Prof. J. Danon (Riu de Janeiru) said: Accurate measurements of isomer shifts show that this hyperfine interaction is sensitive to small changes arising from chemical bonding. Precisely with the iron (11) chlorides it has been found a systematic variation of the isomer shift with the hydration number.l These results indicate that the degree of covalency decreases with the increase in the number of coordinated water molecules around the iron atom.One of the difficulties for interpreting the hyperfine fields in terms of covalency has been pointed out by van Wieringen with the Mn2+ salts : a small mixture of excited states involving s-shells such as (3s-4~) gives a very large contribution to the hyperfine field. The temperature variation of the electric field gradient as used in Johnson's treatment is not sensitive to the presence of such states which involve s-electrons.
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