield derivatives of tetrazolo[5,1-f][1,2]azaborinin as a new fused ring system involving a bridge-head nitrogen atom. The new products have been synthesized and characterized by X-ray analysis, solution and solid-state NMR. 相似文献
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Ludmila A. Ramos Daniella L. Morgado Omar A. El Seoud Valdinéia C. da Silva Elisabete Frollini 《Cellulose (London, England)》2011,18(2):385-392
The acylation of three cellulose samples by acetic anhydride, Ac2O, in the solvent system LiCl/N,N-dimethylacetamide, DMAc (4 h, 110 °C), has been revisited in order to investigate the dependence of the reaction efficiency
on the structural characteristics of cellulose, and its aggregation in solution. The cellulose samples employed included microcrystalline,
MCC; mercerized cotton linters, M-cotton, and mercerized sisal, M-sisal. The reaction efficiency expresses the relationship
between the degree of substitution, DS, of the ester obtained, and the molar ratio Ac2O/AGU (anhydroglucose unit of the biopolymer); 100% efficiency means obtaining DS = 3 at Ac2O/AGU = 3. For all celluloses, the dependence of DS on Ac2O/AGU is described by an exponential decay equation: DS = DSo − Ae−[(Ac2O/AGU)/B]; (A) and (B) are regression coefficients, and DSo is the calculated maximum degree of substitution, achieved under the conditions of each experiment. Values of (B) are clearly
dependent on the cellulose employed: B(M-cotton) > B(M-sisal) > B(MCC); they correlate qualitatively with the degree of polymerization of cellulose, and linearly with the aggregation number, Nagg, of the dissolved biopolymer, as calculated from static light scattering measurements: (B) = 1.709 + 0.034 Nagg. To our knowledge, this is the first report on the latter correlation; it shows the importance of the physical state of dissolved
cellulose, and serves to explain, in part, the need to use distinct reaction conditions for MCC and fibrous celluloses, in particular Ac2O/AGU, time, temperature. 相似文献
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Banerjee D Paniagua JC Mugnaini V Veciana J Feintuch A Pons M Goldfarb D 《Physical chemistry chemical physics : PCCP》2011,13(41):18626-18637
Water soluble perchlorinated trityl (PTM) radicals were found to be effective 95 GHz DNP (dynamic nuclear polarization) polarizers in ex situ (dissolution) (13)C DNP (Gabellieri et al., Angew Chem., Int. Ed. 2010, 49, 3360). The degree of the nuclear polarization obtained was reported to be dependent on the position of the chlorine substituents on the trityl skeleton. In addition, on the basis of the DNP frequency sweeps it was suggested that the (13)C NMR signal enhancement is mediated by the Cl nuclei. To understand the DNP mechanism of the PTM radicals we have explored the 95 GHz EPR characteristics of these radicals that are relevant to their performance as DNP polarizers. The EPR spectra of the radicals revealed axially symmetric g-tensors. A comparison of the spectra with the (13)C DNP frequency sweeps showed that although the solid effect mechanism is operational the DNP frequency sweeps reveal some extra width suggesting that contributions from EPR forbidden transitions involving (35,37)Cl nuclear flips are likely. This was substantiated experimentally by ELDOR (electron-electron double resonance) detected NMR measurements, which map the EPR forbidden transitions, and ELDOR experiments that follow the depolarization of the electron spin upon irradiation of the forbidden EPR transitions. DFT (density functional theory) calculations helped to assign the observed transitions and provided the relevant spin Hamiltonian parameters. These results show that the (35,37)Cl hyperfine and nuclear quadrupolar interactions cause a considerable nuclear state mixing at 95 GHz thus facilitating the polarization of the Cl nuclei upon microwave irradiation. Overlap of Cl nuclear frequencies and the (13)C Larmor frequency further facilitates the polarization of the (13)C nuclei by spin diffusion. Calculation of the (13)C DNP frequency sweep based on the Cl nuclear polarization showed that it does lead to an increase in the width of the spectra, improving the agreement with the experimental sweeps, thus supporting the existence of a new heteronuclear assisted DNP mechanism. 相似文献