Bond angle deformation

Glassy state In amorphous plastics, below the Tg, cooperative molecular chain motions are frozen , so that only limited local motions are possible. Material behaves mainly elastically since stress causes only limited bond angle deformations and stretching. Thus, it is hard, rigid, and often brittle. 

Chain scission is the ultimate fate of a stressed bond. At some value below the critical stress for chain rupture, bond angle deformation may result in an increase in reactivity. As stated in Sect. 3.1, mechanically activated hydrolysis of polymers containing ester groups can lead to the scission of the bond this concurrent reaction should be differentiated from homolytic chain scission, for example by looking at any pH-dependence as was found to be the case during shear degradation of DNA [84]. 

A regular PI proceeds without breaking and making of bonds, by bond angle deformations or internal rotations. 

Buttressing effects are known to raise the barrier to rotation in the biphenyl series by preventing bond angle deformations of a substituent involved in direct interaction in the transition state. Similar effects were found in the 9-arylfluorene series (108). The barrier to rotation of 9-(3-bromo-6-methoxy-2,4-dimethyl-phenyl)fluorene (67, X = H) in chloroform-d at 56.3°C is 25.7 kcal/mol for... 

TABLE 4. Comparison between observed and calculated wavenumbers (cm ) for Li(C2H4)(N2) using a 6 X 6 harmonic model including the C=C stretch and two CH2 bond angle deformations of the ethylene subunits ... 

Viscoelastic behavior can be divided into five subclassifications (Figure 14.4). From 1 to 2, the material behaves as a viscous glass or Hookean elastic or glass, where chain segmental motion is quite restricted and involves mainly only bond bending and bond angle deformation. 

Steric strain differences in the monomer and polymer arising from bond angle deformation, bond stretching, or interactions between nonbonded atoms. 

Stability. The four-membered ring in the diazabiphenylene series of compounds is strained due to bond angle deformation . The conventional ring strain energy for cyclobutane is 111 kj moP alkyl substituents stabilize the ring by only a few kilojoules per mole. 

Bond angle deformation is one of the more common sources of strain. A C—C—C bond in an open-chain alkane has an angle of 111°. For relatively small angular deviations, the increase in energy would approximately be given by... 

The strain energies of cyclopropane (26.5 kcaFmol) and cyclobutane (26 kcal/ mol) are nearly the same despite the apparent much greater bond angle deformation with the former. One reason is that the weak C—C bonds in cyclopropane are compensated in part by the stronger C—H bonds. The increased strength results from the greater s character, and it is known that C—H bond strengths increase with... 

Many of the unique properties of cyclopropanes, and to a lesser extent, cyclobutanes, are derived from the formation of bent bonds. They may act in a fashion similar to 7r-bonds in interacting with electron-deficient centers, and are more easily cleaved thermally via electrophilic attack than are ordinary C-C bonds. The strain energy associated with bond angle deformation is also an important quantity, especially when considering thermal reactions. 

Vibrational spectroscopy reveals that, for small displacements from equilibrium, energy variations associated with bond angle deformation are as well modeled by polynomial expansions as are variations associated with bond stretching. Thus, the typical force field function for angle strain energy is... 

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