Chain with restricted rotations

A more complicated relationship for coil molecules in good solvents exists between the chain end-to-end distance and the radius of gyration for valence chains with restricted rotation ... 

Model Chains with Restricted Rotation and No Interaction Among the s... 

The structural requirements in the Du Pont patents are shown in Table IV. The repeating unit contains 4 rigid subunits connected by noncolinear bonds (n), 4 bonds with restricted rotation and 22/29 of the chain atoms are aromatic. 

The repeating unit has 4 N subunits, 2 bonds with restricted rotation, 3 of the N subunits have at least one bond with restricted rotation, and 18/27 of the atoms in the chain are aromatic.. ... 

Model Chains with Freely-Rotating Bonds in Restrictive... 

One additional piece of evidence supporting the formation of a mid-chain radical was obtained from an examination of hydrogen abstraction from polyacrylates. This mid-chain radical can be formed by hydrogen abstraction from polyacrylates by oxygen centered radicals. PolyfBA and fert-butyl peroxide (tBPO) were dissolved in benzene, and the mixture exhibited the ESR spectra shown in Fig. 17a under irradiation. The spectrum was similar to both the spectra observed in the polymerization system (Fig. d) and that reported by Westtnoreland et al. Furthermore, it was reasonably simulated by considering two sets of methylene protons with restricted rotation at both sides of the mid-chain radical, as shown in Fig. lib. Consequently, the radical observed at high temperatures (Fig. lib) is due to the formation of midchain radicals. 

Taking the length per repeat unit (i.e., bond angles already considered) as 0.78 nm in each instance, evaluate the factors (1 + cos 0)/(l - cos (p) and cos (p for each polymer. Ignoring the difference between 130 and 140°C, do you find the difference in steric hindrance between the tributyrate and tri-caprylate to be what you expected Is the effect of temperature on the 1q value of cellulose tributyrate what you expected Briefly explain each answer. For each polymer, calculate r if n = 10 also do this for the hypothetical chain with no restrictions to rotation and having the same repeat length. 

While conformation II (Fig. 2.34) of Uke-y -amino acids is found in the 2.614-helical structure, conformation I, which similarly does not suffer from sy -pen-tane interaction, should be an appropriate alternative for the construction of sheet-like structures. However, sheet-like arrangement have not been reported so far for y-peptides composed of acyclic y " -amino acid residues. Nevertheless, other conformational biases (such as a,/9-unsaturation, cyclization between C(a) and C(y)) have been introduced into the y-amino acid backbone to restrict rotation around ethylene bonds and to promote extended conformation with formation of sheets in model peptides. Examples of such short chain y-peptides forming antiparallel (e.g. 152 [208]) and parallel (e.g. 153-155 [205, 208]) sheet-hke structures are shown in Fig. 2.38. 

Dipolar couplings of side chains can provide valuable structural information. Dipolar couplings for methylene and methyl sites have been applied to protein structure refinement [49]. In a side chain with a C-CH3 moiety, rapid rotation of the methyl group results in an averaged C-H dipolar vector aligned in the direction of the C-C bond. This rapid rotation scales the real value of the C-H dipolar by a factor of —1/3. Therefore, when including methyl dipolar couplings in protein structure refinement, the orientation of the C-CH3 bond is the one that is restricted, rather than the C-H bond. 

At 60 °C, 50% of the ester groups, trapped in constrained environments with high activation energy barriers, do not performed 7r-flips at a frequency higher than 10 kHz and their motions are limited to restricted rotations (rocking) around the local chain axis with an average amplitude of 7°. 

It is a familiar fact that the skeletal factor s defined in Eq. (2) can very often be decomposed into two independent parts, depending respectively on the restriction of bond angles and on the hindrance to internal rotations. In the simplest case of the polymethylene chain with an internal rotational potential that is symmetric about the irons conformation, we have... 

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