Mossbauer computed parameters

Figure 23 Computed conversion curves of [Fe(bpym)(NCSe)2]2bpym (2), [Fe(bt)(NCS)2]2bpym (3) and [Fe(bt)(NCSe)2]2bpym (4), fitted to e experimental Mossbauer data parameter values are listed in Table 3. Reproduced with permission from ref. 81.

Sandala GM, Hopmann KH et al (2011) Calibration of DFT functionals for the prediction of Fe Mossbauer spectral parameters in iron-nitrosyl and iron-sulfur complexes accurate geometries prove essential. J Chem Theory Comput 7 3232-3247... 

The EFG parameters Vzz and described by (4.42a) and (4.42b) do not represent the actual EFG felt by the Mossbauer nucleus. Instead, the electron shell of the Mossbauer atom will be distorted by electrostatic interaction with the noncubic distribution of the external charges, such that the EFG becomes amplified. This phenomenon has been treated by Stemheimer [54—58], who introduced an anti-shielding factor (1 —y 00) for computation of the so-called lattice contribution to the EFG, which arises from (point) charges located on the atoms surrounding the Mossbauer atom in a crystal lattice (or a molecule). In this approach,the actual lattice contribution is given by... 

There may, however, be a number of other reasons to pursue a predictive first principles theory of Mossbauer spectroscopy. For example, one may want to elucidate structure/spectroscopy correlations in the cleanest way. To this end one may construct in the computer a number of models with systematic variations in oxidation states, spin states, coordination numbers, and identity of hgands to name only a few chemical degrees of freedom. In such studies it is immaterial whether these molecules have been made or could be made what matters is that one can find out which structural details the Mossbauer parameters are most sensitive to. This can provide insight into the effects of geometry or covalency that are very difficult to obtain by any other means. 

A CD-ROM is attached containing a teaching course of Mossbauer spectroscopy (ca. 300 ppt frames), a selection of examples of applications of Mossbauer spectroscopy in various fields (ca. 500 ppt frames), review articles on computation and interpretation of Mossbauer parameters using modem quantum-mechanical methods, list of properties of isotopes relevant to Mossbauer spectroscopy, appendices refering to book chapters, and the first edition of this book which appeared in 1978. In subsequent printmns files are available via springer.extra.com (see imprint page). 

Recently, the stochastic models for the Mossbauer line shape problem have been discussed by several investigators.20 Such models can be treated in a systematic way as we have described in the above. For example, in a 57Fe nucleus, the spin in the excited state is / = and that in the ground state is / = i, so that the Hamiltonian is a 6 x 6 matrix. If a two-state-jump model is adopted, the dimension of the matrix equation, Eq. (63), is 6 x 2 = 12. If the stochastic operator is of the type (26), then the equation is a set of six differential equations. These equations can be solved, if necessary, by computers to yield the line shape functions for various values of parameters. 

The parameters, IS and QS, shown in Table 5 are measured at 4.2 °K with zero applied field. The value of rj and the sign of QS are derived by matching computed spectra to the Mossbauer data for the oxidized proteins taken at 4.2 °K in 46 kilogauss applied magnetic field (Fig. 8). The above parameters do not exhibit any measurable temperature dependence over the temperature range from 4.2 °K to 77 °K. 

Magnetic nanoparticles in the sol-gel silica glass were obtained by heat treatment at 1000°C in air during 6 h and identified by X-ray and Mossbauer spectroscopy as y-Fe203 (maghemite) [11], The magnetic parameters of maghemite used in computer simulations of the SPR spectra, are Ms = 370 kA m 1 and = —4.64 kJ m"3 (cubic symmetry) [35],... 

Fig. 9b shows the experimental Mossbauer spectrum of Hb at 4.2 K with H0 =30 kOe and R y. So far, it has not been possible to compute a theoretical Mossbauer spectrum, which fits the experimental pattern of Fig. 9b, mainly due to the fact that most of the parameters required for such a fit have not been available. 

The asymmetry parameters rj(T) (from Table 8) which are related with the AEq(T) and fit curve b of Fig. 28a also agree with the Mb single crystal Mossbauer results. In Ref. (42) the angle a = 40° 8° was derived assuming rj to be zero. A most recent computational analysis of our former Mb single crystal Mossbauer data by Maeda et al. (62), however, indicates that a =40° 8° is consistent also with an -parameter in the range of 0< <0.3. 

The Mossbauer spectrometer was equipped with a 10 mC 57Co/Rh source maintained at room temperature, A Northern NS-900 multichannel analyser was used for taking the spectra. The Mttssbauer parameters were determined by least square computer programme. The isomer shift (5) was calculated with reference to a-Fe, The characteristics of iron in various iron molybdates and iron antimonate are given in table I. 

The Lamb-Mossbauer factor /lm, which is difficult to determine experimentally as well as computationally, does not belong to the most common Mossbauer parameters in the field of inorganic chemistry. It can be of importance, however, when Mossbauer spectra are used to determine for a given sample the fractions of different species containing the same Mossbauer isotope. This is the case, for instance, for iron spin-crossover complexes where Mossbauer spectroscopy can be used to measure the temperature- or pressure-dependent fraction of different spin isomers. The first approach to estimate the change of the Lamb-Mossbauer factor upon spin crossover has been restricted to molecular vibrations, neglecting the important contribution that arises from intermolecular vibrations. ... 

The Mossbauer parameters are derived from the peak parameters (base line parameters, peak position, peak width, and peak area/height) via the fitting process by computer evaluation of spectra in the case of the so-called model-dependent evaluation. In this case, an exact a priori knowledge about the spectrum decomposition (peak-shape function, number, and type of subspectra corresponding to the interactions assumed for each microenvironment in the model) is inevitably necessary. (Incorrectly chosen number of peaks renders the analysis itself incorrect.)... 

A versatile Mossbauer Data Analysis (MDA) user oriented computer simulation program was developed by Jernberg and Sundqvist (1983) in which the calculations consider a number of experimental situations and the comparisons is made by least squares sums or by plotting the simulated and the measured spectrum. The fitting routine minimizes the least square sums to find the parameters characterizing the measured spectrum. 

The samples were characterized by X-ray fluorescence spectroscopy (XRFS), X-ray powder dififiaction (XRD), scanning electron microscopy(SEM) and Mossbauer effect spectroscopy(MES). For MES measurements the Co in chromium matrix was served as the source. Isomeric shift values were given in relation to metallic iron. Spectra were computer-fitted and Mossbauer parameters were calculated. 

The Mossbauer spectral lineshapes in the case of thin absorbers can be represented fairly accurately by Lorentzian-type functions. The values of the isomer shift, quadrupole splitting, linewidth, strength of the effective magnetic field at the nucleus, and other parameters of interest can be determined by a statistical treatment of the data. A number of computer routines have become available which enable such analyses to be performed in a rather straightforward fashion. 

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