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  • 1
    Electronic Resource
    Electronic Resource
    Orientation-selective 14N electron spin echo envelope modulation (ESEEM): The determination of 14N quadrupole coupling tensor principal axis orientations in orientationally disordered solids (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 2162-2168 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present an orientation-selective ESEEM (electron spin echo envelope modulation) technique. From a measurement of the variation of the amplitudes of modulation components at 14N pure quadrupole frequencies as a function of irradiation position within an EPR (electron paramagnetic resonance) powder pattern, we determine the orientation of the principal axes of the quadrupole coupling tensor relative to the principal axes of the tensor which governs the dispersion of the EPR spectrum in an orientationally disordered sample. The pure quadrupole frequencies appear when the EPR frequency is selected such that the nuclear Zeeman and the hyperfine interaction are approximately equal in magnitude. We have applied the method to a mercaptoethanol complex of myoglobin in which the pure quadrupole frequencies originate from 14N in the proximal imidazole ring. Our results enable us to validate the assignment of the quadrupole modulations to the metal-coordinated nitrogen of the imidazole ring, to correlate the principal axes with the principal values of the quadrupole coupling tensor, and to determine the orientation of the imidazole ring relative to the principal axes of the (low-spin) Fe(III) g matrix. These findings are discussed and compared with results of previous studies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454049
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  • 2
    Electronic Resource
    Electronic Resource
    Multifrequency electron spin echo envelope modulation in S=1/2, I=1/2 systems: Analysis of the spectral amplitudes, line shapes, and linewidths (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 7161-7166 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The nuclear modulation effect in S=1/2, I=1/2 systems is analyzed with particular emphasis on the impact of variation of the external field strength. We show that the modulation depth parameter k reaches its maximum possible value when the Zeeman and hyperfine interactions are made equal in magnitude, or "matched,'' and that this matching plays the dominant role in determining the ESEEM (electron spin echo envelope modulation) line shapes; the distinctive ESEEM spectral features are readily understood on this basis. Through multifrequency ESEEM, it is possible to interpret the features and to utilize them to measure hyperfine interactions. We discuss several experimental strategies, involving the field dependences of ESEEM amplitudes, line shapes, and linewidths, for determining hyperfine coupling constants.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455293
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  • 3
    Electronic Resource
    Electronic Resource
    Electron spin echo envelope modulation amplitudes: A perturbation treatment of I=1 nuclei in extreme quadrupole coupling limits (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 2585-2586 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: ESEEM (electron spin echo envelope modulation) amplitudes for an S=1/2, I=1 system with isotropic hyperfine interaction in an orientationally disordered solid are derived by perturbation treatment in the limits of extreme quadrupole coupling. The derived expressions are shown to account for peak absences and other features noted in simulated spectra [H. L. Flanagan and D. J. Singel, J. Chem. Phys. 87, 5606 (1987)].
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455054
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  • 4
    Electronic Resource
    Electronic Resource
    Analysis of 14N ESEEM patterns of randomly oriented solids (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 5606-5616 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Simulations of 14N ESEEM patterns for an S=1/2, I=1 spin system with an isotropic hyperfine coupling, in a nonordered solid are reported. The dependence of the simulated patterns on the quadrupole and hyperfine coupling constants, and on the external field strength is examined and discussed. Three distinct types of powder patterns are identified, all of which exhibit discrete peaks. The dependence of the frequency, width, and amplitude of these peaks on the coupling constants and field strength is elucidated in simple terms. The results provide a basis for the interpretation of ESEEM patterns and for the development of experimental tactics: the variation of field strength (electron spin-excitation frequency) can be used to manipulate modulation depths, select specific nuclei, and ascertain accurate quadrupole coupling constants. The anisotropy of the amplitude of modulation components, its effect on ESEEM powder patterns, and its implications as regards orientation selection experiments are also considered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453532
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    Multifrequency electron spin-echo envelope modulation: The determination of nitro group 14N hyperfine and quadrupole interactions of DPPH in frozen solution (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 20-24 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Two- and three-pulse electron spin-echo modulations from 14N in the nitro groups of DPPH in toluene at 77 K have been measured at electron spin excitation frequencies of 4.45, 4.94, 5.47, 6.68, and 9.10 GHz. Analysis of the frequencies, amplitudes, and linewidths of the modulation components, together with their variation with excitation frequency, establishes that the 14N hyperfine interactions are approximately isotropic and enables the evaluation of the isotropic coupling. Distinctive spectral trends apparent in multifrequency ESEEM are employed to identify the cancellation of hyperfine and Zeeman interactions; modulation components accordingly assigned as pure quadrupole frequencies afford accurate determination of quadrupole coupling constants. Multifrequency ESEEM is shown to be a vital tool of general applicability in 14N ESEEM spectroscopy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454638
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  • 6
    Electronic Resource
    Electronic Resource
    Isotope effects in molecular mechanics (MM2). Calculations deuterium compounds (1983)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 4 (1983), S. 399-403 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Parameters have been derived so as to enable the inclusion of deuterium in the MM2 molecular mechanics force field. Several compounds were studied and the results are compared with experiment. The results are never qualitatively wrong, but the accuracy ranges from excellent to only fair. They are quite good for hydrocarbons, but less so for ketones.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540040315
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    Paper (German National Licenses)
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