[en] A program, written for the calculation of Moessbauer spectra of ⁵⁷Fe in the presence of relaxation effects in magnetically ordered polycrystalline materials has been extended to treat the case of an applied external magnetic field parallel or perpendicular to the gamma rays. The program uses the Liouville operator formalism with the usual nuclear spin Hamiltonian and an electronic spin Hamiltonian including axial and rhombic distortions in the general case of non coincident crystal field and e.f.g. principal axis systems. Model calculations with typical values of the spin Hamiltonian parameters for S=3/2 were obtained within reasonable computer times. The calculation has been applied to two cases of Fe³⁺ S=3/2 paramagnets. It was shown that satisfactory fits to the experimental spectra could be obtained only with the introduction of relaxation effects

[en] Crystalline choline chloride is known to exhibit sensitivity to ionizing radiation. Measurements are reported for two other choline salts choline bromide and iodide on a differential scanning calorimeter and are extended to low temperatures to elucidate the radiation sensitivity of these salts

[en] The organic salt choline chloride exists in two crystalline polymorphs. One (α-form) which is present at ambient temperature, is extraordinarily sensitive to ionizing radiation. The other (β-form), which exists above 78⁰C, is radiation stable. The present work shows that radiation sensitivity increases rapidly above 25⁰C, but reaches a maximum around 65⁰C, well below the α- to β- crystal phase transition. This behavior is correlated with the onset of various rotational motions that are indicated by recent NMR studies. It has also been found that H atom irradiations of the β-form, and of the radiation stable choline iodide, produce as much decomposition as in the α-form, a result that is in accord with previously published theories of the mechanism of the α-form's unique radiolysis. (author)

[en] The organic salt choline chloride exists in two crystalline polymorphs. One (the α form) is extraordinarily sensitive to ionizing radiation, the other (the β form) is not. The present report describes an x-ray diffraction study of the β form. The structure has been found to be highly disordered face centered cubic. A reasonable least-square refinement of the intensity data has been achieved in the centrosymmetric space group Fm3 or Fm3m by use of a molecular model with restrained bond lengths. The results show that in the β form the electronic density due to the choline cation is closely spaced around the N, so that hydrogen bonding to the chloride is unlikely. Comparison with infrared and NMR data indicates that the disordering is dynamic and can be ascribed to rotations of the choline ion around crystallographic symmetry axes. Possible connections of these results with the radiation stability of the β form are discussed

[en] Measurements of three high-spin Fe^IIS₄ complexes at various temperatures and applied magnetic fields are reported and the results are compared with those of reduced rubredoxin. Simulations of the measured spectra by means of the fully developed stochastic theory of lineshape prove that intermediate relaxation behavior is present and is determined essentially by the phonon energy distribution and by one single parameter W₀, the scaling factor, which contains the strength of spin-lattice coupling. (Auth.)

[en] The authors report the results of a Moessbauer study of the low-potential iron-sulfur cluster F_X in the Photosystem I core protein of Synechococcus 6301. The Moessbauer spectrum of F_X in the oxidized state shows an isomer shift of 0.42 mm/s, which is in good agreement with the 0.43 mm/s isomer shift found in [4Fe-4S] proteins but not with the isomer shift of 0.26 mm/s found in [2Fe-2S] proteins. In the reduced state the spectrum is asymmetrically broadened at 80 K, indicating the presence of two very closely spaced doublets with an average isomer shift of 0.55 mm/s, which is also in agreement with [4Fe-4S] proteins. At 4.2 K, the spectrum exhibits broadening and magnetic splitting similar to what is observed for [4Fe-4S] proteins and quite unlike [2Fe-2S] proteins. Given the assumption that the iron atoms of F_X are tetrahedrally coordinated with sulfur ligands, the data strongly support the assignment of F_X as a [4Fe-4S] cluster