Chemical isomer shift elements

In this series there is an approximately linear correlation between the chemical isomer shift and the electronegativity differences between tin and the other element in each compound, but this is not as regular as was previously supposed. Deviations no doubt reflect the substantial variation in bond type throughout the series. 

Promethium is another poor candidate for chemical Mossbauer spectroscopy as it is not a naturally occurring element. A resonance has been observed for the Pm isotope (half-life 2-62 y) using sources of Nd prepared by an (n, y) reaction on NdzOa [6]. Nine compounds as absorbers all gave a 91-06-keV single resonance line at 4-2 K with no detectable chemical isomer shift. No further details are available. 

The total electron density at the nucleus p(0) depends on the nature of the chemical bonding. Positive contributions to p(0) arise from the atomic 6s populations of the molecular orbitals on the gold ion, while a decrease in p(0) is caused by the atomic bd populations due to their shielding effects on s electrons. The relative magnitudes of the various contributions are known from the results of relativistic free-ion self-consistent field calculations for gold and for other d transition elements. The atomic 6p populations, on the other hand, yield only small contributions both by shielding of s electrons and by their relativistic density at the nucleus. Hence p(0) and consequently the isomer shift will mainly depend on the atomic 6s and 5[Pg.281]

The principal limitation of the method is the lack of suitable isotopes. It is unfortunate that elements such as carbon, oxygen, and silicon do not show a Mossbauer effect. In addition to iron, Mossbauer effects are readily observed in isotopes of tin, xenon, iodine, europium, gold, and a few others. Isomer shifts make it possible to distinguish the valence states of iron and tin, but most of the other nuclei have greater resonance linewidths, making it difficult to observe chemical effects. 

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