Structural Influences

On the molecular level, energy-elasticity is concerned with changes in bond lengths and valence angles. Consequently, with the aid of X-ray 

The lattice moduli of elasticity of poly(ethylene) are quite high since a deformation can only result from a valence angle deformation because of the 2i -trans conformation in the chain direction. On the other hand, it-poly(propylene), poly(oxymethylene), and poly(oxyethylene) occur in helix conformations. Here, deformation occurs as a result of rotation about the valence angles. Consequently, the lattice moduli of elasticity of helixforming polymers are smaller than those of polymers in the diW-trans conformation, i.e., their compliance (extensibility) is higher. 

In general, the moduli of elasticity are much lower than the lattice moduli of the same compounds. There are many reasons for this. For one thing, most chains of a sample do not normally lie in the strain direction. Consequently, deformation can occur through increase in the interchain distance, that is, through other conformational positions (entropy-elasticity). Deformations can also occur through irreversible slippage of chains past each other (viscoelasticity). 

in contrast to lattice moduli, moduli of elasticity obtained from stress/strain measurements are not measures of the energy elasticity, because of the effects of entropy-elasticity and viscoelasticity. Moduli of elasticity have more the character of being solely proportionality constants in a Hooke-type law. The proportionality limits are 0.05 % extension for steel and 0.1-0.2% for polymers. Above these so-called proportionality 

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Where more than one of the structural influences on a particular carbonyl group is operating, the net effect is usually close to additive. 

More important than the mechanism by which die tertiary amine catalysts function is how their molecular structure influences catalytic activity and selectivity... 

A fundament of the quantum chemical standpoint is that structure and reactivity are correlated. When using quantum chemical reactivity parameters for quantifying relationships between structure and reactivity one has the advantage of being able to describe the nature of the structural influences in a direct manner, without empirical assumptions. This is especially valid for the so-called Salem-Klopman equation. It allows the differentiation between the charge and the orbital controlled portions of the interaction between reactants. This was shown by the investigation of the interaction between the Lewis acid with complex counterions 18> (see part 4.4). 

When one compares the brutto polymerization rate constants, a measure of the reactivity of monomers during cationic homopolymerizations is obtained. It was found for p-substituted styrenes that lg kBr increased parallel to the reactivity, which the monomers show versus a constant acceptor 93). The reactivity graduation of the cationic chain ends is apparently overcomed by the structural influence on the monomers during the entire process of the cationic polymerization. The quantitative treatment of the substituent influences with the assistance of the LFE principle leads to the following Hammett-type equations for the brutto polymerization rate constants ... 

Kennedy and co-workers 2 117) used the changing effect of the initiation ability of the Lewis acids according to Eq. (17) and the termination tendency of the anion formed according to Eq. (18) in order to obtain telechelic polymers , block copolymers and graft copolymers in a controlled manner. Quantum chemical calculations provide the possibility to discuss structural influences which work on the equilibrium Eq. (19) and therefore on the stability of the two adjacent ions. 

The interactions among solute molecules are a reflection of their chemical structure. The manifestations of this structural influence are the physical properties associated with their intermolecular binding. As examples, the melting point reflects the ease of crystal disruption to bring about a state change. The solubility is another phenomenon derived from the structural influences on the binding of solute molecules in a crystal. 

Learning to recognize the properties of a substance by examining a chemical formula is an important part of mastering chemistry. Fortunately, common acids and bases fall into a small number of structural categories. In this section we describe how to recognize acids and bases, hi Section 17-1 we explain how chemical structure influences acidity. 

How do particle size and structure influence catalytic activity ... 

FIG. 4 Schematic representation of various lipid phase structures influenced by lipid molecular shape. 

Another feature to be noted about the structures of TltTp1 ] is concerned with the structural influence of the 5-alkyl substituent. For example, whereas the 5-methyl derivative Tl[TpBut,Me] exhibits approximate C3u symmetry (Fig. 8), the 5-<-butyl derivative Tl[TpBut2] displays a pronounced distortion such that the molecule possesses only C3 symmetry (Figs. 9 and 10) (27). 

Unlike mass transport across membranes, which relates to chemical structure in predictable ways, the potencies of drugs as seen in pharmacological, pharmacodynamic, or other tests are highly structurally specific within a class of drugs and are without commonality across classes. A drug s activity involves a complex merging of these separate structural influences, with bioavailability always one of the concerns. Such concern is minimal when a truly superficial effect is involved, however. For example, the most potent antiseptic as measured in the test tube is likely to have... 

Balaiah V, Seshadri TR, Venkateswarlu V (1942) Visible fluorescence and chemical constitution of compounds of the benzopyrone group. Part III. Further study of structural influences in coumarins. Proc Ind Acad Sci 16A 68-82... 

Futter, M.N. 1994. Pelagic food-web structure influences probability of mercury contamination in lake trout (,Salvelinus namaycush). Sci. Total Environ. 145 7-12. 

Hapeman, C J., S. Bilboulian, B.G. Anderson, and A. Torrents. 1998. Structural influences of low-molecular weight dissolved organic carbon mimics on the photolytic fate of atrazine. Environ. Toxicol. Chem. 17 975-981. 

Their direct mutagenicity makes them excellent vehicles for investigating mechanisms of mutagenesis and structural influences upon the mutagenic process and we have utilised many analogues as probes for drug-DNA interactions.46,49,50... 

Structural influences such as conjugation or configuration and intra and inter molecular attractions hydrogen bonding in particular shift the IR bands of the groups involved. 

Atoms of S and Se can sufficiently structurally influence fragments of CH3 that are frequently located on the ends of hydrocarbon chains or in the form of free radicals. The data given confirm high reactivity of sulfur and selenium atoms as retardants of chain reactions of free radicals as elements drawing back impaired valence electrons of free radicals, but at the same time preserving the basic structure of hydrocarbon chain. 

Oxide composition and lattice structure influences the coordin-ative environment of surface sites, and should have an impact on rates of ligand substitution. Hematite (Fe203), goethite (a-FeOOH), and lepidocrocite (y-FeOOH), for example, are all Fe(III) oxide/ hydroxides, but may exhibit different rates of surface chemical... 

The relative product yields can be rationalized in terms of crystal-structural influences (247). In both crystal forms the packing of the nitrile groups is well-suited for optimizing the dipole-dipole attraction between molecules. For the reaction 183 — 185 to occur, one free radical would need to rotate about an axis perpendicular to the plane, thus allowing the nitrile nitrogen to approach... 

C resonance is subject to a variety of structural influences, and often it is very... 

There is a structural influence that can affect the magnitude of a-SCS(OH) and invert the normal sequence a -SCS > a -SCS. 

For Type B correlations the proportionality between the structure influence on a catalytic and (usually) noncatalytic process is strong evidence of mechanistic kinship. The only condition for the value of the slope is its significant difference from zero the absolute value is of no consequence. 

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