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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.

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The synthesis and characterization of the trifluoromethanesulfonate salt of bis(2,6-xylyl isocyanide)tetrakis(p-tolyl)porphyrinatoiron(III), 2 ]CF 3 SO 3 (1), is reported. The crystal structure shows that the porphyrinate ring is strongly ruffled. The equatorial Fe-N bond distances average to 1.961(7) Å for 1, which is quite short for low-spin iron(III) porphyrinate derivatives. Two additional complexes, having TPP (2) and m-TTP (3) as the porphyrinates, were also synthesized and studied by NMR, EPR, and Mo ¨ssbauer spectroscopy. All physical properties are consistent with a low-spin iron(III) porphyrinate with the less-common ground-state configuration (d xz ,d yz ) 4 (d xy ) 1 . The 1 H NMR chemical shift of the pyrrole protons at 297 K is +10.7 ppm for 1. The EPR spectrum of 1 in solution is axial, with g ⊥ ) 2.15 and g || ) 1.94, ∑g 2 ) 13.0, while in the solid state g ⊥ ) 2.2 and g || ) 1.94, ∑g 2 ) 13.4. The Mo ¨ssbauer spectrum of 1 at 190 K has an isomer shift of 0.14 mm/s and quadrupole splitting of 1.81 mm/s. Magnetic Mo ¨ssbauer spectra analyzed in the intermediate spin-spin relaxation regime by the dynamic line-shape formalism of Blume and Clauser confirm this electron configuration and yield large negative quadrupole splittings, ∆E Q ) -1.8 to -2.0 mm/s for the three complexes. To our knowledge, this is the first case in which the Mo ¨ssbauer spectra of low-spin ferriheme systems have been analyzed in terms of the effect of intermediate rates of spin fluctuations on the appearance of the spectra. Analysis of the temperature dependence of the quadrupole splitting, ∆E Q , for 2 yielded a different estimate of the energy separation between the (d xz ,d yz ) 4 (d xy ) 1 ground state and an excited state than did the temperature dependence of the NMR isotropic shifts. It is postulated that the excited state is actually the planar transition state between the two ruffled conformations of the porphyrinate that are related by "inversion". To explain the temperature dependence of both NMR isotropic shifts and Mo ¨ssbauer quadupole splittings, the planar transition state must have the (d xy ) 2 (d xz ,d yz ) 3 electron configuration. The energy barrier appears to be smaller in homogeneous solution than in the solid state and is considerably lower than that predicted for the (d xz ,d yz ) 3 (d xy ) 2 excited electronic state of the ruffled conformation on the basis of the EPR g values.

Mössbauer spectroscopy not only offers information about the structural and electronic properties of iron centres in biomolecules but also about their dynamic behaviour. In order to apply this nuclear method to biologically relevant iron centres knowledge of nuclear and molecular physics is required. This review introduces the basic physical concepts of 57 Fe-Mössbauer spectroscopy. The various oxidation and spin states of iron are discussed in a simple orbital frame. Aspects of the ligand field theory of paramagnetic molecules are introduced, and a review covering the iron centres in proteins which have been presently characterized is given. The review covers Mössbauer studies starting from heme proteins, continuing with the abovementioned proteins containing di-nuclear iron clusters, iron storage proteins and iron-sulfur proteins, and concludes with the complex iron clusters of nitrogenase. The physical properties of the different forms of iron centres as well as their biological relevance are elaborated upon. The effects of electronic spin dynamics on the Mössbauer spectra are reviewed. In addition, the dynamics of the iron ion itself and how it can be studied by Mössbauer spectroscopy is discussed. An introduction into the recently developed technique using synchroton radiation which is also called Mössbauer spectroscopy in the time domain shall give an impression about the future developments in this field of spectroscopy.

A procedure is presented that allows us to simulate from first principles the normalized spectra of nuclear inelastic scattering ͑NIS͒ of synchrotron radiation by molecular crystals containing a Mössbauer isotope. Neglecting intermolecular vibrations the NIS spectrum is derived from the normal modes of the free molecule, that are calculated with the density-functional method B3LYP. At low temperatures the inelastic part of the calculated NIS spectrum is a superposition of peaks that correspond to the individual vibrational modes of the molecule. The area of each peak is proportional to that part of the mean-square displacement of the Mössbauer isotope that is due to the corresponding vibrational mode. Angular-dependent NIS spectra have been recorded for a guanidinium nitroprusside single crystal and temperature-dependent NIS spectra for the spin-crossover system ͓Fe(tpa)(NCS) 2 ͔ ͓tpaϭtris͑2-pyridylmethyl͒amine͔. Qualitative agreement is achieved between measured and simulated spectra for different crystal orientations of guanidinium nitroprusside. A remarkable increase of the iron-ligand bond stretching upon spin crossover has unambiguously been identified by comparing the measured NIS spectra of ͓Fe(tpa)(NCS) 2 ͔ with the theoretical simulations.