Diffraction molecular conformation

The melting points, optical rotations, and uv spectral data for selected prostanoids are provided in Table 1. Additional physical properties for the primary PGs have been summarized in the Hterature and the physical methods have been reviewed (47). The molecular conformations of PGE2 and PGA have been determined in the soHd state by x-ray diffraction, and special H and nuclear magnetic resonance (nmr) spectral studies of several PGs have been reported (11,48—53). Mass spectral data have also been compiled (54) (see Mass spectrometry Spectroscopy). 

Keywords Smectic liquid crystals, Molecular conformations, polar and steric frustrations, Polyphilic and perfluorinated mesogens. X-ray diffraction... 

Chiu, N. S., H. L. Sellers, L. Schafer, and K. Kohata. 1979. Molecular Orbital Constrained Electron Diffraction Studies. Conformational Behavior of 1,2-Dimethylhydrazine. J. Am. Chem. Soc. 101, 5883-5889. 

Key words gas diffraction, molecular geometry, conformational equilibria, electron diffraction... 

First of all, we analyzed the 2-dimensional structure projected on the ab-plane in order to confirm the result of the analysis by electron diffraction. The 17 independent reflections on the equator were used (R=35%, B=0.075nm (isotropic)). The molecular conformation of the p-form was adopted because it lowered R. The mutual positions and orientations of molecular chains are almost identical to those analyzed by electron diffraction. In the 3-dimensional analysis, therefore, this structure in the aft-plane projection was basically fixed. 

Molecular conformation is highly related to functional properties. Since the conformation of the crystalline solids can be precisely determined by diffraction methods, molecular modeling is most important for interpreting molecular structures in solution. This is, however, even more difficult for theoreticians. While carbohydrates dissolve in a variety of solvents, the important solvent for biological systems is water and this solvent deserves special emphasis. 

Molecules in the gas phase provide an electron diffraction pattern which can be analyzed in order to obtain relative interatomic distances in molecules. Some of the distances depend on molecular conformation and, in principle, it is possible to extract conformational data (conformer structure and population). Rough estimates of energy barriers may also be obtained from the peak widths by comparing calculated and experimental distribution functions. Uncertainties on populations are rather high ( 10-15%). 

These changes are generally considered to be first-order crystalline transitions. The molecular conformation below and a-bove the 19° C transition are shown in drawings of molecular models in Fig.3 and the changes in the j I diffraction patterns of crystalline fibers at these transitions are illustrated in Fig. 4. Below 19° C the... 

The determination of crystal structure in synthetic polymers is often made difficult by the lack of resolution in the diffraction data. The diffuseness of the reflections observed in most x-ray fiber patterns results from the small size and imperfect lattice nature of the polymer crystallites. Resolution of individual reflections is also made difficult from misorientation of the crystallites about the fiber axis. This lack of resolution leads to poor accuracy in measurement of peak positions. In particular, this lack of accuracy makes determination of layer line heights difficult with a corresponding loss of significant figures in evaluation of the repeat distance for the molecular conformation. In the case of helical conformations, the repeat distance may be of considerable length or, as we shall show, indeterminate and, in effect, nonperiodic. This evaluation requires high accuracy in measurements of layer line heights. 

In this paper we examine electron diffraction fiber patterns of the homopolymer polytetrafluoroethylene (-CF2 CF2-)n PTFE, in which the resolution is sufficient to yield much more accurate values of layer line heights than were available from the previous x-ray diffraction experiments (1) on the crystal structure of Phase II, the phase below the 19°C transition (2). On the basis of x-ray data, the molecule was assigned the conformation 13/6 or thirteen CF2 motifs regularly spaced along six turns of the helix. This is equivalent to a 132 screw axis. The relationship between the molecular conformation and the helical symmetry has been studied by Clark and Muus (3) and is illustrated in Figure 1. The electron diffraction data of high resolution enabled us to determine if this unusual 13-fold symmetry was exact or an approximation of the true symmetry. We have also... 

A convenient method for defining helical symmetry and calculating the distribution of Intensity In a fiber pattern was devised by Cochran, Crick and Vand (CCV) (4). As Indicated in Figure 1, the molecular conformation is treated as a regular series of diffraction units uniformly spaced along a helix of pitch, P, with axial separation, s. In PTFE, one helix defines the carbon positions two helices define fluorine positions. If there is a meaningful translational Identity, c, It follows that P/s will be the ratio of small whole numbers ... 

Both in theory and in practice there exist eight gluco-pyranose homopolymers, and some of the molecular conformations of three of these, i.e. cellulose and amylose (l.,2., 3,4), and (1+3)-8-D-glucan (5.,6.,.7) have been established by x-ray analysis. Although (1+3)-a-D-glucan is among the five homopolymers previously unsolved by x-ray diffraction, possible chain conformations were predicted with computers to be an extended ribbon (8.,9.) a single helix (9.), or a double or triple helix (10). 

Clark ES, Weeks JJ, Eby RK (1980) Diffraction from non-periodic structures, the molecular conformation of polytetrafluoroethylne (phase II). In French AD, Gardner KH (eds) Fiber diffraction ACS symposium series 141. American Chemical Society, Washington, DC, p 183... 

The molecular structures of 7a (99JOC3113), 38, and 39 (98JST55) have been determined by X-ray single crystal diffraction. The conformations of both 38 and 39 in the crystalline phase are mainly determined by the properties of the C-bonded Cl atoms. These substituents are found in a more or less orthogonal position relative to the pyridinium ring. The acidity of the methylene hydrogens promotes the formation of the intermolecular C-H... Cl hydrogen bonds in 38 and 39. Further specific properties are exemplified for 7a (Scheme 12). 

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