Metal ions electron relaxation

The Cua site, common in biology (inset in Fig. 5.42), is dinuclear with two copper atoms bridged by the thiolate sulfurs of two cysteine ligands. One unpaired electron is delocalized over two metals, which are thus Cul 5+. The NMR spectra show narrow lines from the copper ligands (Fig. 5.42) [120,121], corresponding to an electron relaxation time of 10 11 s, as in Cu2+-Cu2+ dimers (see Section 6.3.2). However, in Cua there is no magnetic coupling between the two centers, as they contain only one unpaired electron just as an isolated Cu2+ ion. Electron relaxation of Cua may be fast because the orbital overlap between the two copper centers provides new relaxation mechanisms not available to a monomer (as Orbach or Raman relaxation). 

The electron spin(s) of a given metal ion may relax faster if coupled to another metal ion experiencing more efficient relaxation mechanisms. This electron relaxation enhancement depends on the electron relaxation time of the more rapidly relaxing metal ion and on the magnitude of the coupling constant. In the case of isotropic coupling between two metal ions, Eq. (11) has been proposed, which describes the increase in the electron relaxation rates of the more slowly relaxing metal ion (Banci et al., 1991),... 

Electronic absorption spectra are produced when electromagnetic radiation promotes the ions from their ground state to excited states. For the lanthanides the most common of such transitions involve excited states which are either components of the ground term or else belong to excited terms which arise from the same 4f" configuration as the ground term. In either case the transitions therefore involve only a redistribution of electrons within the 4f orbitals (i.e. f—>f transitions) and so are orbitally forbidden just like d—>d transitions. In the case of the latter the rule is partially relaxed by a mechanism which depends on the effect of the crystal field in distorting the symmetry of the metal ion. However, it has already been pointed out that crystal field effects are very much smaller in the case of ions and they... 

The principle of ICP-AES is that atoms (or sometimes ions) are thermally excited, in a plasma torch, to higher energy levels, these atoms or ions then relax back to lower electronic energy levels by emitting radiation in the UV-visible region. The emitted radiation is detected and used to determine which elements are present, and their concentration. Analysis of organometallic and inorganic additives, based on the ICP-AES determination of specific metal ions, is routinely undertaken. 

The electron spin resonance (ESR) technique has been extensively used to study paramagnetic species that exist on various solid surfaces. These species may be supported metal ions, surface defects, or adsorbed molecules, ions, etc. Of course, each surface entity must have one or more unpaired electrons. In addition, other factors such as spin-spin interactions, the crystal field interaction, and the relaxation time will have a significant effect upon the spectrum. The extent of information obtainable from ESR data varies from a simple confirmation that an unknown paramagnetic species is present to a detailed description of the bonding and orientation of the surface complex. Of particular importance to the catalytic chemist... 

In this section, we discuss the work inquiring into the meaning of r/s, the distance between the two dipoles in Eqs. (12) and (13). The simplest approximation is to assume that r/s is equal to the internuclear distance between the nucleus in the ligand, the relaxation of which is being studied, and the metal ion. This amounts to the point-dipole approximation for both the nuclear and the electron spins. While such an approximation is perfectly... 

I J XgJ, Xg2- In this case, the two metal ions can be considered to have a single set of electron spin relaxation rates. If no additional relaxation mechanisms are established, such common relaxation rates are about equal to the fastest relaxation rates of the uncoupled spins. Actually, calculations indicate the presence of different electron relaxation rates for each level and for each transition. The electron relaxation rates for the pair are the sum of the rates of the two spins, weighted by coefficients depending on the transition 108). 

The electron relaxes through modulation of the A and g anisotropy. Typical examples are copper(II), oxovanadium(IV) and silver(II) aqua ions. The electronic relaxation times are relatively long (10 -10 ° s at room temperature) and the hyperfine coupling with the metal nuclear spin is usually present. No field dependence of the electron relaxation time is usually evident up to 100 MHz. 

As an example on the relationship between proton relaxivity, electron relaxation and coordination environment, we report the case of azurin and its mutants. The relaxivity of wild type azurin is very low (Fig. 6) due to a solvent-protected copper site, the closest water being found at a distance of more than 5 A from the copper ion. The fit, performed with the Florence NMRD program, able to take into account the presence of hyperfine coupling with the metal nucleus (Ay = 62 x 0 cm , see Section II.B) indicates Tie values of 8 X 10 s. Although the metal site in azurin is relatively inaccessible, several mutations of the copper ligands open it up to the solvent. The H NMRD profiles indicate the presence of water coordination for the... 

As an example of tetra-coordinate cobalt(II) systems, the NMRD profile of cobalt(II)-substituted carbonic anhydrase (MW 30,000) at high pH is reported (Fig. 14). The metal ion is coordinated to three histidines and to a hydroxide ion (48). The NMRD profile shows a cos Cg dispersion centered around 10 MHz, which qualitatively sets the correlation time around 10 s. As the reorientational correlation time of the molecule is much longer, this value is a measure of the effective electronic relaxation time. A quantitative... 

In the previous discussion, the electron-nucleus spin system was assumed to be rigidly held within a molecule isotropically rotating in solution. If the molecule cannot be treated as a rigid sphere, its motion is in general anisotropic, and three or five different reorientational correlation times have to be considered 79). Furthermore, it was calculated that free rotation of water protons about the metal ion-oxygen bond decreases the proton relaxation time in aqua ions of about 20% 79). A general treatment for considering the presence of internal motions faster than the reorientational correlation time of the whole molecule is the Lipari Szabo model free treatment 80). Relaxation is calculated as the sum of two terms 8J), of the type... 

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