Electronic dipolar field

Restricting ourselves to metal-centered nuclear relaxation, the fluctuation of the electron dipolar field at the nucleus due to electron relaxation can be easily visualized as depicted in Fig. 3.1 A. We now want to mention other mechanisms, besides electron relaxation, which occur in solution and through which unpaired electrons cause nuclear relaxation. All of these mechanisms will be discussed in detail in this chapter. 

Fig. 3.1. Pictorial representation of the motions causing nuclear relaxation electron spin relaxation (A), molecular rotation (B) and chemical exchange (C). It can be seen that the electron dipolar field at the nucleus fluctuates with time in direction (A), intensity (C) or both (B).

Adsorption of a neutral (n ) onto a metal surface leads to a heat of adsorption of Q, as the electrons and nuclei of the neutral and metal attract or repel each other. Partial positive and negative charges are induced on each with the formation of a dipolar field (Figure 7.4). 

Schematic diagram showing the development of a dipolar field and ionization on the surface of a metal filament, (a) As a neutral atom or molecule approaches the surface of the metal, the negative electrons and positive nuclei of the neutral and metal attract each other, causing dipoles to be set up in each, (b) When the neutral particle reaches the surface, it is attracted there by the dipolar field with an energy Q,. (c) If the values of 1 and <() are opposite, an electron can leave the neutral completely and produce an ion on the surface, and the heat of adsorption becomes Q,. Similarly, an ion alighting on the surface can produce a neutral, depending on the values of I and <(), On a hot filament the relative numbers of ions and neutrals that desorb are given by Equation 7.1,which includes the difference, I - <(), and the temperature, T,...

Schematic diagram showing how placing a thin layer of highly dispersed carbon onto the surface of a metal filament leads to an induced dipolar field having positive and negative image charges. The positive side is always on the metal, which is much less electronegative than carbon. This positive charge makes it much more difficult to remove electrons from the metal surface. The higher the value of a work function, the more difficult it is to remove an electron. Effectively, the layer of carbon increases the work function of the filament metal. Very finely divided silicon dioxide can be used in place of carbon.

In the spin-correlated RP the two radicals interact via electron-electron dipolar and exchange interaction which leads to line splitting. The ET process creates the RP in a strongly spin-polarized state with a characteristic intensity pattern of the lines that occur either in enhanced absorption (A) or emission (E).144 145 The spectrum is therefore very intense and can directly be observed with cw EPR (transient EPR) or by pulse methods (field-swept ESE).14 To study the RPs high field EPR with its increased Zeeman resolution proved to be very useful the first experiment on an RP was performed by Prisner et al. in 1995146. From the analysis of the RP structure detailed information about the relative orientation of the two radicals can be extracted from the interaction parameters. In addition kinetic information about the formation and decay of the RP and the polarization are available (see references 145,147). 

As Figure 2 shows, the two models differ only by a vertical displacement, which is a measure of the difference in the mean square interparticle dipolar fields (second moments) operative in the two models. The values of these mean square local fields can be calculated easily from the minimum value of Ti or from the rigid lattice values of T2. Because of the 1000-fold ratio between electronic and nuclear magnetic moments, even... 

By measuring zero-field splitting (zfs) from electron-electron dipolar coupling in triplet radical pairs and proton hyperfine splitting (hfs), Segmuller developed a detailed picture of the motions of these radicals. The sequence of... 

Two different nuclei in TDAE-C60 have been investigated so far by NMR protons of methyl groups of the TDAE molecule and 13C nuclei of the Qq ion. The main difference between these two nuclei (in addition to their relative sensitivity) is that methyl protons experience mostly the dipolar fields of the Qq magnetic moments. On the other hand 13C nuclei on each Cgo ion will, in addition to dipolar fields, also feel the hyperfine contact field of the unpaired electron spin. Details of the 13C NMR results will be given in the next section. Here we... 

This unitary transformation redefines the variables of the field (ak, E, A, etc.) and displaces the momentum operator of each charge. Retaining only the electronic dipolar terms, the total hamiltonian becomes... 

The pseudocontact interaction (perhaps more appropriately called a dipolar interaction) arises from the magnetic dipolar fields experienced by a nucleus near a paramagnetic ion. The effect is entirely analogous to the magnetic anisotropy discussed in Section 4.5. It arises only when the g tensor of the electron is anisotropic that is, for an axially symmetric case, j> g . The g value for an electron is defined as... 

Two nitroxides, separated by the distance r, are coupled through space via electron-electron dipolar interactions arising from the unpaired electrons. Spin coupling induces line splitting dependent on their separation and their orientation with respect to the magnetic field according to... 

The orientation of electron spins in this manner influences the electron-electron dipolar interaction described above. For strong magnetic fields, the diffusive... 

A development in the theory of nuclear relaxation in macromolecules by paramagnetic ions has been suggested by Gueron. (675) In the case of heme proteins there is a net polarization of the iron electronic spin magnetic moment which is oriented along the direction of the magnetic field. Modulation of this dipolar field due to the spin polarization (Curie spin) by rotational diffusion introduces an additional term into the expression for transverse relaxation [equation (18)] giving ... 

Figure 2.7 Artistic view of electron-phonon scattering. Lattice motions involving the displacement of polar modes can scatter the electron inelastically. The polar fluctuations create dipolar fields that can modulate the electron distribution. The electron responds to these stochastic fluctuations in local fields with a change in its energy and effective momentum transfer to the lattice. This process is depicted by comparing (a) and (b) to visualise the motion of the lattice atoms, leading to a change in direction or momentum of the electron from its initial path shown in (a).

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