Electronic, Magnetic, and Vibrational Properties

Summary of EPR -Values, Fg-Hyperfine Coupling Constants, Isomer Shifts, and Quadrupole Splittings for Some Representative [Fg3S4] Clusters 

NMR is the only technique capable of assigning the cysteines ligating specific Fe atoms, since the asymmetric coupling of the three Fe + ions results in different temperature dependence for the contact shifts 

The multifrequency EPR and Mdssbauer properties of the [FesSJ in C. vinosum NiFe-hydrogenase are particularly interesting since they provide evidence of magnetic interactions with nearby paramagnetic species (151, 154, 155). The magnetically isolated form exhibits a well-resolved, almost axial EPR signal, g = 2.018, 2.016, 2.002, indicative of minimal conformational heterogeneity. However, a com- 

The relatively broad and featureless absorption spectra of [Fe3S4l clusters belies their complex excited-state electronic structure. This is illustrated in Fig. 5 using P. furiosus 3Fe Fd as an example (42). In addition to the protein band centered at 280 nm, the UV-visible 

Vibrational Frequencies (cm ), Isotope Downshifts (cm in Parentheses), and Assignments for the Fe-S Stretching Modes of Oxidized FeaSJSa Centers in Beef Heart Aconitase, a. vinelandii Fdl, T. thermophilus Fd, P. furiosus Fd, D. gigas Fdll, and 

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The electronic, magnetic, and vibrational properties of [2Fe-2S] + + clusters have been extensively characterized using the combination of absorption, CD, VTMCD, EPR, ENDOR, Mbssbauer, NMR and resonance Raman spectroscopies, and the ground-state electronic properties are summarized in Figure 3. In the oxidized state, the two 5 =5/2 Fe ions in the [2Fe-2S] + cluster are antiferromagnetically coupled (/ 360 cm for plant-type Fds and >550 cm for... 

Considerable exploratory work on amorphous and liquid semiconductors was done by the Leningrad School since the early fifties. In recent years, much research in several countries was directed to deepen the understanding of the structural, electronic, optical, vibrational, magnetic and other properties of these materials and to possibly approach the present level of understanding of crystalline semiconductors. This effort was stimulated not only by purely scientific interest but also by the possibility of new applications from which memory devices in the general sense are perhaps the most challenging. 

The second part contains the theoretical calculation of different properties of polymers based on the methods systematically introduced in the first part. The properties calculated include the electronic and vibrational spectra of polymers, and the computation of their transport, magnetic, and mechanical properties. In cases where reliable experimental data are available, the theoretical results are compared with them. 

NiS Vibrational, magnetic, and electronic properties in semimetalhc antiferromagnetic and in metallic phase phase transition study... 

An electric dipole operator, of importance in electronic (visible and uv) and in vibrational spectroscopy (infrared) has the same symmetry properties as Ta. Magnetic dipoles, of importance in rotational (microwave), nmr (radio frequency) and epr (microwave) spectroscopies, have an operator with symmetry properties of Ra. Raman (visible) spectra relate to polarizability and the operator has the same symmetry properties as terms such as x2, xy, etc. In the study of optically active species, that cause helical movement of charge density, the important symmetry property of a helix to note, is that it corresponds to simultaneous translation and rotation. Optically active molecules must therefore have a symmetry such that Ta and Ra (a = x, y, z) transform as the same i.r. It only occurs for molecules with an alternating or improper rotation axis, Sn. 

Molecular dynamics (MD) simulations fill a significant niche in the study of chemical structure. While nuclear magnetic resonance (NMR) yields the structure of a molecule in atomic detail, this structure is the time-averaged composite of several conformations. Electronic and vibrational circular dichroism spectroscopy and more general ultraviolet/visible and infrared (IR) spectroscopy yield the secondary structure of the molecule, but at low resolution. MD simulations, on the other hand, yield a large set of individual structures in high detail and can describe the dynamic properties of these structures in solution. Movement and energy details of individual atoms can then be easily obtained from these studies. 

Solvent can affect the electronic structure of the solute and, hence, its magnetic properties either directly (e.g. favouring more polar resonance forms) or indirectly through geometry changes. Furthermore, it can influence the dynamical behaviour of the molecule for example, viscous and/or oriented solvents (such as liquid crystals) can strongly damp the rotational and vibrational motions of the radical. Static aspects will be treated in the following, whereas the last aspect will be tackled in the section devoted to all the dynamical effects. 

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