Lanthanide series electron system

There is a remarkable discontinuity in magnitude of the crystal field parameters, especially the sixth-rank parameters (k = 6), between the first and second half of the lanthanide series in systems like LaFs and LaCb (Camall et al. 1989). According to Judd (1979) the drop in the k = 6 parameters in going Ifom LaCl3 Eu to LaCbiTb " is an indication for the need to include two-electron operators in the crystal-field Hamiltonian (which is done in correlation crystal-field theory, see sect. 4.5). 

The actinide element series, like the lanthanide series, is characterized by the filling of an f-electron shell. The chemical and physical properties, however, are quite different between these two series of f-electron elements, especially in the first half of the series. The differences are mainly due to the different radial extension of the 4f- and 5f-electron wavefunctions. For the rare-earth ions, even in metallic systems, the 4f electrons are spatially well localized near the ion sites. Photoemission spectra of the f electrons in lanthanide elements and compounds always show "final state multiplet" structure (3), spectra that result from partially filled shells of localized electrons. In contrast, the 5f electrons are not so well localized. They experience a smaller coulomb correlation interaction than the 4f electrons in the rare earths and stronger hybridization with the 6d- and 7s-derived conduction bands. The 5f s thus... 

Both through-bond and pseudocontact contributions can be easily factorized into a series of products of two terms, each term depending either on the nucleus i (topologic and geometric location) or from the lanthanide j (electronic structure and crystal-field effects). For axial complexes, that is, possessing at least a three-fold axis as found in triple-stranded helicates, the molecular magnetic susceptibility tensor written in the principal magnetic axes system is symmetrical xx = mag-... 

The oxidation states of the lanthanides in oxides The combination of the lanthanide elements with oxygen results in a formal oxidation state of the metal atom that depends upon the chemical potential of oxygen and the different electronic conditions of the atoms within the compound. The oxidation state of the lanthanide atoms varies between (II), as in EuO, and (IV), as in CeOj. All the lanthanides exhibit the (III) oxidation state in their oxide systems, as in La203. Furthermore, mixed-valence states exist in the series between these extremes both as stoichiometric and as nonstoichiometric compounds. Hund s first rule is followed in the lanthanide series which explains the stability of oxidation states other than three. 

The role of the crystal-field interaction remain obscure in all these systems, both for actinides and cerium. Whereas in the other lanthanide NaCl-type compounds, LnX and LnZ, the easy directions are given by straightforward crystal-field considerations and the value of the crystal-field potential varies in a systematic way across the lanthanide series, this is not the case in the materials discussed here. The crystal-field energy levels can be measured in the cerium compounds with neutron inelastic scattering, but have not been observed in the uranium (or higher) actinides. It is assumed that this inability to observe directly the crystal-field levels is because they are strongly broadened by the interaction between the 5f and conduction electrons. This has been the subject of much work by Cooper and his collaborators. 

In order to characterize the active site structure of Ca ATPase from sarcoplasmic reticulum, we have employed Gd + as a paramagnetic probe of this system in a series of NMR and EPR investigations. Gadolinium and several other lanthanide ions have been used in recent years to characterize Ca + (and in some cases Mg2+) binding sites on proteins and enzymes using a variety of techniques, including water proton nuclear relaxation rate measurements (35,36,37), fluorescence (38) and electron spin resonance (39). In particular Dwek and Richards (35) as well as Cottam and his coworkers (36,37) have employed a series of nuclear relaxation measurements of both metal-bound water protons and substrate nuclei to characterize the interaction of Gd + with several enzyme systems. 

The lanthanides form a series of ions of closely related size and bonding characteristics and in many respects resemble Ca +, for which they often substitute isomorphously in biological systems. Since different Ln ions can be probed with particular spectroscopic techniques (e.g. Eu + and Tb +, fluorescence Gd +, ESR Nd +, electronic spectra), in favourable circumstances it should be possible to obtain information about the binding site of spectroscopically inactive Ca + in several ways. Systems smdied include the calciumbinding sites in calmodulin, trypsin, parvalbumin, and the Satellite tobacco necrosis virus. 

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