Mossbauer quadrupole splittings

Sn Mossbauer Quadrupole Splittings for Organotin Compounds OF Known Stereochemistry... 

Table 1. The 72-atom model examined by different theoretical methods. The energy differences (AE in kcal/mol) are calculated with respect to the lowest SCF energy. q(Fe) stands for Mulliken population charges on the Fe atoms q(S) and SS(b.i.) are the Mulliken population charges and the bond index for the bridging S atoms, respectively AEq is the calculated Mossbauer quadrupole splitting constant [mm/sec]. The PUHF spin states are those projected from the UHF wavefunction with 5 = 5,. 

PUHF, yield the lowest energies as well as reasonable Mossbauer quadrupole splitting constants, A Jq. 

As indicated in the previous discussion, Mossbauer spectroscopy provides information that when coupled with results using other structural techniques assists in determining the structure of the complex under analysis. The relationships between the various techniques are summarized in Table II. The Mossbauer chemical shift provides information about the 4 electron contribution to the bond between the metal and the ligands in a complex. Similar estimates can be obtained from the results of measurements on the fine structure in the x-ray absorption edge and nuclear magnetic resonance data. The number of unpaired electrons can be evaluated from magnetic susceptibility data, electron spin resonance, and the temperature coeflScient of the Mossbauer quadrupole splitting (Pr). 

Abstract. We report the results of experimental and theoretical studies of 13C and 15N shifts in proteins and model systems, together with 57Fe shifts and Mossbauer quadrupole splittings (electric field gradients) in metalloporphyrins and metalloproteins. The ability to relate these spectroscopic observables to structure by using quantum chemical methods opens up new opportunities for predicting and refining protein structure. 

The distorted octahedron of Me2SnCl2 agrees also with the Mossbauer quadrupole splitting found for the compound (Table V), lying between the tetrahedral (2.3 mm/sec) and octahedral (4.1 mm/sec) values. 

Because quadrupolar interactions depend on the coordination environment of the ion, changes in Mossbauer quadrupole splittings are sensitive to the sites at which the cations are located, whereas differences in the isomer shift are sensitive to the oxidation state of the ion. For reduced samples, the ferrous cations in site I have been assigned to the outer doublet, while those in sites P, IP and/or II have been assigned to the inner doublet (2). 

A comparison of the Fe sites for (TPPFe)20 and (TPPFe)2N are shown in Fig. 325 41). In addition, whereas the Fe-O-Fe bond angle is 174.5°, the Fe-N-Fe bonds are exactly linear. The shorter Fe-N vs Fe-0 bond is consistent with greater ir bonding. The closer position of the FeCIH to the porphyrin plane in (TPPFe)2N vs that of Fe(III) in (TPPFe)20 may indicate that (TPPFe)2N contains low spin iron. But the Mossbauer quadrupole splitting (A) is only 1.08 mm/s whereas low spin Fe(III)... 

The 57Fe Mossbauer quadrupole splitting (AEq) arises from the nonspherical nuclear charge distribution in the I = 3/2 excited state in the presence of an electric field gradient (EFG) at the 57Fe nucleus, while the isomer shift (6Fe) arises from differences in the electron density at the nucleus between the absorber (the molecule or system of interest) and a reference compound (usually a-Fe at 300 K). The former effect is related to the components of the EFG tensor at the nucleus as follows [45] ... 

This hydrogen bonding also impacts Mossbauer spectroscopic properties as shown in Table 3. It should be noted that the use of the B3LYP method has enabled accurate predictions of the Mossbauer quadrupole splittings and isomer shifts in a wide variety of iron proteins and model systems covering all iron spin states and coordination states [8-14,23], with a theory-versus-experiment correlation coefficient R2=0.98 over an experimental range of 8.80 mm/s in 47 systems... 

Zhang, Y., Mao, J.H., Godbout, N., Oldfield, E. Mossbauer quadrupole splittings and electronic structure in heme proteins and model systems A density functional theory investigation. J. Am. Chem. Soc. 2002,124,13921-30. 

Zhang, Y., Oldfield, E. An investigation of the unusual 57Fe Mossbauer quadrupole splittings and isomer shifts in 2 and 3-coordinate Fe(II) complexes. J. Phys. Chem. B 2003,107, 7180-8. 

The oxidation of the iron(III) complexes [Fe(DAS)2X2][FeX4] (DAS = o-phenylenebis-dimethylarsine, X = Cl, Br) with concentrated nitric acid followed by addition of the appropriate anion led to the isolation of black [Fe(DAS)2X2]Y2 (Y = BF, CIO, ReO ). The magnetic properties indicate two unpaired electrons consistent with a low-spin configuration in a tetragonal trans halide) structure. Mossbauer quadrupole splittings are large ( 3.2mms ) and temperature independent. ... 

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