Electron spin-echo envelope modulation ESEEM spectra

The local structure of iron sites in Fe-mazzite and Fe-ZSM-5, in which iron was incorporated during zeolite synthesis, was studied by X- and Q-band ESR, electron spin echo detected ESR (ED-ESR), electron spin echo envelope modulation (ESEEM), and diffuse reflectance UV-vis [94G1]. The X-band ESR spectra of Fe-MAZ (100 Fe/(Fe + A1 + Si) = 1.20) render three signals at g = 4.3, g = 2.3, and g = 2.0 - Table 14a. The Q-band spectra testifies only the signal atg = 2.0. The linewidths of the g = 2.0 signals are smaller in the Q-band spectra - Table 14. This narrowing indicates that the linewidth is at least partially due to the second-order broadening of the -l/2> l/2> transition. The X-band spectrum of Fe-MAZ with 100 Fe/(Fe + A1 + Si) = 0.07 exhibits the... 

More advanced experiments, such as ENDOR, electron spin echo envelope modulation (ESEEM), or relaxation measurements by pulsed ESR rely on a selective excitation of spins close to the resonance field. Usually, the powder ESR spectrum is much broader than the excitation bandwidth of the pulses, which is in the range between 2 and 10 G. In cases where one anisotropic interaction dominates the spectrum, the experiments thus select contributions only from certain orientations of the molecule with respect to the external magnetic field. Such orientation selection is more efficient and easier to interpret at a field that is high enough for the g anisotropy to dominate. Finally, the size of mw resonators scales with wavelength and thus scales inversely with frequency. At higher frequency, spectra can thus be measured with much smaller sample volumes, yet the concentration does not need to be significantly increased. 

Double-resonance spectroscopy involves the use of two different sources of radiation. In the context of EPR, these usually are a microwave and a radiowave or (less common) a microwave and another microwave. The two combinations were originally called ENDOR (electron nuclear double resonance) and ELDOR (electron electron double resonance), but the development of many variations on this theme has led to a wide spectrum of derived techniques and associated acronyms, such as ESEEM (electron spin echo envelope modulation), which is a pulsed variant of ENDOR, or DEER (double electron electron spin resonance), which is a pulsed variant of ELDOR. The basic principle involves the saturation (partially or wholly) of an EPR absorption and the subsequent transfer of spin energy to a different absorption by means of the second radiation, leading to the detection of the difference signal. The requirement of saturability implies operation at close to liquid helium, or even lower, temperatures, which, combined with long experimentation times, produces a... 

To determine whether PLP was actually associated with the lysine radical, [4 - H]PLP was synthesized and exchanged into the enzyme, and the [4 - H]PLP-enzyme was used to prepare a sample of the putative product radical 3. The EPR spectrum of the sample containing [4 - H]PLP proved to be identical with that of a matched sample containing PLP. The two samples were submitted to electron spin echo envelope modulation spectroscopy (ESEEM). The ESEEM spectra revealed a signal corresponding to the Larmor frequency for deuterium in the sample containing [4 - H]PLP (Fig. 5) and no signal in the PLP sample. This meant that the deuterium in [4 - H]PLP must be... 

Electron spin echo spectroscopy (ESE) monitors the spontaneous generation of microwave energy as a function of the timing of a specific excitation scheme, i.e. two or more short resonant microwave pulses. This is illustrated in Fig. 7. In a typical two-pulse excitation, the initial n/2 pulse places the spin system in a coherent state. Subsequently, the spin packets, each characterized by their own Larmor precession frequency m, start to dephase. A second rx-pulse at time r effectively reverses the time evolution of the spin packet magnetizations, i.e. the spin packets start to rephase, and an emission of microwave energy (the primary echo) occurs at time 2r. The echo ampHtude, as a fvmction of r, constitutes the ESE spectrum and relaxation processes lead to an irreversible loss of phase correlation. The characteristic time for the ampHtude decay is called the phase memory time T. This decay is often accompanied by a modulation of the echo amplitude, which is due to weak electron-nuclear hyperfine interactions. The analysis of the modulation frequencies and ampHtudes forms the basis of the electron spin echo envelope modulation spectroscopy (ESEEM). 

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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