Flexible bonds

Two-component systems consist of (1) polyol or polyamine, and (2) isocyanate. The hardening starts with the mixing of the two components. Due to the low viscosities of the two components, they can be used without addition of solvents. The mass ratio between the two components determines the properties of the bond line. Linear polyols and a lower surplus of isocyanates give flexible bond lines, whereas branched polyols and higher amounts of isocyanates lead to hard and brittle bond lines. The pot life of the two-component systems is determined by the reactivity of the two components, the temperature and the addition of catalysts. The pot life can vary between 0.5 and 24 h. The cure at room temperature is completed within 3 to 20 h. 

In previous studies (56,57) a bilayer was modeled with atomic detail of the hydrocarbon chain, including the proper dihedral angle functions and using flexible bond angles, but the aqueous layer was not represented. 

A second type of solid phase is also possible, and commonly found in pharmaceutical systems. In this case, molecules in the solid phase are randomly distributed like those of the liquid phase, and do not posses long range order. A solid of this type is called amorphous or glassy, and is often found in organic molecules that have several flexible bonds. 

Crystal interactions may lead to a number of readily observable consequences. The molecular geometries may be different in different polymorphic forms of a given compound or, as in the case above, in symmetry-independent sites in a given crystal a change of molecular substituents remote from the torsionally flexible bonds may change the molecular conformation, as a result of modification... 

Molecular mechanics is the modeling of the molecule not as electrons obeying quantum mechanics, but as a number of atoms connected together by flexible bonds, which move according to Newtonian mechanics and empirical force fields. For instance, in a simple diatomic molecule like H2, the length of the bond between the two atoms is assumed to have an energy equal to... 

The term cycloethane , as used here and in Reference 20d, generally makes more immediate sense to individuals used to molecular models with flexible bonds, whether they be metal springs or plastic tubes, than to those individuals who are used to model sets with lobes or fans to convey orbitals. The term cycloethane is also in interesting historical and linguistic counterpoint to the early chemical literature wherein trimethylene was often used in lieu of cyclopropane. For... 

Processes of charge separation in porphyrin-quinone compounds with flexible bonding... 

The high conformational mobility of porphyrin-quinone compounds with flexible bonding makes it difficult to elucidate in sufficient detail the mechanism of electron transfer between porphyrin and quinone fragments. Far greater possibilities for determining the role of mutual orientation of P and Q and the distance between them are offered by P-Q compounds in which the P and Q fragments are linked by several bridges. A P-Q compound of this... 

The model of Robertson (Robertson, 1966 Escaig and G Sell, 1982 Cook et al., 1998) is based on a statistical distribution of stiff and flexible bonds (gauche —> trans conformation). The applied stress improves the density of flexible bonds up to a critical value where the material becomes equivalent to a liquid. 

Algorithm GASP uses a genetic algorithm that iteratively optimizes a population of chromosomes according to the fitness function described here below [16]. Each chromosome encodes angles of rotations about flexible bonds and mappings between features. 

Figure 17 A sketch of the rigid units of an oligomeric PFPE molecule (a) the flexible bonds with freely jointed beads and springs for coarse-grained bead-spring model and (b) SRS model with polarity (red arrow).

Processes of Charge Separation in Porphyrin-Quinone Compounds with Flexible Bonding... 

Fig. 3. 5 -AMP showing major flexible bonds but note that the ribose ring has preferred conformations...

The process of drug discovery amounts to the search for optimality in a hyper-dimensional, multi-response surface area, and thus is a complex process. As discussed earlier, log P and the other R05 parameters, together with other properties such as PSA, and the number of flexible bonds, are significant contributors to a large number of models for different ADME/T properties. The open question is whether the use of such models provides an added value, compared to the simple and easily interpretable R05 criteria or the recent trends for the drug-likeness formulated by Gleeson. But can these models provide added value ... 

Influence of Flexible Bond Angle on Torsional Conformations, Hexamethyldisiloxane was used as a test compound for several procedures applied in conformational analysis. This compound is relatively simple, and structural data are available in the literature. The experimental value for the Si-O-Si bond angle in hexamethyldisiloxane is 148 3°, as measured by electron diffraction (12). The electron diffraction data indicate a staggered conformation 2 symmetry), although a model with twist angles around Si-O bonds of about 30° cannot be excluded. 

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