Electron relaxation mechanisms

I. From the NMRD profile to the electron relaxation mechanism 105... 

I. From the NMRD Profile to the Electron Relaxation Mechanism... 

As stated in Section II.B of Chapter 2, the actual correlation time for electron-nuclear dipole-dipole relaxation, is dominated by the fastest process among proton exchange, rotation, and electron spin relaxation. It follows that if electron relaxation is the fastest process, the proton correlation time Xc is given by electron-spin relaxation times Tie, and the field dependence of proton relaxation rates allows us to obtain the electron relaxation times and their field dependence, thus providing information on electron relaxation mechanisms. If motions faster than electron relaxation dominate Xc, it is only possible to set lower limits for the electron relaxation time, but we learn about some aspects on the dynamics of the system. In the remainder of this section we will deal with systems where electron relaxation determines the correlation time. 

Tie are also expected to be field-dependent. Their field dependence can be described by two parameters the electron relaxation time at low fields Tso, and the correlation time for the electron relaxation mechanism Ty (see Eq. (14) of Chapter 2) (5). However, Tso usually depends on (see Eq. (52) of Chapter 2). Therefore, it is preferable to select two different parameters for describing the field dependence of electron relaxation. For S > 1/2 systems, in case the electron relaxation is due to modulation of a time dependent transient zero-field splitting, A, (pseudorotational model), the Bloembergen-Morgan equations are obtained 5,6) ... 

There are complexes which display a field dependence of the electron relaxation rate, in others the field dependence is not evident. The availability of this information, which comes from NMRD experiments, may thus permit to obtain indications on the most efficient electron relaxation mechanisms in the different systems. 

In symmetric complexes where the excited electronic levels are high in energy, for S > 1/2, the most efficient electron relaxation mechanism seems to be due to the modulation of transient ZFS with a correlation time independent of xr. As already seen, this time is ascribed to the correlation time for the collisions of the solvent molecules, responsible for the deformation of the coordination polyhedron causing transient ZFS. In complexes where a static ZFS is also present, modulation of this ZFS with a correlation time related to xr is another possible electron relaxation mechanism. 

Chromium(III) has a ground state in pseudo-octahedral symmetry. The absence of low-lying excited states excludes fast electron relaxation, which is in fact of the order of 10 -10 ° s. The main electron relaxation mechanism is ascribed to the modulation of transient ZFS. Figure 18 shows the NMRD profiles of hexaaqua chromium(III) at different temperatures (62). The position of the first dispersion, in the 333 K profile, indicates a correlation time of 5 X 10 ° s. Since it is too long to be the reorientational time and too fast to be the water proton lifetime, it must correspond to the electron relaxation time, and such a dispersion must be due to contact relaxation. The high field dispersion is the oos dispersion due to dipolar relaxation, modulated by the reorientational correlation time = 3 x 10 s. According to the Stokes-Einstein law, increases with decreasing temperature, and... 

A phenomenon closely related to electronic relaxation is the existence of diffuseness in the absorption spectra of the higher excited electronic states of molecules. It has been known for some time that very fast electronic relaxation processes occur when the higher excited states of molecules are caused to interact with radiation. It is remarkable then that only in relatively recent work has the association between these fast processes and spectral diffuseness been clearly focused upon. These spectral results provide some of the most definitive features that may be associated with the electronic relaxation mechanisms. First, the results from solid-state spectra 62 ... 

The sole presence of an electron spin causes nuclear relaxation. The correlation time for the electron nucleus interaction is presented as well as equations valid for dipolar and contact interaction. To do so, electron relaxation mechanisms need to be quickly reviewed. All the subtleties of nuclear relaxation enhancements are presented pictorially and quantitatively. 

Copper(II) has a 3d9 electronic configuration. In principle, pure octahedral and tetrahedral symmetries can never be observed because Jahn-Teller distortions (see Section 3.3.1) remove the orbital degeneracy of the ground state. The separation of the electronic energy levels depends on the coordination number and stereochemistry, as well as on the nature of the ligands. However, the ground state orbital is always well isolated from the excited states, and therefore the electronic relaxation mechanisms are relatively inefficient. Copperfll) complexes have thus relatively sharp EPR signals, and it is often possible to record these spectra at room temperature. 

In this case t, = generally, however, in this article we refer to r, = T,. It is possible, however, that T, = for slowly relaxing systems as a result of rotation independent electronic relaxation mechanisms. 

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