ENDOR electron-nucleus double

ESR and ENDOR (electron-nucleus double resonance) spectroscopies110. 

ENDOR = electron nuclear double resonance EPR = electron-paramagnetic resonance ESR = electron-spin resonance NMR = nuclear magnetic resonance MA = modulation amplitude SOFT = second-order perturbation theory s-o = spin-orbit zfs = zero-field splitting (for S > 1 /2) D = uniaxial zfs E = rhombic zfs g = g-factor with principal components gy, and g ge = free electron g-factor a = hyperfrne splitting constant A = hyperftne coupling constant for a given nucleus N (nuclear spin 7>0). 

ENDOR, Electron Nuclear Double Resonance, is a combination of electron- and nuclear magnetic resonance developed in 1956. The method was apparently originally intended for applications in physics to achieve nuclear polarisation [6], e.g. to obtain more nuclei with wj/ = -Vi than with nij = +Vi. But its main application became to resolve hyperfine structure that is unresolved in regular ESR in liquid and solid samples. Two lines appear in the ENDOR spectmm of a radical (5 = Vi), containing a single nucleus, one corresponding to the ms = +Vi the other to the ms = -Vi electronic level. 

Deuterium quadrupole coupling constants can also be obtained from electron nuclear double resonance (ENDOR).19 30 An observation of the hyperfine structure caused by quadrupole coupling in the electron paramagnetic resonance (EPR) spectrum, as for many lanthanide complexes, has not been reported for deuterium. The determination of nuclear quadrupole coupling constants from Mossbauer spectroscopy is not applicable to the deuterium nucleus. 

There are also pulse EPR methods that probe the chemical or rather magnetic environment. These are pulse electron nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopy, which allow measuring hyperfine couplings from the unpaired electron spin to surrounding magnetically active nuclei ([20] in Fig. 3 this is a P nucleus). As these experiments are performed in frozen solution (e.g., in all examples of this chapter) or in solids, from the anisotropy and orientation dependence of the hyperfine coupling one can obtain valuable information on the structure up to 1 nm. 

Later [38, 39], oxygen vacancies (Fig. 2.2) and E point defects present in glassy Si02 could be studied in great detail, including also full ab-initio calculations of the hyperfine parameters experimentally detected by electron-nuclear double resonance (ENDOR) experiments. Indeed, these types of measurements are nowadays routinely done to identify this class of paramagnetic defects. In the ENDOR technique, some Si atoms are substituted with their isotopes Si. This confers anon-zero nuclear spin I to the atomic nucleus that couples to the electron spin S via a tensor A. On the theoretical front, the calculation from first principles DFT approaches does not pose particular problems since the hyperfine interaction is still a ground state property which can be expressed in terms of the electronic density p x). The interaction between an electron spin (S) and a nuclear (I) spin is in fact described by the Hamiltonian... 

Nucleus Double Resonance (ENDOR), NMR study previously reported for the radical anions of 7r-extended TCNQs [27], the added electrons are accommodated in the dicyanomethylene units, the extra negative charges taken by each C(CN>2 unit being 0.38 e for the radical anion and 0.83 e for the dianion. The stability of the radical anion of the TCNQ justifies the two one-electron reduction waves observed for the parent TCNQ in the cyclic voltammetric studies. 

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