Mdssbauer temperature dependence

Figure 17. The molecular structure and temperature dependence of the "Fe Mdssbauer parameters (IS and QS) and half-width of the absorption line (h) and some parameters of the nonprimitive unit cell of the FeNx i(BHd)2 crystal [67],...

To verify this assumption, the temperature dependence of the Mdssbauer parameters (Figs. 21 and 22) were measured for the clathrochelate FeNx3(BOH)2, Fe(4MNx)3(BOH)2, and FeGx3(BOH)2 complexes and their nonmacrocyclic [Fe(H2Nx)3](S04) analog [56]. 

Fig. 18. Temperature-dependent Fe Mdssbauer spectra of mature limpet teeth,...

Precision measurements of the temperature dependence of QS are not common in tin Mdssbauer spectroscopy, primarily because the splittings vary little with temperature, and most chemical structural interpretations can be based on the QS value at a single temperature, typically 77 K. The theory needed for the interpretation of the QS parameter in tin compounds has not been developed to such a point as to make precision measurements worthwhile.21-23 However, the thermal variation of the QS is commonly studied for two main reasons for monitoring structural phase transitions and for the study of the motional anisotropy... 

Equation (1.10) indicates that the probability of zero-phonon emission decreases exponentially with the square of the y-ray energy. This places an upper limit on the usable values of Ey, and the highest transition energy for which a measurable Mdssbauer effect has been reported is 155 keV for Os. Equation 1.10 also shows that/increases exponentially with decrease in which in turn depends on the firmness of binding and on the temperature. The displacement of the nucleus must be small compared to the wavelength X of the y-ray. This is why the Mossbauer effect is not detectable in gases and non-viscous liquids. Clearly, however, a study of the temperature dependence of the recoil-free fraction affords a valuable means of studying the lattice dynamics of crystals. 

The nature of other ligands in this dinuclear complex can be deduced from other spectroscopic data. It is clear that the tyrosyl radical is NOT a ligand. Reduction of the tyrosyl radical with hydroxyurea does not alter the features of the Mdssbauer (26) and EXAFS (40) data. Indeed, the only hint that the radical is in the vicinity of the dinuclear iron unit is the temperature dependence of the EPR relaxation properties of the radical signal. At liquid helium temperatures, the tyrosyl radical behaves like an isolated organic radical. But, at temperatures where the dinuclear iron complex becomes paramagnetic, the radical signal becomes harder to saturate relative to an isolated radical (114). Based on these effects, the radical is estimated to be within 10 A of the diferric site. 

Mossbauer Spectroscopy. Mdssbauer spectroscopy has been used as a powerful technique to probe the electronic structure of the five- and six-coordinate ferrous nitrosyl porphyrins. The isomer shifts of both types of complexes (5 0.35 mm s ) are similar and show temperature dependences that are consistent with a second-order Doppler effect, but are slightly smaller than those of most other iron(II) porphyrin complexes (5 0.45 mm Unlike the isomer... 

There are few direct spectroscopic probes for iron for instance, in most complexes charge transfer bands obscure the d-d transitions and magnetic measurements cannot provide information on low-spin iron(II) complexes. Therefore when the Mdssbauer technique became generally available in the late 1960s, it was widely used for the study of iron complexes. The effect arises from the resonant absorption of y-rays by the Fe nucleus, which has a natural abundance of 2%. When the y-ray source and sample nuclei are in different environments, the shift in the energy of resonant absorption is expressed as a velocity relative to some arbitrary zero such as stainless steel or Na2[Fe(CN)5N0] 2H20. Whilst the value of the shift is temperature dependent, it is also affected by the electronic environment. 

According to the full shell model, the Aujs duster core consists of a central Au atom surrounded by 12 further Au atoms which form the inner core of the Aujj nudeus. The 42 outer shell atoms are divided into 12 which bear phosphine ligands, 6 which link to chlorine atoms, and 24 which are uncoordinat si ace gold atoms. In addition to the temperature dependence and the absorption inten-aty results, the Mdssbauer spectra also give information on the Debye-Waller factors (f-factors or Mdssbauer fractions) which are affected by intraduster vibrations whose values can also be calculated. [83] Since inequivalent gold atoms can... 

Isomer shifts show a slight temperature dependence which has been ignored in this compilation. Data from N. N. Greenwood and T. C. Gibb Mdssbauer Spectroscopy Chapman Hall, London, 1971. 

R. Grimm, P. Giitlich, E. Kankeleit, R. Link, Time and temperature dependence of aftereffects in [ Co(phen)3](C104)2-2H20 from time-differential Mdssbauer emission spectroscopy. J. Chem. Phys. 67, 5491 (1977)... 

Since the magnetic ordering temperatures of these salts are either very low or non-existent, the Mdssbauer effect spectrum of the lanthanide nucleus reflects the temperature dependence of the quadrupole interaction, except that in cases where the ionic relaxation rate is sufficiently slow, magnetic hyperfine structure may also appear at low temperatures. 

The Fe Mdssbauer parameters for the clathrochelate [FeMn(trom)](BF4) and [Fe2(trom)](BF4) complexes are characteristic of high-spin iron complexes. The spectrum of the [FeMn(trom)](BF4) clathrochelate exhibits one doublet line, which indicates the sole type of coordination centre. The QS value increases regularly with a temperature decrease. A major contribution to the QS value is made by the valence component (the lattice component is small and practically independent of temperature). For [FeMn(trom)](BF4) clathrochelate, all three spin-quintet states are presumably within kT, and the QS value depends on temperature [192]. 

The thermal shift 6j of the Mdssbauer spectrum is the sum of a contribution due to the second-order Doppler effect (6sod) and a possible contribution due to an intrinsic dependence of the isomer shift (8j) on temperature. The second-order Doppler shift is proportional to the mean square velocity of the Mdssbauer nucleus. For the purpose of a comparison with thermodynamic data, the SOD shift may be described in terms of the Debye approximation,6... 

Unfortunately, such a one-to-one correspondence between M5ssbauer species and individual patterns may be nonexistent under the given conditions, i.e., for that particular set of experimental parameters at which the Mdssbauer spectrum was recorded. However, when the whole range of the possible conditions is considered, one may find points in the space of experimental parameters at which only one pattern is associated with one species and vice versa and thus one can get round the problem of ambiguity. One can also do simultaneous evaluation of a whole series of Mbsshauer spectra taken from the same sample under different conditions. Such a serial evaluation is a very effective tool, because the known dependences of the Mbsshauer parameters (e.g., isomer shift) on the externally set experimental parameters (e.g., temperature) can be used as constraints for the serial fit. 

---