Electronic relaxation, rate

Electronic relaxation rates that are also dependent on field strength... 

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). 

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. 

The electron relaxation is usually field dependent and the main mechanism for electron relaxation is the modulation of transient ZFS caused by collisions with solvent molecules. Small static ZFS have been estimated for several manganese(II) and gadolinium(III) proteins, and somewhat larger ones for iron(III) compounds. In such low symmetry systems, it is reasonable to expect the magnitude of transient ZFS to be related to that of the static ZFS, as the former can be seen as a perturbation of the latter. As a consequence, systems with increasing static ZFS experience faster electron relaxation rates. Modulation of static ZFS by rotation could be an additional mechanism for relaxation, which may coexist with the collisional mechanism. 

An electron spin can relax by coupling with a neighboring electron spin. Therefore, when a paramagnetic metal ion interacts with a second paramagnetic metal ion, the electron relaxation rates of the two metal ions may be dramatically affected. If Si and S2 are the two spins coupled by a scalar interaction, new spin levels will be established due to the interaction, with total S varying in unitary steps from Si — S2I to Si + S2. The energies of these spin levels are given by )... 

The values of the electron relaxation rates of the coupled metal ion strongly depend both on the relative electron relaxation rates of the isolated ions and on the value of the magnetic coupling constant J. When the absolute value of J (expressed as J /K) is smaller than both electronic relaxation rates, no effect on the electronic relaxation of the pair is expected. When J /H > (electron relaxation rate of the first ion) but smaller than T[ 2) (electron relaxation rate of the second ion), from first order perturbation... 

We have seen that copper(II) is a slowly relaxing metal ion. Magnetic coupling of copper to a fast relaxing metal ion increases the electron relaxation rate of copper, as clearly shown by the NMRD profiles of tetragonal copper(II) complexes reacting with ferricyanide (105) (Fig. 38). The electron relaxation time, estimated from the relaxation rate of the water protons coordinated to the copper ion, is 3 x 10 ° s, a factor of 10 shorter than in the absence of ferricyanide. 

Oxidized Fe2S2 ferredoxins, containing two equivalent iron atoms, with J = 400 cm , show sharper NMR lines with respect to the monomeric iron model provided by oxidized rubredoxin (107-109), due to the decreased Boltzmann population of the paramagnetic excited states. For reduced ferredoxins (Si = 5/2, S2 = 2), with J = 200 cm , the ground state is paramagnetic (S = 1/2) (110). A smaller decrease in linewidth is expected. However, the fast electron relaxation rates of the iron(II) ion cause both ions to relax faster, and the linewidths in the dimer are sharp. 

A first example is represented by the Mn(III)/Mn(II) redox switch. The complexes of Mn(II) and Mn(III) with the water-soluble tetraphenylsulpho-nate porphyrin (TPPS, Chart 13) display significantly different ri values at low magnetic field strength (lower than 1 MHz), but very similar values at the fields currently used in the clinical practice (> 10 MHz) (141). However, the longer electronic relaxation rates of the Mn(II) complex makes its relaxivity dependent on the rotational mobility of the chelate. In fact, upon interacting with a poly-p-cyclodextrin, a 4-fold enhancement of the relaxivity of [Mn(H)-TPPS(H20)2] at 20 MHz has been detected, whereas little effect has been observed for the Mn(III)-complex. The ability of the Mn(II)/Mn(III)... 

Unlike the lanthanides, the actinides U, Np, Pu, and Am have a tendency to form linear actinyl dioxo cations with formula MeO and/or Me02. All these ions are paramagnetic except UO and they all have a non-spherical distribution of their unpaired electronic spins. Hence their electronic relaxation rates are expected to be very fast and their relaxivities, quite low. However, two ions, namely NpO and PuOl", stand out because of their unusual relaxation properties. This chapter will be essentially devoted to these ions that are both 5/. Some comments will be included later about UOi (5/°) and NpOi (5/ ). One should note here that there is some confusion in the literature about the nomenclature of the actinyl cations. The yl ending of plutonyl is often used indiscriminately for PuO and PuOl and the name neptunyl is applied to both NpO and NpOi. For instance, SciFinder Scholar" makes no difference between yl compounds in different oxidation states. Here, the names neptunyl and plutonyl designate two ions of the same 5f electronic structure but of different electric charge and... 

A major aspect of the problem is the relationship between J and the effect on the electronic relaxation rates of the slow—relaxing metal ion. More experimental data are probably necessary to make general statements. Finally, we have always assumed the zero field splitting of uncoupled ions to... 

These experimental studies in mixed crystals, crystals, and in low-pressure vapors provide compelling indications that electronic relaxation and spectral diffuseness are related phenomena, and the implications are that electronic relaxation rates are directly obtainable by application of the uncertainty principle. 

Electron relaxation times have been reported in the range 10-11 s to 10-9 s. Spin-orbit coupling with an excited quartet causes the splitting of the S = 5h state [1] (Fig. 5.2). The larger the splitting, the faster the electron relaxation rate and the... 

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