Unpaired electron spins spectroscopy

The real power of ESR spectroscopy for identification of radical structure is based on the interaction of the unpaired electron spin with nuclear spins. This interaction splits the energy levels and often allows determination of the atomic or molecular structure of species containing unpaired electrons. The more complete Hamiltonian is given in Equation (6) for a species containing one unpaired electron, where the summations are over all the nuclei, n, interacting with the electron spin. 

ESR spectroscopy can be transformed into an imaging method, ESRI, for samples containing unpaired electron spins, if the spectra are measured in the presence of magnetic field gradients. In an ESRI experiment the microwave power is absorbed by the unpaired electrons located at point x when the resonance condition, Equation (10), is fulfilled. 

In contrast to ESR spectroscopy, which can only be used to study species with unpaired electrons, NMR spectroscopy is applicable to the investigation of all polymer samples. Nuclei with non-zero total nuclear spin (e.g., 1H, l3C, 19F, 14N) will have a magnetic moment which will interact with an external magnetic field resulting in quantized energy levels. Transitions between these energy levels form the basis of NMR spectroscopy. 1H and 13C... 

As will be explained in Chapter 7, spectroscopic methods are a powerful way to probe the active sites of the hydrogenases. Often spectroscopic methods are greatly enhanced by judicious enrichment of the active sites with a stable isotope. For example, Mossbauer spectroscopy detects only the isotope Fe, which is present at only 2.2 per cent abundance in natural iron. Hydrogen atoms, which cannot be seen by X-ray diffraction for example, can be studied by EPR and ENDOR spectroscopy, which exploit the hyperfine interactions between the unpaired electron spin and nuclear spins. More detailed information has been derived from hyperfine interactions with nuclei such as Ni and Se, in the active sites. In FTTR spec-... 

We shall herein be dealing with magnetic resonance spectroscopy. When unpaired-electron spins and nuclear spins both are present, one cannot always distinguish between EPR and NMR for example, see atomic hydrogen at sufficiently low applied magnetic fields.15... 

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. 

The two possible orientations of an electron s spin and its associated spin magnetic moment with respect to an axis produce two energy levels in an externally applied magnetic field. In electron-spin-resonance (ESR) spectroscopy, one observes transitions between these two levels. ESR spectroscopy is applicable to species such as free radicals and transition-metal ions that have one or more unpaired electron spins and hence have a nonzero total electron spin and spin magnetic moment. 

Duxbury (1985) has recently published an extensive review of laser Stark spectroscopy. Table 1 lists most of the molecules that have been studied in this way. The majority of these have closed-shell singlet ground states, and their Stark splittings follow eqns 2-5, at least for weak fields. However, one of the more reactive species, HCO, has a doublet ground state. The presence of the unpaired electron spin complicates the energy level pattern, but the same principles hold as for a singlet state. 

Unpaired electron spin densities on ligand atoms as measured by NMR spectroscopy, discussed in Section 12.5.1. 

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