Nuclear from ENDOR spectrum

ENDOR spectroscopy offers enhanced resolution compared with conventional ESR for example, isotropic hyperfine interactions as small as 0.004 mT can be measured. This enhanced resolution is achieved partly because the technique lies between ESR and nuclear magnetic resonance (NMR) and also because redundant lines are eliminated from the spectrum essentially the ESR signal is monitored while sweeping through NMR frequencies. The simplification of the spectrum arises from the fact that each fl-value produces only two lines in the spectrum, irrespective of how many nuclei contribute to that hyperfine coupling constant. Other advantages of the method are ... 

In an ENDOR experiment, magnetization from electron spins has to be transferred to nuclear spin transitions. Resonant radio frequency (rf) irradiation can then change this magnetization and the change can be detected again on an electron spin transition. By plotting the ESR signal as a function of the irradiated rf, one obtains the spectrum of the nuclear spin transitions (ENDOR spectrum). 

The Gdaq O ENDOR spectrum is presented in Figure 13. Apart from the central broad feature that may be, at least in part, attributed to distant matrix oxygens, this spectrum can be described as consisting of two narrow peaks with a splitting of -1.33 MHz between them, and two sets of shoulders with splittings of about 3.3 and 5.1 MHz. In view of the preceding theoretical considerations, which have shown that -1/2 1/2 nuclear transitions are not affected by nqi (to first order),... 

If the hfs of the nucleus under consideration is not resolved in the EPR spectrum, all nuclear spin states are simultaneously saturated and a sign determination using ENDOR line intensities is not possible. In this case the relative signs may sometimes be determined from second order hf contributions. This method has been applied by DuVarney and Spaeth74) to determine the sign of the 41K electric quadrupole moment using F centres in KC1. 

Fig. 27a-c. Electron spin echo envelope modulation of Co(acacen), temperature 4K. a) Nuclear modulation pattern of Co(acacen) diluted into a Ni(acacen) 1/2 H20 single crystal. Crystal setting rotation axis I,

Fourier transform of the nuclear modulation pattern (From R. de Beer1 4)) c) Stick spectrum ENDOR frequencies (AmN = 1, 2) calculated from the hfs and quadruple tensors in Ref. 59 dashed lines ms = - 1/2, full lines ms = 1/2... 

Fe-ENDOR. 57Fe hf interactions in Mb and Hb have been reported by Scholes et al.24<5). The samples were enriched to about 90% in 57Fe which has a nuclear spin of I = 1/2. Since the tensor AFe has to be (nearly) axially symmetric and coaxial with g, single crystal-like ENDOR spectra may be recorded at any B0 field position in the EPR spectrum thus, the complete hfs tensor can be determined from a frozen solution sample. ENDOR frequencies with the (intrinsic) hf coupling parameters Afe = -27.77 MHz (-27.15 MHz) and A[e = -27.05 MHz (-26.70 MHz) have been found for Mb(Hb). 

ESR methods unambiguously establishes the presence of species bearing unpaired electrons (ion-radicals and radicals). The ESR spectrum quantitatively characterizes the distribution of electron density within the paramagnetic particle by a hyperfine structure of ESR spectra. This establishes the nature and electronic configuration of the particle. A review by Davies (2001) is highly recommended as a guide to current practice for ESR spectroscopic studies (this quotation is from the title of the review). The ESR method dominates in ion-radical studies. Its modern modifications, namely, ENDOR and electron-nuclear-nuclear triple resonance (TRIPLE) and special methods to observe ion-radicals by swiftness or stealth are described in special literatures (Moebius and Biehl 1979, Kurreck et al. 1988, Werst and Trifunac 1998). 

ESEEM is a pulsed EPR technique which is complementary to both conventional EPR and ENDOR spectroscopy(74.75). In the ESEEM experiment, one selects a field (effective g value) in the EPR spectrum and through a sequence of microwave pulses generates a spin echo whose intensity is monitored as a function of the delay time between the pulses. This resulting echo envelope decay pattern is amplitude modulated due to the magnetic interaction of nuclear spins that are coupled to the electron spin. Cosine Fourier transformation of this envelope yields an ENDOR-like spectrum from which nuclear hyperfine and quadrupole splittings can be determined. 

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