Lanthanide spectra systems

Excitation of the Eu3+ or Tb3+ ions has traditionally been indirect, by broad-band UV excitation of a conjugated organic ligand which is followed by intramolecular energy transfer to the lanthanide ion / system, followed in turn by /- / emission.614 However, more recently, following the advent of tunable dye lasers, direct excitation of an excited / level is in many cases preferable. By scanning this frequency, an excitation spectrum can be obtained whose energy values are independent of the resolution of a monochromator and not subject to spectral interferences. 

When 3 equiv of hexamethylphosphoric triamide (HMPA) ligand is added to the (BNA)2—O2—Sc system, the O2 /Sc complex becomes significantly stable and the ESR spectrum of the O2 -Sc (HMPA) complex can be detected even at 60°C under irradiation with light (78). The lanthanide complexes of radical anions of aromatic ketones are stabilized by the presence of HMPA ligand (79, 80). Oxygen enriched in can provide valuable information about the inequivalency... 

Organic-soluble lanthanide chelates have been used to probe lipophillic systems. The compound 4-(4-dipentylamino-( )- S-styryl)-l-(2,2,2-trifluoroethyl)pyridinium perchlorate (22) was employed as a probe in dimyristoylphosphatidylcholine vesicles. Probe molecules assembled in the inner and outer shells of the vesicle as evidenced by the presence of two signals in the NMR spectrum (376 MHz). Even though addition of Eu(fod)3 promoted vesicle fusion, only one of the signals shifted. The shifted signal was likely from the probe molecule on the outer shell, as the internal P signal of the phospholipid did not shift in the presence of Eu(fod)3 ". 

The enantiotopic protons of the prochiral methyl groups in the iminium salt 36 exhibited distinct resonances in the presence of Eu(hfc)3 . As already discussed for achiral lanthanide S-drketonates, the system likely forms an ion pair between the organic cation and the species [Ln( S-dik)3X]. The spectrum of racemic 37, which as its bromide salt has been studied as an ionic liquid, exhibits nonequivalence in the presence of Eu(tfc)3 and Eu(hfc)3. No splitting of the resonance occurs in the presence of Eu(fod)3. In addition to the likely ion-pairing interaction of 37 with [Ln(/ -dik)3X] , rather substantial shifts of some of the OCH2 protons implied that the ether oxygen atoms also likely coordinated with the europium ion. A similar ion-paired system explains the enantiomeric discrimination observed in the spectrum of the tris(phenanthroline) complexes of Ru(II) ([Ru(phen)3]Cl2) in the presence of Eu(tfc)3 . 

Achiral binuclear reagents have been added to mixtures of a chiral crown ether (74) and chloroform-soluble amino acid ester hydrochlorides to enhance the chiral discrimination in the NMR spectrum. The [Ln(fod)4] preferentially associated with the enantiomer in the bulk solution such that the enantiomer with lower association with 74 showed the larger lanthanide-induced shifts. The system also enhanced the chiral discrimination in acetonitrile-fifs, although [Pr(fod)4] was needed because it causes larger shifts than the... 

Figure 2c shows the near-infrared luminescence spectrum of [Gd(hfac)3NIT-BzImH] compared to its lowest-energy absorption band system. At 5 K, both spectra show well-resolved structure that is similar to the patterns observed for the uncoordinated radical, as summarized in Tables 1 and 2. The corresponding electronic transitions can be observed for many other complexes of lanthanide or d-block metal ions with radical ligands [24-27, 30]. In general, the spectra for lanthanide complexes are very similar to those of the uncoordinated radicals. 

The first measurement of the temperature dependence of an optical line width in an actinide system, Np + in LaC, was recently completed (47). The fluorescence transitions at 671.4 and 677.2 nm were studied from 10 to 200 K. The low temperature limit for the line width of the 677.2 nm transition is 16.5 GHz and is a measure of the width of the first excited crystal-field level of the ground manifold. The 671.4 nm transition has a line width of 0.55 GHz at 10 K. Its temperature dependence is described in terms of an effective three-level scheme for the excited manifold. The parameters are comparable to those found for Pr + in LaF. Further comparison depends upon the details of the phonon spectrum and the electronic states. At low temperatures, the residual width of the 671.4 nm transition was limited by the laser line width. This is consistent with the very narrow line widths observed in Pr +. Additional detailed studies of this type and proper contrast and comparison between lanthanides and actinides may provide the additional information needed to describe the electron-phonon and electron-ligand interactions of the actinides. 

The present method to study heavy element compounds in new and our experience is so far limited to atoms and some small molecules. It has the virtue that we can now use the machinery of CASSCF/CASPT2 for the entire periodic system. The method has been tested for all alkaline, alkaline earth, main group, transition metal atoms and in addition for some of the lanthanides and actinides. The results are promising. One example It has recently been possible to assign the electronic spectrum of the UO2 molecule (more than 150 electronic levels were computed) [33]. A drawback is that for the heaviest elements one has to include a large number of electronic states in order to fully account for the effects of spin-orbit coupling. 

The rate of this interconversion is fast on the n.m.r. time-scale, and hence, the actual n.m.r. spectrum observed for the substrate will be the weighted time-average of the spectra of the free and bound substrate. Insofar as there is a substantial change between the shifts of the free and bound substrate (up to - 40 p.p.m. ), it is only necessary for a small proportion of the substrate to bind with the lanthanide in order to produce a substantial change in the shifts observed. Nevertheless, it is mandatory that the substrate shall associate with the lanthanide, and hence, it is necessary for the substrate to have some suitable donor-site as lanthanide shift-reagents are hard Lewis acids, this means that the donor atom must be a hard Lewis base. Fortunately, many functional groups commonly found in carbohydrate systems, such as the hydroxyl, acetoxyl, amino, and acetamido groups, form donor bonds with lanthanides. 

---