Hypersensitive bands

Nevertheless, crystal fields cannot be completely ignored. The intensities of a number of bands ( hypersensitive bands) show a distinct dependence on the actual ligands which are coordinated. Also, in the same way that crystal fields lift some of the orbital degeneracy (2L -)- 1) of the terms of d" ions, so they lift some of the 2J -)- 1 degeneracy of the sates of P ions, though in this case only by the order of 100cm This produces fine structure in some bands of Ln spectra. 

In practice, the hypersensitive transitions are often used for the determination of stability constants in aqueous solution. Lanthanide absorption bands in solution do not normally change in position on complexation to such an extent that bands due to the complexed and uncomplexed ion can be clearly observed independently, as is often the case for d transition metal ions, but the marked change of intensity of the hypersensitive bands is sufficient to allow determination of K values, for example as demonstrated for various adducts of [Ho(dpm)3].6U... 

Fig. 8.11. Plot of optical electronegativities [741 vs. hypersensitive band oscillator strengths for...

More intense bands in the 4f-4f region were observed with lanthanide complexes of mandelic, salicylic, thiosalicylic, furoic, and thiophenic acids than the acetate and haloacetate analogues [235]. Both increase in intensity and nephelauxetic effect have been observed. It seems plausible that the aromatic groups in the hydroxy acids such as benzene, thiophene or furan contribute significantly to the intensity of the hypersensitive bands of the complexes. It appears that pH has an important role in determining the stoichiometry of the complex formed. Only ML species is formed up to pH 4 and both ML2+ and ML2 are formed at a pH of 6.0. 

Intramonomer bands other than are also sensitive to H-bonds, although in a much less spectacular way. This sensitivity proves to be most useful in chemistry and biology, and, as will be seen in the case of the H2O molecule, allows precise measurements of the number of H-bonds from which much structural and dynamic information can be obtained. It will prove useful to follow the development of the H-bond network of a macromolecule during hydration (Ch. 10). In physics and chemistry, the hypersensitive band ensures the... 

The solvent effect on intensities of hypersensitive transitions of R(C104)3 (R=Nd, Ho, Er) has been investigated by Legendziewicz et al. (1982). The molar refiactivities and oscillator strengths of hypersensitive bands are reported for water, alcohols, DMSO, DMF and diethylformamide. Intensities of the f-f bands and the nephelauxetic effect of the alcoholic solvates of the heavy-lanthanide chlorides with Ho(III) and Er(III) have also been reported (Keller et al. 1982). The intensity T2 parameter increases as follows ... 

According to Jorgensen and Judd (1964) the hypersensitive transition intensities increase when the lanthanide is surrounded by a field of decreasing symmetry. When the lanthanide itself lies on a center of symmetry, the hypersensitive band intensity is zero (apart from vibronic intensity). This has been established from studies of lanthanide crystals of high symmetry (Dieke 1968). 

Finally, experimental results point to the following outline behavior of the hypersensitive band intensities as affected from the chemical environment of the lanthanide ions (Henrie et al. 1976, Peacock 1975) (i) the intensity of a given hypersensitive transition in a halide compound of constant structure appears to increase in the order r >Br > Cr > F (ii) hypersensitivity is proportional to the nephelauxetic ratio and possibly to the R-X covalency (Henrie and Choppin 1968, Mehta et al. 1971) and (iii) a correlation exists between the hypersensitive intensities and the electron donating ability (basicity) of the ligand according to which the more basic the ligand the more intense the hypersensitive band. 

During attempts to study the resonance Raman spectra of the R-A-Cl (A=A1, Ga, In) vapor complexes by laser excitation near hypersensitive bands, Papatheodorou and Berg (1980) and Boghosian et al. (1995) have observed resonance fluorescence due to 4f" transitions of R + (R=Er, Nd, Dy, Ho) in the vapor complexes. Raman scattering from the A2Cl6(g)-ACl3(g) carrier gas molecules which are the predominant species in the equilibrium vapor mixture was also observed. Thus laser excitation of the Er-A-Cl (A=Al, Ga, In) vapor mixture with the 647.1, 530.9 and 520.8 nm Kr" laser lines in the... 

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