19 As the X- and Q-band spectra are collected in different spectrometers at different frequencies and in different detection modes – unsaturated absorption mode at X-band with derivative display through field modulation, rapid-passage dispersion mode with absorption-display at Q-band – the lineshapes and g-values determined by EPR simulations differ very slightly (third significant figure in g-values) for the two types of spectra. The 35 GHz CW EPR and ENDOR spectra were recorded on a modified Varian E-110 spectrometer equipped with a helium immersion dewar. 11, 12Īll CW (continuous wave) X-band EPR measurements were performed on a Bruker ESP-300 spectrometer with a liquid helium flow Oxford Instruments ESR-900 cryostat. 12 The presence of a covalent Fe-C bond in each was established by the observation of 13C and 1H hyperfine couplings to the alkyl that show isotropic components indicative of Fe-C covalency. The S = ½ spin of each was shown to reside on an 3+ cluster by its axial g-tensor with average g-value, g iso > 2, and Ω furthermore was shown to exhibit the large 57Fe hyperfine couplings associated with such a cluster. 15 Both intermediates were established by EPR and ENDOR spectroscopies to contain an 3+−alkyl cluster.
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