Rotation of bonds

Fig. 15. Simulation of conformational changes in elongational flow by preferred rotation of bonds perpendicular to flow direction (according to Ref. [70])...

It is worth noting that whilst we have restricted discussion in this section to conformational interconversion based on the slow rotation of bonds, the concept of the NMR timescale is equally applicable to other types of interconversion, such as can sometimes be seen in cyclic systems which may exist in two different conformational forms. 

A second-order phase transition is one in which the enthalpy and first derivatives are continuous, but the second derivatives are discontinuous. The Cp versus T curve is often shaped like the Greek letter X. Hence, these transitions are also called -transitions (Figure 2-15b Thompson and Perkins, 1981). The structure change is minor in second-order phase transitions, such as the rotation of bonds and order-disorder of some ions. Examples include melt to glass transition, X-transition in fayalite, and magnetic transitions. Second-order phase transitions often do not require nucleation and are rapid. On some characteristics, these transitions may be viewed as a homogeneous reaction or many simultaneous homogeneous reactions. 

In order that the contribution from random-coil sequences might be more correctly evaluated, Miller and Flory (43) carried out a calculation, taking into account the hindrance to internal rotation of bonds, van der Waals interactions of non-bonded atoms, dipole-dipole interactions between atomic groups, and so forth. However, no excluded-volume effect was allowed for. Actually, they... 

The chain conformation of a macromolecule is determined by the torsional angles assumed by the backbone bonds. By convention, the angles 0°, 0° are used to define a trans-trans-planar conformation as shown in Figure 3.9a. Torsion (rotation) of bonds 2 and 4 in Figure 3.9a by 180° generates the cis-trans-plmar conformation (Figure 3.9b). 

Convention for torsion angles 0° when bonds A-B and C-D arecvs clockwise rotation of bond C-D relative to A-B is positive. 

These considerations, thus, lay the groundwork for tests among several semi-empirical approaches to the estimation of optical rotation of bond systems regarded as helices. Should it be necessary to use Eq. (lb) rather than (la), then a sweeping reassessment of the use of the helical conductor model will be required. However that test turns out, a test between that model and the simple conformational dissymmetry model becomes possible on the basis of the material shown in Table 1. At this point it should be said that our calculations on twistane 16> support the helical conductor model but that the results obtained by Pino and his co-workers 17 18> on the chiroptical properties of isotactic polymers prepared from chiral a-olefins support the conformational dissymmetry model. [We are not able, at present anyhow, to account for their results with the helical conductor model]. 

The analysis given for d = 2 can also be applied to higher dimensions. As before, the deformation of the chain may be represented by a sequence of transformations, where the zth transformation includes the stretching of the bond b, and the rotation of bonds bn about R,=. ... 

The substituents have been exchanged in a way that is impossible through any rotation of bonds or rotation through space. If this molecule is rotated in space in the direction shown, the mirror image nature of enantiomers is seen by comparison with the first of the three molecules above ... 

Non-rotation of bonds - the C=C bond cannot rotate and is the most common cause of diastereomerism. Other causes are cyclic compounds and steric hindrance. 

The formation of ds-propenyl ether is probably a consequence of a cyclic intermediary structure. The formation of trans-propenyl ether is probably a consequence of free rotation of bonds in the transition state. The rate of the transformation of allyl ether groups in propenyl ether groups is higher at higher temperatures (150-160 °C). This rearrangement of ally ethers to propenyl ether groups is catalysed not only by alkaline alcoholates [60, 77, 78] but also by some complex compounds of ruthenium (such as ruthenium dichloride-triphenylphosphine complex [89]) ... 

The C=C Bond and Geometric (cis-trans) Isomerism There are two major structural differences between alkenes and alkanes. First, alkanes have a tetrahedral geometry (bond angles of —109.5°) around each C atom, whereas the double-bonded C atoms in alkenes are. trigonal planar (—120°). Second, the C—C bond allows rotation of bonded groups, so the atoms in an alkane continually change their relative positions. In contrast, the -n bond of the C=C bond restricts rotation, which fixes the relative positions of the atoms bonded to it. 

The existence of different stereochemical configurations, which by definition cannot be converted from one to another by rotation of bonds, is dependent on the inclusion of a chiral carbon in the molecule. A chiral carbon is one that has four different substituents attached, and therefore, can exist in two mirror-image forms, as illustrated in Fig. 3.27. 

It is important to be able to regulate the degree of chain stiffness, as rigid chains are preferred for fiber formation whereas flexible chains make better elastom. The flexibility of a polymer depends on the ease with which the backbone chain bonds can rotate. Highly flexible chains will be able to rotate easily into the various available conformations, whereas the internal rotations of bonds in a stiff chain arc hindered and impeded. 

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