Distance modulation spectroscopy

Finally, two potential models for the Mo and Fe sites of sulfite oxidase have been prepared in which a hydrotris(3,5-dimethylpyrazolyl)borate complex of oxomolybdenum(V) is covalently attached to one of the phenyl rings of a tetraphenylporphinatoiron(III) derivative in which three phenyl rings carry / -Me groups and the other is either a 2,3- or 3,4-catecholate. Although the metals are not directly bridged in these complexes, they are held at fixed Mo - Fe distances. EPR spectroscopy at 4K shows distance-dependent static dipolar coupling between the Mo and Fe centers. At 77 K the dipolar coupling is modulated by rapid relaxation of the Fe center. Coordination of two A-methylimidazoles to the Fe centers produce S = l/2)Fe-(5 = 1/2)mo coupled systems that are not only... 

Figure 6. Interfacially confined hexadecane measured by SFM shear modulation spectroscopy. The shear response is measured simultaneously with the normal force deflection of the cantilever as function of the cantilever-siicon sample distance. The difference in the bending onset of the two curves defines the interfacially confined boundary layer thickness.

Microwave spectroscopy is a powerful tool for the determination of molecular structure. Thiazoles and thiadiazoles have been studied by this technique, but it was not until 1976 that a paper on the microwave spectrum of 1,2,3-thiadiazole appeared. Bond distances and angles for 1,2,3-thiadiazole (7) are listed in Table 4 (76MI42400). The success of this project is owed in part to the development of double resonance modulated (DRM) microwave spectroscopy which allows for quick analysis of an individual spectrum. 

Another valuable tool is extended x-ray absorption fine structure (EXAFS) spectroscopy. This absorption technique makes use of the modulation in absorption coefficient above an element absorption edge, due to backscatterlng of the ejected photoelectron by neighboring atoms. By suitable data treatment, it is possible to determine the elemental type, number, and distance of the atoms coordinated to the absorbing cation. Thus, EXAFS probes the atomic-scale structure in the material, while SAXS and ASAXS probe the microdomain-scale structure. By using both scattering and absorption techniques, a full picture of ionomer morphology may be developed. 

Electron diffraction is the other of the two important sources of gas-phase structural data. As discussed by Hedberg (this volume), the intensity of electrons scattered by molecules is modulated by the interatomic distances, both bonded and nonbonded. Since interatomic distances enter explicitly into electron diffraction determinations, the method is in some ways more direct than spectroscopy. Moments of inertia are functions of Cartesian coordinates of individual atoms rather than distances between atoms. On the other hand, electron diffraction is much more susceptible to experimental error than spectroscopic techniques.15 Problems with structural determinations by spectroscopic methods often stem almost entirely from model error, whereas in electron diffraction both experimental and model error are important. Experimental and model error in electron diffraction are discussed elsewhere in this volume by Hedberg, and we shall confine ourselves here to definitions of the various structural parameters that arise in electron diffraction studies and the relationships among them and spectroscopic quantities. 

A determination of the complete structure of the four thiadiazoles by double resonance modulation microwave spectroscopy has provided precise information of the bond distances and angles of each isomer. The results show that closest comparability exists between 1,2,4- and 1,2,5-thiadiazole on the one hand, and between the 1,2,3- and 1,3,4-isomers on the other.271... 

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