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.
Newcastle upon Tyne) said: Two of the most fascinating results in Wickman's paper are (i) that a change from chlorine to bromine in these compounds alters the sign of D, the splitting of the lowest quartet level ; and (ii) that in the chloro-derivative the diethyl compound is ferromagnetic whereas the dimethyl and di-isopropyl compounds are paramagnetic at low temperatures. Is there any explanation of these observations and, in particular, is it possible to give them any intuitive chemical rationalization ? Dr. H. M. Wickman (Bell Telephone Lab., N.J.) said. In reply to Greenwood, a possible explanation of the change in sign of D has been suggested by preliminary results of a crystal field calculation for C,, symmetry, within the 3d5 configuration, representing penta-co-ordinate Fe (111). There are many limitations in this approach, and the calculation, though a straightforward application of results of Tanabe, Sugano and Kamimura, is tedious. However, for a reasonable choice of parameters specifying the ligand field potential, Racah parameters, etc., I find that a cross-over occurs between two ground quartet terms as the parameter representing halide charge is varied in a systematic fashion. As spin-orbit interaction was neglected in the calculation, the actual splitting within these quartet terms has not been directly computed. On the basis of the cross over, however, it appears likely that the change in sign of D in the S = 3 spin Hamiltonian in the ethyl-chloro and ethyl-bromo derivatives arises from the appearance of a different ground quartet in the two cases. This situation may be contrasted with the high spin Fe3+, 6S, where inversion in sign of D must always be due to variable contributions to the zero-field splitting with the (unique) S = 3 ground term. It should be emphasized that it is not yet possible completely to rule out this possibility in the quartet case. To do this explicitly would require additional data such as optical studies.
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