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Nuclear inelastic scattering measurements were performed on a guanidium nitroprusside ((CN3H6) (Sub 2) [Fe(CN)(Sub 5)NO], GNP) monocrystals at 77 K after the sample was illuminated with blue light at 50 K to populate the two metastable states, MS1 and MS2, of the nitroprusside anion. Comparison of experimental nuclear inelastic scattering spectra, which have been recorded under conditions excluding and including the metastable MS1 state, with simulated NIS spectra using density functional theory, strongly supports the isonitrosyl structure of the metastable MS1 state as proposed by

A correlation between data from Mossbauer quadrupole splittings and NMR is helpful in detecting the 57Fe NMR signals for valence electron cloud asymmetry. The 11 197 ppm chemical shift reported in literature for ferrocytochrome, c20, which is based on Mossbauer-NMR correlation, is acceptable. This correlation also indicates chemical shifts of 11 000 to 13 000 ppm for most bis-pyridine and nonhindered imidazole complexes of model heme and cytochrome b5.

The syntheses of the ferric and ferrous forms of the (acetato)iron 'pivalimide-picket-fence' porphyrin are subjected to structural, Mossbauer and EPR analyses. Specifically, the ferrous form is subjected to x-ray diffraction. Results indicate that there is a large positive quadropole splitting in the ferrous form of the acetato-porphyrinato complex. The crystal-field model is also used to explain the effect of distortions in one of the synthesis.

A set of iron (II) complexes that exhibit a temperature dependent spin-crossover between he low-spin and the high-spin is examined with the help of density functional methods. While the calculated total energy differences between the two spin states have significant errors, qualitative agreement is achieved for the zero-point viibrational energy differences and the entropy differences.

Crystalline samples of four low-spin Fe(III) actaalyltetraphenylprophyrinate and two low-spin Fe(III) tetramesitylprophyrinate complexes are investigated in detail by Mossbauer spectroscopy. Results reveal that relationship exist between increasing dihedralangle of the axial ligand planes, from 0 degree to 90 degree and EPR and Mossbauer spectroscopic properties.

The spin cross over relaxation for [Fe(btpa)](Pf (sub 6))(sub 2) and the analogous system, [Fe(b(bdpa))]-(PF(sub 6))(sub 2), where the uncorrelated pyridyl groups are replaced by benzyl groups by using the laser photopertubaration method coupled with time-resolved resonance Raman spectroscopy (TR sub 3) is explained. It is found that [Fe(btpa)](Pf (sub 6))(sub 2) complex contains one-high spin and one low-spin molecule in the unit cell is differing significantly in their molecular geometries.

In this paper we present the characterization by UV-VIS, Mössbauer, and EPR spectroscopy of [L2Fe2(μ-OH)3](ClO4)2·2CH3OH·2H2O, with L=N,N’,N‘-trimethyl-1,4,7-triazacyclononane, a novel dimeric iron compound, which is shown to possess a central exchange-coupled delocalized-valence Fe(II/III) pair. Complete delocalization of the excess electron in the dimeric iron center is concluded from the indistinguishability of the two iron sites in Mössbauer spectroscopy. Mössbauer, EPR, and magnetic susceptibility data imply a system spin St =9/2 for the ground state. This finding is explained as being a consequence of the double-exchange interaction which is generated by the delocalized electron. Experimental values obtained from UV-VIS, Mössbauer, and EPR spectroscopy are for the double-exchange parameter B=1300 cm-1, the g factors gx,y =2.04 and gz =2.3, the parameters for zero-field splitting D=4 cm-1 and E≊0 cm-1, and for the hyperfine parameters ΔEQ =-2.14 mm s-1, Ax,y =-21.2 T, Az =-27 T, and δ=0.74 mm s-1. From our temperature-dependent studies we assign to the first excited state a spin-octet with an excitation energy Δ>175 cm-1. From this value a lower bound of -235 cm-1 has been deduced for the exchange-coupling constant J. In the framework of a simplified description of the iron atoms by unperturbed 3d orbitals, the values of the A tensor components as well as the quadrupole splitting ΔEQ can be interpreted in a consistent manner by assuming the excess electron being delocalized over two dσ orbitals centered at the two iron sites of the dimer and directed along the iron–iron axis as the z direction. [ABSTRACT FROM AUTHOR]

Research is presented describing the low-spin ground state characterization of tetraphenylporphyrinate compounds.

The interaction of the antitumor agent daunomycin (DN) with ferric iron has been analysed by Mōssbauer spectroscopy, EPR, extended X-ray absorption fine structure (EXAFS), and magnetic susceptibility measurements. In contrast to literature data, at millimolar iron and anthracycline concentrations no solitary Fc(DN)3 complexes are formed in appreciable amounts. The Mössbauer spectroscopic analysis revealed severe dependencies on temperature, on the preparation procedure, the time allowed for equilibration, and on the metal/ligand ratio. The Mössbauer spectra exhibit two components: a broad magnetic sextet and a quadrupole doublet at an Fe/DN molar ratio of 1:3 and exclusively a doublet at a molar ratio of 1:20, indicating an equilibrium of these two spectral components. The EPR spectra are dominated by a signal at geff = 2. Double integration of the EPR signals enabled the determination of their spin density and a correlation between EPR and Mössbauer spectra. The Mössbauer sextet species is EPR invisible and corresponds to magnetically ordered polynuclear aggregates with high magnetic anisotropy. EXAFS and susceptibility measurements provide additional evidence for the formation of polynuclear aggregates of ferric daunomycin. The quadrupole doublet species in the Mössbauer spectra correlates with the g = 2 signal in EPR. This species is also related to a magnetically ordered system, exhibiting, however, superparamagnetic behavior due to less magnetic anisotropy. Since daunomycin forms dimers in aqueous solution at millimolar concentrations, we conclude that the cooperative phenomena observed in EPR and Mössbauer spectra are a consequence of its stacking effects. [ABSTRACT FROM AUTHOR]

Oxoferrylporphyrin cation radical complexes were generated by m-chloroperoxybenzoic acid oxidation of the chloro and trifluoromethanesulfonato complexes of tetramesitylporphyrinatoiron(III) [(TMP)Fe] and the trifluoromethanesulfonato complex of tetra(2,6-dichlorophenyl)porphyrinatoiron(III) [TPP(2,6-Cl)Fe]. Coupling between ferryl iron (S = 1) and porphyrin radical (S' = 1/2) spin systems was investigated by Mössbauer and EPR spectroscopy. The oxoferrylporphyrin cation radical systems generated from the TMP complexes show strong ferromagnetic coupling. Analysis of the magnetic Mössbauer spectra, using a spin Hamiltonian explicitly including a coupling tensor J, suggests an exchange-coupling constant J > 80 cm-1. The EPR spectra show non-zero rhombicity, the origin of which is discussed in terms of contributions from the usual zero-field effects of iron and from iron-radical spin-dipolar interaction. A consistent estimate of zero-field splitting parameter D ≈ + 6 cm-1 was obtained by EPR and Mössbauer measurements. EPR and Mössbauer parameters are shown to be slightly dependent on solvent, but not on the axial ligand in the starting (TMP)Fe complex. In contrast to the TMP complex, the oxoferrylporphyrin cation radical system generated from [TPP(2,6-CI)FeOSO2CF3] exhibits Mössbauer and EPR spectra consistent with weak iron-porphyrin radical coupling of J ≈ 1 cm-1. [ABSTRACT FROM AUTHOR]

The catalytic Zn(II) ion of horse liver alcohol dehydrogenase (EE isozyme) was replaced by Fe(II), providing a novel iron protein with the unusual coordination of Fe(II) to two cysteines residues, one histine residue and water. The electronic structure of iron in this system was characterized by Mrssbauer spectroscopy at various temperatures as well as applied magnetic fields and analysed in terms of the spin Hamiltonian formalism. The novelty we found is an unusually weak spin coupling (¦J¦ < 0.1 cm-1) of a paramagnet (S = 1) with iron (S = 2). From EPR and biochemical studies we conclude that the corresponding chemical species is triplet oxygen (O2). The quantitative determination of the coupling energy was possible utilizing the competition between Zeemann interaction and spin coupling at weak magnetic fields and low temperature. Oxidation experiments followed by Mössbauer spectroscopy showed that the spin-coupled system is an outer-sphere Fe(II)·(O2)aq complex occurring as an intermediate during a Fe(II)-catalyzed dioxygen activation. We observed two additional Fe(II) species after treatment with O2 and dithionite. The spin Hamiltonian parameters of iron in the coupled system are presented. The results are compared with those of iron in other non-heme iron proteins. [ABSTRACT FROM AUTHOR]

Two dioxygen adducts of thiolato‐iron(II) porphyrins, [K(⊂ 222)][Fe(TPpivP)(SC6HF4)(O2)] 1a and [Na(⊂ 18c.6)][Fe(TPpivP)(SC6HF4)(O2)] 2 were synthesized by reaction of O2 with five‐coordinate, high‐spin, cryptated alkali metal thiolato‐iron(II) ‘picket fence’ porphyrinate. They were characterized by visible and infrared spectroscopy: λmax (log ɛ) = 360 nm (4), 427 nm (4.69), 560 nm (3.69), 610 nm (3.40) for both compounds; v(16O‐16O) = 1139 cm−1 in chlorobenzene and fluorobenzene for 1a and 2. Single crystals of composition [K(⊂ 222)][Fe(TPpivP)(SC6HF4)(O2)] · [K(⊂ 222)](SC6HF4)(C6H5Cl)(H2O) 1b were obtained by diffusion of pentane/xylene mixtures into chlorobenzene solutions of 1a at – 5°C. Single crystals of composition [Na(⊂ 18c.6)][Fe(TPpivP)(SC6HF4)(O2)] were obtained by slow diffusion of pentane into benzene solutions of 2. Structures of 1b and 2 were studied at 20°C (1b) and – 100°C (1b and 2). 1b: space group P21/c (monoclinic), a= 16.806(5) Å (1.6806 nm), b= 14.331(4) Å (1.4331 nm), c= 52.000(15) Å (5.2000 nm), β= 92.95(2)°, V= 12507 Å3 (12.507 nm3), Z= 4, Dcal= 1.28 g · cm−3 (t= 20°C). The final R1 factor was 0.085 for 5238 reflections having I > 3 σ(I). 2: space group P21/c (monoclinic), a= 13.107(3) Å (1.3107 nm), b= 27.055(4) Å (2.7055 nm), c= 25.029(4) Å (2.5029 nm), β= 96.84(2)°, V= 8812 Å3 (8.812 nm3), Z= 4, Dcal= 1.18 g · cm−3 (t=−100°C). The final R1 factor was 0.088 for 6587 reflections having I > 3 σ(I). The iron atom is, in both compounds, bonded to the four porphyrinato nitrogens (Np), the sulfur atom of the axial thiolate and one oxygen atom of the axially end‐on bonded dioxygen molecule. The average Fe‐Np distance found in 1b [1.994(4) Å, 0.1994 nm] is not significantly different from that found in 2 [1.993(3) Å, 0.1993 nm]. The Fe‐S bond length is 2.367(3) Å (0.2367 nm) in 1b and 2.365(2) Å (0.2365 nm) in 2. The Fe‐O1 distances with the oxygen atom of O2 bonded to iron are respectively 1.837(9) Å (0.1837 nm) and 1.850(4) Å (0.1850 nm). The end‐on bonded O2 molecule is disordered in both complexes 1b and 2. This disorder is, however, slightly different from one structure to the other. Whereas the terminal oxygen atom of O2 occupies two positions in 1b [τ(O2A) = 78%, τ(O2B) = 22% at 20°C], it occupies three sites in 2 [τ(O2A) = 51%, τ(O2B) =τ(O2C) = 24.5% at – 100°C] (τ= occupancy). The estimate of the magnitude of the non‐bonded interactions between O2 and its environment located in a sphere of 7‐Å (0.7‐nm) radius in 1b and 2 is consistent with a static disorder of O2 in 1b and a dynamic distribution in 2. Mössbauer measurements were performed with polycrystals of 1b and 2 in the temperature ranges 5 – 197 K and 4.2 – 295 K, respectively, and under externally applied magnetic field (6.76 T) at 4.2 K (1b and 2) and at 173 K (2). The temperature variation of the Lamb‐Mössbauer factor indicates that the iron atom in these oxycytochrome P450 models is not very firmly bonded, more like that in pentacoordinated than in other hexacoordinated hemes. From the specific temperature behavior of quadrupole splitting, asymmetry parameter and line width obtained for 2, the structural disorder of the terminal oxygen atom of O2 is identified as dynamic distribution over three different sites. Corresponding values obtained for 1b are consistent with a static disorder of O2. Magnetic spectra reveal that the oxycytochrome P450 models are purely diamagnetic. [ABSTRACT FROM AUTHOR]

Mononuclear non-heme cofactor iron of putidamonooxin has been investigated in the binary oxidized ‘enzyme · substrate’ complex and in the ternary ‘enzyme · substrate · NO’ complex via Mössbauer spectroscopy. The experimental spectra were analyzed on the basis of the spin-Hamiltonian formalism. The resulting fine and hyperfine structure parameters are compared with literature values of similar compounds. From this comparison we conclude that in the binary complex (reduced and oxidized) the mononuclear non-heme cofactor iron has a coordination number higher than four. Additionally, the cofactor iron shows remarkable spectral similarities with iron in protocatechuate 3.4-dioxygenase, though the catalytic properties of the iron sites in the two proteins are different. The data obtained form the ternary ‘enzyme · substrate · NO’ complex indicate that the cofactor iron (a) is in the ferric intermediate spin state (S = 3/2) and (b) is pentacoordinated, which means that upon NO binding to the reduced cofactor iron at least one ligand has to be released. Comparing our data with literature values suggests that the cofactor iron in the binary as well as in the ternary NO complex is not directly bound to a sulfur atom, though biochemical arguments seem to indicate the opposite. [ABSTRACT FROM AUTHOR]

Crystalline samples of four low-spin Fe(III) octaalkyltetraphenylporphyrinate and two low-spin Fe(III) tetramesitylporphyrinate complexes, all of which are models of the bis-histidine-coordinated cytochrorres of mitochondrial complexes II, III, and IV and chloroplast complex b6 f, and whose molecular structures and EPR spectra have been reported previously, have been investigated in detail by Mössbauer spectroscopy. The six complexes and the dihedral angles between axial ligand planes of each are [(TMP)-Fe(1-Melm)2]CIO4 (0°), paral-[(OMTPP)Fe(1-Melm)2]Cl (19.5°), paral-((TMP)Fe(5-MeHlm)2]ClO4 (26°, 30° for two molecules in the unit cell whose EPR spectra overlap), [(OETPP)Fe(4-Me2NPy)2]Cl (70°), perp-[(OETPP)Fe(1-Melm)2]Cl (73°), and perp-[(OMTPP)Fe(1-Melm)2]Cl (90°). Of these, the first three have been shown to exhibit normal rhombic EPR spectra, each with three clearly resolved g-values, while the last three have been shown to exhibit "large gmax" EPR spectra at 4.2 K. It is found that the hyperfine coupling constants of the complexes are consistent with those reported previously for low-spin ferriheme systems, with the largest-magnitude hyperfine coupling constant, Azz, being considerably smaller for the "parallel" complexes (400-540 kG) than for the strictly perpendicular complex (902 kG), Axx being negative for all six complexes, and Azz and Axx being of similar magnitude for the "parallel" complexes (for example, for [(TMP)Fe(1-Melm)2]Cl, Azz = 400 kG, Axx = -400 kG). In all cases, Ayy is small but difficult to estimate with accuracy. With results for six structurally characterized model systems, we find for the first time qualitative correlations of gzz, Azz and ΔEQ with axial ligand plane dihedral angle Δ. [ABSTRACT FROM AUTHOR]

The vibrational modes of the low-spin and high-spin isomers of the spin crossover complex [Fe(phen)(2)(NCS)(2)] (phen = 1,10-phenanthroline) have been measured by IR and Raman spectroscopy and by nuclear inelastic scattering. The vibrational frequencies and normal modes and the IR and Raman intensities have been calculated by density functional methods. The vibrational entropy difference between the two isomers, DeltaS(vib), which is--together with the electronic entropy difference DeltaS(el)--the driving force for the spin-transition, has been determined from the measured and from the calculated frequencies. The calculated difference (DeltaS(vib) = 57-70 J mol(-1) K(-1), depending on the method) is in qualitative agreement with experimental values (20-36 J mol(-1) K(-1)). Only the low energy vibrational modes (20% of the 147 modes of the free molecule) contribute to the entropy difference and about three quarters of the vibrational entropy difference are due to the 15 modes of the central FeN(6) octahedron.