134 INSTRUMENTAL METHODS
Asymmetry in the ligand environment, either geometric or in charge distri-
bution (or both), affect the asymmetry parameter, η. An η = 0 value corre-
sponds to complete axial symmetry, whereas η = 1 corresponds to pure rhombic
symmetry. Electric monopole interactions between the nuclear charge distri-
butions and the electrons at the nucleus cause a shift of the nuclear ground
and excited states. These interactions are known as the isomer shift, δ. Both
the M ö ssbauer source and the absorber (the sample of interest) experience
an isomer shift, and it is customary to quote δ relative to a standard, usually
Fe metal or Na 2 [Fe(CN) 5 NO] · 2 H 2 O at 298 K.
3.6.2 Quadrupole Splitting and the Isomer Shift,
The isomer shift, δ , is a measure of the s - electron density at the iron nucleus.
Infl uences on this parameter include changes in the s population of a valence
shell or shielding effects caused by increasing or decreasingp or d electron
density. Because the radial distributions of s and d electrons overlap, δ can be
a good measure of iron ’ s oxidation state in the sample. Values for the isomer
shift may also yield information on the spin state (several high - or low - spin
states exist for, Fe(IV) d^4 , Fe(III) d^5 , or Fe(II) d^6 ), the coordination sphere of
the metal ion, and the degree of covalency in the metal – ligand bond being
studied. High - spin Fe(II) has a unique range of δ values higher than those for
low - spin Fe(II) and higher than those for high - or low - spin Fe(III) or Fe(IV).
The trend can be understood through the following reasoning. A decrease in
the number ofd electrons with increasing iron oxidation state decreases the
screening ofs electron density at the nucleus (increases s electron density at
the nucleus). This, in turn, tends to decrease the value of the isomer shift, δ.
ΔEQ values depend on the spherical symmetry or asymmetry of the electric
fi eld gradients at the nucleus as explained in more detail below. In general,
the higher the symmetry of electric fi eld about the nucleus, the smaller the
value ofΔEQ. For instance, ΔEQ (Fe 2+ ) > ΔEQ (Fe 3+ ) for the high - spin case
because thed^5 high - spin Fe 3+ ( S = 5/2) ion has spherical symmetry and the d^6
high - spin Fe 2+ ( S = 2) does not, as indicated in Figure 3.26. Comparison of ΔEQ
values for heme ligands in Table 3.2 illustrates the trend: For Fe 3+ - heme ( S = 5/2)
we haveΔEQ = 0.5 – 1.5, while for Fe 2+ - heme ( S = 2) we have ΔEQ = 1.5 – 3.0.
Figure 3.26 Spherical symmetry and asymmetry of electric fi eld gradients affecting
ΔEQ.
Fe3+
high spin
Td caseFe2+
high spin
Td case