Partial chemical isomer shifts

Another semi-empirical observation is that the chemical isomer shift can be calculated as the sum of partial chemical isomer shifts for each ligand... 

The early work by Kistner showed partially resolved quadrupole splittings and large chemical isomer shifts in RUO2 and Ru( i-C5H5)2 [21]. More... 

Recently, in a study of a large number of organo-iron compounds 120), it has been found possible to treat the isomer shift as being due to a sum of partial isomer shifts 8,. from individual ligands which for 7r-cyclopentadienyl derivatives could be correlated with proton NMR chemical shifts in the same compounds. The Mossbauer spectra of two cases of theoretical interest are worthy of mention. First, the spectra of both the mono- and binuclear iron tricarbonyl derivatives of cyclooctatetraene 115) show similar 8 and A values. The small 8 values are consistent with zero oxidation state for the iron atom and essentially complete covalent bonding between the iron and the tt electrons of the ring, so that, at least for the mononuclear... 

The isomer shift provides information on the electronic charge density at the nucleus, while the quadrupole splitting is related to the symmetry of the surrounding electron charge distribution. The changes which occur in these parameters when the Mossbauer nuclei are in different chemical environments can be interpreted in terms of changes in hybridization and the partial ionic character of the chemical bonds. 

The electronic density at the nucleus may be separated into contributions due to inner-core filled orbitals and to partially filled valence orbitals, and thus the isomer shift may be expected to be sensitive to the specific nature of the chemical bonds involving the Mossbauer-active species and adjacent atoms in the molecule or lattice. Furthermore, the value of 8 may be modified not only by varying the 5-orbital occupation but also by a shielding effect associated with orbitals of other types. The removal of a d-electron from an iron ion, for instance, will lead to a contraction of the occupied s-orbitals and thus to a shift of 8 in the negative direction. A correlation between the isomer shift and oxidation states, which may be useful in the analysis of certain spectral features associated with iron in molecules and other materials, is shown in Figure 3. 

A comparison of IGLO/DZ//MP2/6-31G(d) calculated and measured 13C NMR chemical shifts demonstrates that the partially methyl bridged isomer is the preferred species.36 It was demonstrated that the calculated 13C chemical shifts are highly sensitive toward hyperconjugational distortion, i.e. the degree of bridging (ca. 6ppm/deg from 68° to 98°). 

Research on cis-trans isomerization (CTI) of lipid double bonds focused both on the conversion that occurs in some bacteria enzymatically and on trans isomers that are present in mammalian cells after a dietary supplementation of chemically modified fats [3,4]. It is known that cis/trans isomeric mixtures of fats result from vegetable and fish oils manipulated through partial hydrogenation or deodoriza-tion processes that are frequently utilized in the food industry. Nutritional and epidemiological studies revealed some harmful effects of these unnatural lipids for human health. However, it must be pointed out that in the chemical manipulation of oils the structures of trans fatty acid residues consist of geometrical and positional isomers with unshifted and shifted double bonds compared with the natural cis compounds. With the name trans lipids we indicate these unnatural geometrical and positional isomers. It has to be mentioned for clarity that there... 

X-Ray crystallographic analysis of (35) confirmed a sy -diaxial conformation in a distorted chair. The strain introduced by the repulsive interactions between the polarized S—O bonds is partially relieved by an elongated S-l - S-2 distance of 3.14 A across the ring as compared with the normal value of 3.01 A in the unstrained isomer trans-(3S) [77,78]. No explanation is advanced by the authors regarding the surprising finding of diaxial cis- 3S). In fact, they caution [77] that the x-ray results should not be extrapolated to solution, and that even the NMR data (unusually upheld chemical shift for C-5) could be misleading. 

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