Rigid molecular structure

NMR measurements can distinguish hydrogen In rigid molecular structures of coals, l.e. structures that do not undergo appreciable reorientation and/or translation during time Intervals < 10 s, from hydrogen In mobile structures which possess more rapid molecular motions characteristic of fused or rubbery materials. 

The data provided by these measurements are presented In this paper In terms of a parameter that measures the extent and degree of molecular mobility. M2J is an empirical second moment of the frequency spectrum of the NMR signal (25), truncated here at 16 kHz (26), and Is Inversely related to the average molecular mobility of the specimen. Thus the relative decrease In 2 Is a sensitive measure of the extent and degree of mobility acquired by rigid molecular structures as a result of their destabilization by thermal or solvent treatment. 

Solutions of the aromatic polyamides (PpBA, PpPTA and PmPTA), the polybenzazoles (PBT and PBO), poly(benzyl glutamate) (PBG) and hydroxypropylcellulose (HPC) are the most studied main chain lyotropic systems and our understanding of the behaviour of lyotropics is based on investigations of this relatively small number of materials (Moldenaers, 1996). They form main chain liquid crystals because of their rigid molecular structure in the appropriate solvents. Two kinds of solvents are used (Collyer, 1996) ... 

Certain radicals have rigid molecular structures with fixed bond angles and dihedral angles. These are known as bridge head radicals and have pyramidal structure. This has been supported on the basis of physical and chemical evidences. 

The problem is a general one of placing a rigid molecular structure in an electron density map in the most objective fashion. There are a number of different types of such problems in protein crystallography. One particular instance is the placement of small substrate (or equally well an inhibitor) structures in poor electron density maps of protein binding sites. Another case is when homologue structures are used in the initial location of new protein structures within the unit cell of poor electron density maps. The location of a known structure within a poor electron density map from a different space group is yet another variant. 

Centropentaindane 10 is a high-melting compound bearing two residual bridgehead C-H bonds. Similar to tri-/hso-tetraindane 8, centropentaindane has a rigid molecular structure. The presence of two (mutually fused) tribenzo-triquinacene units in 10 gives rise to an almost perfect C2v-symmetrical conformation. From another viewpoint, the benzo bridge across the fenestrindane backbone of 10 fixes its conformation in an almost perfect D2d-symmetrical skeleton, as confirmed by X-ray structure analysis [102]. 

Besides, the actual distance existing between two vital functional moieties may be almost fixed arbitrarily in rigid molecular structural variants. These restructured and strategically positioned newer targetted-drug molecules may be subjected to vigorous and critical examinations by the aid of several sophisticated latest physicochemical analytical devices, such as X-Ray diffraction analysis Optical Rotary Dispersion (ORD) NMR-spectroscopy Mass Spectroscopy FTIR-Spectrophotometry and the like. 

Entropic contributions, one source of the inequality of AH(X) and AG(X), are virtually eliminated by comparing rigid molecular structures of similar size (3). Application of Eqn. 1 in this system allows the direct resolution of specific in situ interaction energies because a) aqueous solvation contributions are nearly eliminated in the hexane system (3), and b) hexane can interact favorably with ligands only through weak, relatively non-specific dispersion forces. 

In the following two subsections, some of the main features of the adsorption of HPAM and xanthan in porous media will be outlined. It is convenient to separate the discussion of these two polymer types because of their structural differences, as explained earlier in this work. HPAM is a flexible coil polyelectrolyte and, for the reasons discussed previously, shows much more sensitivity to solution conditions, pH, salinity, etc., than xanthan, which has a more rigid molecular structure. 

A characteristic of LCP molecular orientation is their anisotropy -a natural consequence of having such a rigid molecular structure. This polymer structure is exploited for the development of high material extension and chain orientation in a fibre-extrusion process. A conventional polymer (e.g. PET) is assumed to be in a random coil con-... 

---