PrSb compounds

Red shifts due to electron delocalization should be associated with a concommitant decrease in bond alternation and thus with a decreased frequency of the C=C stretching mode. As clearly exposed in Fig. 7, this has been observed both for model RSB and PRSB compounds (221,226,244) and for several pigments derived from opsin and bacterioopsin (221,222,225,226). 

At present, we know the 13C chemical shift values for all the unsaturated carbons of the chromophores of Rh and bR before illumination. Figure 2 shows the chemical shift differences between the chromophores and their model compounds (free protonated retinal Schiff bases, hereafter abbreviated as PRSB). As for Rh, the chemical shifts of the carbons from C8 to C13 show significant downfield displacements relative to those for the model. As for bR, the chemical shifts of C5 and C8 are displaced significantly to downfield and upfield, respectively. These anomalous displacements suggest the occurrence of interactions between the chromophore and the surrounding protein matrix, including the resultant conformational change of the chromophore. 

Organoboranes typically used in these reactions are Et3 B, i-PrsB, and alkylated 9-BBN derivatives. Interestingly, multiple organoboration steps may take place with di-, tri-, and tetra-1-alkynyl element compounds. Intermediates such as the... 

As for elastic constants, the CEF effects in thermal expansion and the Schottky specific heat are especially pronounced in TmSb, see fig. 18. Equations (45) and (46) give a good fit to the data, (Ott and Liithi 1976, 1977) with y, = yj = -1.2 as seen from fig. 18. The CEF Griineisen parameters measuring the volume dependence of the CEF levels turn out to be of the order of one. In this way a number of cubic compounds have been studied so far TmSb, PrSb, SmSb, ErSb, CeTe and TmTe (Ott and Liithi 1977), and TmCu, TmZn and TmCd (Morin and Williamson 1984). 

Parks et al. (1984) and Wieliczka et al. (1984b) noticed that the two-peak structure present in cerium systems also appeared in Pr and Nd systems, but the structures there were further removed from Ef. This seems to rule out a Kondo explanation for the upper peak since in that model such a feature is inherently tied to p It is, therefore, of some interest to see whether our approach followed for the Ce compounds would explain the Pr and Nd pnictides in the same manner as it did the Ce pnictides. Comparison will be made with photoemission data on PrSb and NdSb (unfortunately, no resonance data exist). 

Fig. 12. Demonstration of typical chemical shifts in lanthanide Lm absorption. The energy calibration of the spectra recorded from the Pr absorption in the compounds is accurate within 0.2 eV with respect to o fixed at the intersection point of the high-energy absorption with the absorption line in (dhcp) Pr metal (dashed-dotted line). The intersection point shifts to higher energies with decreasing metallic character of the compounds. The maxima of the prominent main lines, however, remain unshifted, just as the onsets of the lines. PrCu crystallizes in orthorhombic FeB structure (a = 7.343 A, 6 = 4.584 A, c = 5.604 A). The semi-metals PrSb, PrBi have fee (NaCl) structure a = 6.366 and 6.463 A, respectively). Pr Oi] is a nonstoichiometric modification of nominally tetravalent Pr02 (fluorite type, Cap2). The bar diagram indicates ligand-field-split absorption lines for both, the tri- and tetravalent valence states in Pr Oij (cf. section 14).

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