Strain torsional defined

Assuming these conditions apply to a torsion experiment, it also becomes possible to define two shear moduli, and G2. The first of these represents that part of the stress that is in phase with the strain, divided by the strain. 

Like many other chemical concepts the concept of strain is only semi-quantitative and lacks precise definition. Molecules are considered strained if they contain internal coordinates (interatomic distances (bond lengths, distances between non-bonded atoms), bond angles, torsion angles) which deviate from values regarded as normal and strain-free . For instance, the normal bond angle at the tetra-coordinated carbon atom is close to the tetrahedral value of 109.47°. In the course of force field calculations these normal values are defined more satisfactorily, though in a somewhat different way, as force field parameters. 

The total torsional strain is then defined by the sum over non-zero contributions... 

For delocalized systems that require eclipsing across a double bond the torsion angle free of strain may be defined as o — 0, now representing an attraction rather than a repulsion between second-neighbour bonds. 

A close connection exists between the presence of a flexible polymer skeleton and the flexibility of the bulk material. Macromolecular flexibility is often defined in terms of the glass-transition temperature, Tg. Below this temperature, the polymer is a glass, and the backbone bonds have insufficient thermal energy to undergo significant torsional motions. As the temperature is raised above the Y g, an onset of torsional motion occurs, such that individual molecules can now twist and yield to stress and strain. In this state the polymer is a quasi-liquid (an elastomer) unless the bulk material is stiffened by microcrystalfite formation. Thus, a polymer with a high Tt is believed to have a backbone that offers more resistance to bond torsion than a polymer with a low 7 g. 

E r is a torsional strain energy term, defined as Etor = pVn (1 cos... 

In the original molecular mechanics work, a steric energy, E, for a molecule was defined as the sum of the potentials for bond stretch, angle bend, Ee, torsional strain, E, nonbonded interactions, Ey f, and other terms, such as Urey-Bradley terms, cross-interaction terms, and electrostatic terms, (8). 

When substituents are attached to large rings, they tend to occupy only exterior positions to minimize the large transannular interactions. A fully substituted atom of a macrocycle will usually occupy a corner position since this is the only position where it does not cause severe transannular strain. For both substituted and unsubstituted macrocycles, the conformation of the ring will impose restraints on the torsional angles available to each unit of carbons (four carbons define a torsional angle). If these four atoms are shown... 

Certain materials may be weak in shear, and for these it is appropriate to measure their strength by torsion tests. For such tests, the material is fabricated into a rod of circular cross-section, and twisted about its longitudinal axis. The angle of twist is proportional to the shearing strain, y, and the applied force or couple is proportional to the shearing stress, r, their ratio being defined as the shear stress, G, i.e. 

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