Quadrupole splitting constants

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. 

Quadrupole splitting constants, 37 129 Quantum mechanical methods, 38 424 Quantum mechanical predication, lattice energies and, 1 181-186 2,2 6, 2 6",2"-Quaterpyridine coordination compounds, 30 104-106 Structure, 30 70 Quenching center, 35 321 organometallics, 19 93-98 chemical, 19 98... 

Finally, we mention that Li T measurements have been used to study the rate of interconversion between tight and loose ion pairs of lithium fluorenide [44]. The quadrupolar relaxation contribution was derived from Li and Li T data, and a change of the Li quadrupole splitting constant, QSC, obtained via Li quadrupolar and dipolar relaxation rates (see Section 4) with ion pair structure, was indicated. 

The quadrupole splitting constant, QSC, of Li has been used by Jackman et al. [151] and Johnels [152] as an empirical parameter to obtain structural information on lithium enolates in solution. Tlie QSC is given by equation (11). 

These parameters are close in the order of magnitude to the quadrupole splitting constants of Li in LiNhOs (K=46 kHz), in lithium nitrate LiNOs (K=39 kHz) [35]. Therefore, it can he assumed that the structure of the first coordination sphere of... 

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

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... 

The experimentally observed quadrupole splitting AEq for Fe in inorganic compounds, metals, and solids reaches from 0 to more than 6 mm s [30, 32]. The range of AEq for other Mossbauer isotopes may be completely different because of the different nuclear quadrupole moment Q of the respective Mossbauer nucleus, and also because the EFG values may be intrinsically different due to markedly different radial distributions of the atomic orbitals (vide infra). As Q is constant for a given isotope, variations in the quadrupole coupling constants eQV can only arise from... 

Fig. 4.14 Magnetically perturbed quadrupole spectra simulated for powder distributions of the EFG (Vzz > 0) with an applied field B = 4T which is fixed in the laboratory system perpendicular to the y-beam/ The value of the quadrupole splitting is kept constant at AEq = +4 mm s For negative quadrupole splitting (V z < 0), the spectra would be inverted on the velocity scale. Note the difference in relative intensities for the spectrum for ry = 0 and the single-crystal type spectrum given in Fig. 4.13. Similar patterns are obtained for B y...

Chemical information from hafnium Mossbauer spectroscopy can primarily be deduced from the quadrupole-splitting parameter. In Table 7.4, we have listed the quadrupole coupling constants eQV for some hafnium compounds. Schafer et al. 

Apart from the determination of the structures of stannylenes by diffraction methods (X-ray or electron diffraction) many other physico-chemical techniques can be exployed to characterize these compounds more completely. Besides the classical methods such as IR-, Raman-, PE-, UV- and NMR-spectroscopy, MoBbauer-119 m-tin spectroscopy is widely used for the determination of the oxidation states of tin atoms and of their coordination 1n8-12°-123>. jt is not in the scope of this report to study the dependence of MoBbauer constants such as isomer shift and quadrupole splitting on structural parameters. Instead, we want to concentrate on one question Which information can we deduce from the structure of stannylenes to evaluate their reactivity ... 

Whereas the Mossbauer spectra at 77 K of CsM13% (M = V, Mn) can be fitted assuming quadrupole splitting at a single metal site, two sites (a/b =2/1), with coupling constants of opposite sign, must be assumed when M = Cr. In the a and b sites the 12T atoms are in asymmetric and symmetric bridging positions respectively, consistent with axially distorted octahedra. The a to jS phase transition (Table 36) is confirmed from X-ray powder diffraction, but at 165 5 K.278... 

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