Quadrupole splittings

A third method uses data for a neutral molecular complex. Under the assumption that ferrocene and cobalticinium perchlorate have similar chemical bonding, the pure nuclear quadrupole resonance data for the latter together with the independent determination of Qg for Co from atom-beam spectroscopy data have been used to estimate 0e( Fe). The method has the advantage that the lattice sums are less than 1% of the total electric field gradient. 

The fifth approach involves a direct comparison of Al and Fe quadrupole coupling constants from nuclear magnetic resonance and Mossbauer data respectively in C0AI2O4 and Z11AI2O4 doped with 5% Fe. Ferric iron substitutes at aluminium sites and it is assumed that there is no additional distortion induced upon replacement of one spherical ion by another. The 

Rosenberg, S. Mandache, H. Niculescu-Majewska, G. Filotti, and V. Gomolea, 

A large number of estimates of Q are available, and these are listed in Table 

3 in approximate chronological order. Assuming that the more recent determinations are the less inaccurate, it appears that Q has a value of between 0-18 and 0-28 bams. 

Molecules as well as atomic nuclei with spin I 1/ can have 

While neutral molecules may also have electric dipole moments the latter property is excluded for atomic nuclei. 

In classical electrostatics the quadrupole moment, commonly denoted by eQ, where e is the unit electric charge, is defined as 

Instructive descriptions of this subject may be found in the texts of Cohen and Reif [23] and Lucken [24], 

By symmetry arguments it can be shown that for nuclei the next multipole term possible is the hexadecapole. The magnitude of a hexadecapole term can be estimated to be of the order of 10 of the quadrupole term. Experimental evidence for the existence of hexadecapole moments for a number of nuclei has been obtained from atomic beam experiments as well as proton scattering studies. 

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Figure 4.54 The effect of an electric field gradient (EFG) creating asymmetry in the electron distribution round a gold nucleus, leading to a quadrupole splitting in the Mossbauer spectrum. (Reproduced with permission from Gold Bull., 1982,15, 53, published by World Gold Council.)...

Partial quadrupole splittings in inorganic chemistry. G. M. Bancroft. Coord. Chem. Rev., 1973,11, 247-262 (53). 

On the basis of the point-charge model formalism, applied on the experimental nuclear quadrupole splitting rationalization, I Agxp I, the results obtained were interpreted in terms of strong complex formation by either Me2Sn(OH)2 or Me3Sn(0H)(H20) with (n = 1 or 2, obtained in phosphate buffer) and... 

The various, solid-state stereochemistries just described may often be distinguished fairly readily by " Sn Mossbauer spectroscopy 5-9, 452), particularly from the value of the quadrupole splitting parameter, AEq (see Table II). 

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

For example, octahedral temperature-dependent Mossbauer measurements have been used in conjunction with Raman spectroscopy to determine molecular weights 453) and lattice rigidity 460) of various organotin compounds. 

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. 

Summary of EPR -Values, Fg-Hyperfine Coupling Constants, Isomer Shifts, and Quadrupole Splittings for Some Representative [Fg3S4] Clusters... 

Most valuable chemical information can be extracted from Mbssbauer parameters such as the isomer shift (5), the quadrupole splitting (AEq), the magnetic splitting (AEm), and the asymmetry parameter (n). 

Figure 2A. Schematic diagram of Mossbauer parameters isomer shift (6), quadrupole splitting (AEq) and magnetic dipole splitting of the nuclear energy states of 57pe leading to various hyperfine splitting in Mossbauer spectra.

The results of the XRD measurement showed that the Fe jAl, jPO catalyst was almost in amorphous state. Only a very broad peak at 29 of ca. 23 degree was observed. The Mossbauer spectroscopic study on this catalyst showed one doublet of iron with the isomeric shift of 0.31 mm s (a-Fe was used as the reference) and the quadrupole splitting of 0.62 mm s. These parameters are very close to those observed for FePO [13, 14], suggesting that the iron cation in the catalyst is tetrahedrally coordinated with oxygen and isolated by four PO tetrahedral units. Such coordination circumstance was suggested to be a key factor for the iron site effective for the oxidation of CH to CHjOH by H -Oj gas mixture [15]. 

Sample Fe state Isomer shift (mm/s) Quadrupol splitting AE(mm/s) Reference... 

Table 2. Isomer shifts (IS) and quadrupole splitting (QS) of Pt309phen 36O30+10, fractionally transferred into isotopes.

A unique situation is encountered if Fe-M6ssbauer spectroscopy is applied for the study of spin-state transitions in iron complexes. The half-life of the excited state of the Fe nucleus involved in the Mossbauer experiment is tj/2 = 0.977 X 10 s which is related to the decay constant k by tj/2 = ln2/fe. The lifetime t = l//c is therefore = 1.410 x 10 s which value is just at the centre of the range estimated for the spin-state lifetime Tl = I/Zclh- Thus both the situations discussed above are expected to appear under suitable conditions in the Mossbauer spectra. The quantity of importance is here the nuclear Larmor precession frequency co . If the spin-state lifetime Tl = 1/feLH is long relative to the nuclear precession time l/co , i.e. Tl > l/o) , individual and sharp resonance lines for the two spin states are observed. On the other hand, if the spin-state lifetime is short and thus < l/o) , averaged spectra with intermediate values of quadrupole splitting A q and isomer shift 5 are found. For the intermediate case where Tl 1/cl , broadened and asymmetric resonance lines are obtained. These may be the subject of a lineshape analysis that will eventually produce values of rate constants for the dynamic spin-state inter-conversion process. The rate constants extracted from the spectra will be necessarily of the order of 10 -10 s"F... 

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