Metal ions paramagnetic species

The addition of paramagnetic species, such as the metal ions Cu ", Mn, or CF", can have dramatic effects on both the observed spectmm and the relaxation behavior of a molecule. The added ion reduces nuclear relaxation times, and permitting more rapid data collection. In addition, faster relaxation rates minimize NOE effects in the spectra, which can be useful in obtaining quantitative intensity data. The most widely used reagent for this purpose is chromium acetylacetonate [13681 -82-8] known as Cr(acac)2. Practically speaking, the use of such reagents requires care, because at... 

However, while transition-metal ions often contain unpaired electrons, they exhibit none of the reactivity that is commonly associated with such radicals outside the d block. There is no behaviour comparable to that of the highly reactive and short lived radicals such as CH3. Also associated with the presence of unpaired electrons in these species is the phenomenon of paramagnetism. The long-term stability of many compounds with unpaired electrons is a characteristic of the transition-metal series. 

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... 

The chapter Electron Spin Resonance in Catalysis by Lunsford was prompted by the extensive activity in this field since the publication of an article on a similar subject in Volume 12 of this serial publication. This chapter is limited to paramagnetic species that are reasonably well defined by means of their spectra. It contains applications of ESR technique to the study of adsorbed atoms and molecules, and also to the evaluation of surface effects. The application of ESR to the determination of the state of transition metal ions in catalytic reactions is also discussed. 

Most stable ground-state molecules contain closed-shell electron configurations with a completely filled valence shell in which all molecular orbitals are doubly occupied or empty. Radicals, on the other hand, have an odd number of electrons and are therefore paramagnetic species. Electron paramagnetic resonance (EPR), sometimes called electron spin resonance (ESR), is a spectroscopic technique used to study species with one or more unpaired electrons, such as those found in free radicals, triplets (in the solid phase) and some inorganic complexes of transition-metal ions. 

Fig. 15 Magnetization curves for paramagnetic species with various J = 5-values. These data represent a landmark in that the magnetization of a hydrocarbon molecule [15 m = 4] surpassed for the first time those of transition-metal and lanthanide ions, represented here by NH4Fe(S04)2-12Hj0 and Gd2(S04)3-8H20, respectively.

The aims of this study were to observe isotropically shifted signals for metal ions like copper(II), which usually give signals broadened beyond detection, and to relate the observed shifts and relaxation times to those of the uncoupled ions in order to understand the phenomena in theoretical terms. This approach allows the power of the NMR technique to fully exploit paramagnetic species and obtain information on spin delocalization, chemical bonding and so on. It is likely that the theory also applies to coupled metal ion-H adical systems like those proposed for derivatives of peroxidases (compound I), which contain iron(IV) and a heme radical (44). 

There are a few examples of spin equilibria with other metal ions which have not been mentioned above. In cobalt(III) chemistry there exist some paramagnetic planar complexes in equilibrium with the usual diamagnetic octahedral species (22). The equilibria are the converse of the diamagnetic-planar to paramagnetic-octahedral equilibria which occur with nickel(II). Their interconversions are also presumably adiabatic. Preliminary observations indicate relaxation times of tens of microseconds, consistent with slower ligand substitution on a metal ion in the higher (III) oxidation state (120). 

Inorganic radicals and transition metal ions typically exhibit broad lines, and hence diminished sensitivity in the EPR method. Consequently, when dealing with small quantities of paramagnetic material, it is often more difficult to detect inorganic species. Several important studies have been reported, however. Kastening s study [64] of the reduction of S02 in dimethylformamide showed that an equilibrium was established between the SO radical ion and the dimer S20, ... 

Paramagnetic species formed by reactions of the radiation-produced transient species in ice and frozen aqueous systems have been studied by ESR technique. The radiation-produced electrons have been found to react e.g. with acidic solutes to form H-atoms and with group 11(b) metal ions to give the corresponding univalent radical ions, while the holes can react with anions such as S04 2 and H2P04 giving the radical ions S04 and HP04. Evidence that the electron and hole are coupled to each other, and may in fact exist in irradiated pure ice primarily in an (exciton-like) bound state has been discussed. The present work provides evidence for the reactions of the radiation-produced positive holes apart from the reactions of the electrons. 

With such short xs values, in 5 or 6 coordinated cobalt(II) proteins, the metal ion provides negligible paramagnetic effects with respect to the diamagnetic contribution. Therefore, NMRD measurements are uninformative. When the protein contains tetracoordinated cobalt(II), then NMRD becomes again relevant. Water H R measurements of the high pH species of cobalt(II)-substituted carbonic an-hydrase (MW 30,000), which is tetrahedral with three coordinated histidines,... 

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