Mossbauer spectroscopy spin crossover

The authors of this book consider it appropriate to include in this section two contributions from their own laboratories, one on Mossbauer spectroscopy of spin crossover (SCO) phenomena in iron(II) compounds and the other on applications to biological systems. Both chapters will demonstrate the effectiveness of Mossbauer spectroscopy in these particular fields. 

Keywords Spin crossover Magnetism Mossbauer spectroscopy Coooperativity Hysteresis... 

Giitlich P (1981) Recent investigations of spin crossover. In Stevens JG, Shenoy GK (eds) Mossbauer spectroscopy and its chemical applications. American Chemical Society Advances in Chemistry Series no. 194, p 405... 

Most interestingly, [Fe(btzp)3](Cl04)2 is the first one-dimensional Fe(II) spin crossover compound, which shows the LIESST effect, detected in this instance by 57Fe Mossbauer spectroscopy (Fig. 19). 

About twenty years ago we reported on the di-isothiocyanato iron(II) complex of the tetradentate ligand tpa (tris(2-pyridylmethyl)amine) [7] (6). It was shown that this complex exhibits the spin crossover phenomenon with a critical temperature Tm of about 170 K. Several different solvated phases of the same system have since been characterized by Chansou et al. [8]. The unsolvated phase which can be isolated from an aqueous solution has been investigated by nuclear forward scattering (NFS), nuclear inelastic scattering (NIS) [9], extended x-ray absorption fine structure (EXAFS) spectroscopy, conventional Mossbauer spectroscopy, and by measurements of the magnetic susceptibility (SQUID) [10-13]. The various measurements consistently show that the transition is complete and abrupt and it exhibits a hysteresis loop between 102 and 110 K. 

Keywords Spin crossover Dinuclear complexes Two-step transition Plateau Magnetic field Mossbauer spectroscopy... 

The special feature of the spin crossover process in all bpym-bridged dinuclear compounds studied so far is the occurrence of a plateau in the spin transition curve. A reasonable assumption to account for this observation is that a thermal spin transition takes place successively in the two metal centres. However, it cannot be excluded that spin transition takes place simultaneously in the dinuclear units leading directly from [HS—HS] pairs to [LS-LS] pairs with decreasing temperature. Therefore, two possible conversion pathways for [HS—HS] pairs with decreasing temperature may be proposed [HS—HS]<->[HS—LS]<->[LS—LS] or [HS-HS] [LS-LS]. The differentiation of the existence of the [LS—LS], [HS—LS], and [HS—HS] spin pairs is not trivial and has recently been solved experimentally by utilisation of magnetisation versus magnetic field measurements as a macroscopic tool [9], and by Mossbauer spectroscopy in an applied magnetic field as a microscopic tool [11]. 

A systematic study of the metal dilution effect on the spin-crossover behaviour in the solid solutions [FexZnj x(2-pic)3 ] Cl2 EtOH (0.0009 < x < 1) has been performed employing S7Fe Mossbauer spectroscopy between 5 and 300 K 84.8S). The purpose was to find support for the cooperative domain model suggested earlier by Sorai and Seki34 87). Some representative Mossbauer spectra of the undiluted system (x = 1) as a function of temperature are displayed in Fig. 19. The spectra demonstrate that... 

The experimental methods used to study Fen spin crossovers include measurement of bulk magnetic susceptibility, vibrational spectroscopy (because M—L bond strengths differ appreciably between the HS and LS states), crystallography, and Mossbauer spectroscopy. 

An interesting case of spin transition behavior was discovered in our laboratories for the pentanuclear complex [Co(tmphen)2]3[Fe(CN)6]2 (175), which adopts a TBP structure (175, 176) (Fig. 34). Strictly speaking, this compound does not undergo spin crossover, but we consider this compound in the current section because its properties are unique among discrete cyanide-bridged clusters. As established by a combination of X-ray crystallography, Mossbauer spectroscopy. 

The Lamb-Mossbauer factor /lm, which is difficult to determine experimentally as well as compntationally, does not belong to the most common Mdssbaner 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 Mdssbaner 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-Mdssbaner factor upon spin crossover has been restricted to molecular vibrations, neglecting the important contribution that arises from intermolecular vibrations. ... 

Significant complementary data on the metal sites in proteins have also been obtained using the technique of nuclear inelastic scattering spectroscopy (NIES), also known as nuclear resonance vibrational spectroscopy. NIES probes the vibrational modes which directly involve motion of Mossbauer-active nuclei such as Fe. Thus the spectra are dominated by Fe-ligand modes associated with actual displacement of the Fe atom, such as the low frequency doming modes of heme.The NIES technique has also had a definitive role in the study of Fe(II) mono- and polymeric spin-crossover species and that work has been well reviewed recently. Spin-crossover complexes are the topic of separate discussion in Section 4 below. 

The HS LS relaxation is basically a unimolecular process, and in diluted mixed crystals corresponding relaxation curves are single exponential. In Figure 5, HS LS relaxation rate constants for several spin-crossover complexes doped into inert host lattices as well as for some LS complexes are plotted as Lhl on a log scale vs. l/T. Figure 5 includes data obtained by optical spectroscopy, by Mossbauer line shape analysis, and by Mossbauer emission. Above 50K, these curves show the classical behavior of a thermally activated process, as is expected based on the energy barrier between the two states, and in agreement with results from... 

Figure 9.12 shows the crystal structure of [Fe(pyrazine) Pt(CN)4 ] [13]. This complex shows a thermally induced spin-crossover transition (Tct = 284 K, T l = 308 K) with a thermal hysteresis of 24 K, which was observed by means of magnetic susceptibility measurement and Raman spectroscopy. The spin-crossover transition has been confirmed by Fe Mossbauer spectroscopy [13]. The Mossbauer spectrum at 300 K in the cooling mode consists of a single doublet with quadrupole splitting (QS) of 1. 159(5) mm s and isomer shift (IS) of 1.047(3) mm s whose values are typical of the HS state ( T2g, S = 2) of Fe(ll). At 80 K, a new doublet with quadrupole splitting of 0.306(4) mm s and isomer shift of 0.439(2) mm s whose values are typical of the LS state ( A g, 5 = 0) of Fe(ll). The photoinduced spin conversion between the LS and HS states around room temperature has been confirmed by means of Raman spectroscopy within the thermal hysteresis loop of spin-crossover transition, which is shown in Fig. 9.13 [13]. In this complex, the frequency of...

Figure 10.9 shows the temperature dependence of the effective magnetic moments of [Fe(OETPP)(L)2] determined by SQUID for the microcrystalline samples [45]. In the solid, spin crossover occurs not only in the Py but also in the 4-CNPy complex, which is further supported by the Mossbauer spectroscopy. Table 10.9 shows the Mossbauer parameters of [Fe(OETPP)(L)2]" and [Fe(OMTPP)(L)2] determined at ambient temperature and at 78-80 K [45,70]. Being consistent with the magnetic data, [Fe(OETPP)(4-CNPy)2]" shows two sets of doublets at 80 K assigned... 

More recently, Lemaire and coworkers observed spin crossover between S = 3/2 and S = 5/2 in nonporphyrin complexes, that is, the iron(lll) complexes of 3-ethynylthienyl-substituted N-(8-quinolyl)salicylaldimine by means of Mossbauer spectroscopy, SQUID, and EPR methods[80]. At 293 K, the Mossbauer spectrum of the PF5 complex shows two sets of doublets with very different QS values IS and QS values for one set of doublets are 0.41 and 0.66 mm s while they are 0.14 and 2.56 mm s for the other with the population ratio of 73.6 26.4. The ratio changes to 23 77 as the temperature is lowered to 5.8 K. The result clearly indicates spin crossover from S = 5/2 to S = 3/2. 

In Chapters I and 2, an introduction is made to the synchrotron Mossbauer spectroscopy with examples. Examples include the/ns/tu Mossbauer spectroscopy with synchrotron radiation on thin films and the study of deep-earth minerals. Investigations of in-beam Mossbauer spectroscopy using a Mn beam at the RIKEN RIBF is presented in Chapter 3. This chapter demonstrates innovative experimental setup for online Mossbauer spectroscopy using the thermal neutron capture reaction, Fe (n, y) Fe. The Mossbauer spectroscopy of radionuclides is described in Chapters 4-7. Chapter 4 gives full description of the latest analysis results of lanthanides Eu and Gd) Mossbauer structure and powder X-ray diffraction (XRD) lattice parameter (oq) data of defect fluorite (DF) oxides with the new defect crystal chemistry (DCC) Oq model. Chapter 5 reviews the Np Mossbauer and magnetic study of neptunyl(+l) complexes, while Chapter 6 describes the Mossbauer spectroscopy of organic complexes of europium and dysprosium. Mossbauer spectroscopy is presented in Chapter 7. There are three chapters on spin-state switching/spin-crossover phenomena (Chapter 8-10). Examples in these chapters are mainly on iron compounds, such as iron(lll) porphyrins. The use of Mossbauer spectroscopy of physical properties of Sn(ll) is discussed in Chapter I I. 

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