Isomer shifts factors affecting

The Mossbauer data from the quinquevalent gold compounds reported in this paper provide a further basis for the identification of the factors which affect the isomer shift. 

The spectroscopic and photochemical properties of the synthetic carotenoid, locked-15,15 -cA-spheroidene, were studied by absorption, fluorescence, CD, fast transient absorption and EPR spectroscopies in solution and after incorporation into the RC of Rb. sphaeroides R-26.1. High performance liquid chromatography (HPLC) purification of the synthetic molecule reveal the presence of several Ai-cis geometric isomers in addition to the mono-c/x isomer of locked-15,15 -c/x-spheroidene. In solution, the absorption spectrum of the purified mono-cA sample was red-shifted and showed a large c/x-peak at 351 nm compared to unlocked all-spheroidene. Spectroscopic studies of the purified locked-15,15 -mono-c/x molecule in solution revealed a more stable manifold of excited states compared to the unlocked spheroidene. Molecular modeling and semi-empirical calculations revealed that geometric isomerization and structural factors affect the room temperature spectra. RCs of Rb. sphaeroides R-26.1 in which the locked-15,15 -c/x-spheroidene was incorporated showed no difference in either the spectroscopic properties or photochemistry compared to RCs in which unlocked spheroidene was incorporated or to Rb. sphaeroides wild type strain 2.4.1 RCs which naturally contain spheroidene. The data indicate that the natural selection of a c/x-isomer of spheroidene for incorporation into native RCs of Rb. sphaeroides wild type strain 2.4.1 was probably more determined by the structure or assembly of the RC protein than by any special quality of the c/x-isomer of the carotenoid that would affect its ability to accept triplet energy from the primary donor or to carry out photoprotection. 

The data reported in Table 5.6 reveal an indisputable correlation between the donicities of the solvents and the isomer shift values. Appreciable effects are also exerted on the isomer shift values, however, by a number of other factors influencing the electronic structure of the reference acceptor, such as dissociation reactions affected by the relative permittivity of the solvent. 

For the elements from tin to xenon the most important single factor affecting the isomer shift is dR/R, and this accounts for the high sensitivity for Sb and Since 5R/R cannot be measured directly, it is not possible to quantify the relationship between isomer shift and electron configuration without making assumptions about the electronic populations in particular systems. The difficulties of such calibration procedures are discussed later. 

Table 2.3. Nuclear factors affecting the isomer shift"...

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