Quadrupole splitting, Aeq

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 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... 

Mossbauer spectroscopy is particularly suitable to study ST since (1) the spectral parameters associated with the HS and LS states of iron(II) clearly differ and (2) the time-scale of the technique ( 10 s) allows the detection of the separate spin states in the course of the transition. Typically, Mossbauer spectra of HS iron(II) show relatively high quadrupole splitting (AEq 2-3 mm s ) and isomer shift (3 1 mm s ), while for LS iron(II), these parameters are generally smaller (AEq < 1 mm s 3 < 0.5 mm s ). Among the early applications of Mossbauer spectroscopy to study ST phenomena in iron(II) complexes is the work of Dezsi et al. [7] on [Fe (phen)2(NCS)2] (phen = 1,10-phenanthroline) as a function of temperature (Fig. 8.2). The transition from the HS ( 12) state (quadrupole doublet of outer two lines with AEq 3 mm s ) to the LS CAi) state (quadrupole... 

Mossbauer parameters isomer shift, 6, and quadrupole splitting, AEq, in units of mm/s. 

Figure 1. Scatter plots of iron compounds. Correlation of differential chemical shift. Soy u)ith quadrupole splitting, AEq, for inorganic compounds and biochemicals at room temperature. Adapted from Ref. 34.

Figure 3.25 shows the Mossbauer spectrum that results from splitting of the Fe excited state, a quadrupole doublet, for a sample containing randomly oriented molecules such as found in polycrystalline solids or frozen solutions. The two doublets are separated in energy by the quadrupole splitting, AEq, dehned by the following equation ... 

Oxide Recording temperature K Isomer shift S mm s" Quadrupole splitting AEq mm Hypetfine field Bhf T... 

Konig and Madeja62-64) have first communicated results from s7Fe Mossbauer effect measurements on I and II at 293 and 77 K. At both temperatures only one doublet was observed, but the quadrupole splitting AEq as well as the isomer shift changed drastically on going from 293 to 77 K in both systems AEq 2.6 mms-1 and 5 — 1.0 mms-1 at 293 K, which is typical of high-spin iron(II)80>81> AEQ ... 

Fig. 9. Temperature dependence of the area fraction xhs, the line width Ths> < the quadrupole splitting AEq(HS) of the high-spin (5T2) quadrupole doublet, and of the magnetic moment Meff of [Fe(phen)2(NCS)2J (prepared after method (B) see text)70)...

Table I. 119Sn Isomer Shifts (AS1) and Quadrupole Splitting (AEQ)for Some Au-Sn Binary Alloys...

The spectrum of CIII japanese radish peroxidase exhibits two widely splitted lines with a quadrupole splitting AEq = 2.37 mm s 1 and an isomer shift 8pe = 0.29 mm s 1 at 77°K [145], very similar to those observed in oxyhemoglobin [144], a horseradish peroxidase [146], The electronic structure of the iron is apparently determined by the Fe-C>2 bond with a marginal contribution of the fifth, axial ligand provided by the protein (Fig. 11.8). 

Complex T(K) Isomer shift (6) (mmr1) Quadrupole splitting (AEQ) (nuns )... 

Complex Spin State Ati BM) Isomerb Shift 4(mms-1) Quadrupole Splitting AEq(mms 1)... 

Complex Isomer shift 6 (mm/s)a Quadrupole splitting AEq (mm/s) Magnetic moment M (Mb) Ref. 

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