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  • 1985-1989  (2)
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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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    Paper (German National Licenses)
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  • 2
    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
    Location Call Number Limitation Availability
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    Paper (German National Licenses)
    Fulltext
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