Various rotations about bonds

A molecule that contains several single bonds may be altered in shape by rotation about these single bonds. Such rotations occur frequently in the liquid or gaseous state, where intermolecular forces that keep the molecule fairly rigid in the crystal have been lost. Different rotational positions about the various bonds are represented by different torsion angles these demonstrate the inherent flexibility of the molecule. Such rotations about bonds axe evident in the different shapes of the same molecule in different crystal structures, or by disorder in the crystal. 

Because of the atoms freedom to rotate about single bonds, a chain of carbon atoms can achieve various positions in space. On one extreme is the zig-zag extended chain and on the other is a coil. Such spatial structures become particularly important in determining properties of very long chained compounds known as polymers (Chapter 5). 

A quite different and complimentary approach is to assume that addition of a nucleophile to an acyl derivative (RCOX) would follow the linear free energy relationship for addition of the nucleophile to the corresponding ketone (RCOR, or aldehyde if R=H) if conjugation between X and the carbonyl could be turned off, while leaving its polar effects unchanged. This can be done if one knows or can estimate the barrier to rotation about the CO-X bond, because the transition state for this rotation is expected to be in a conformation with X rotated by 90° relative to RCO. In this conformation X is no longer conjugated, so one can treat it as a pure polar substituent. Various values determined by this approach are included in the tables in this chapter. 

Restricted rotation about the C—N bond of amides was studied by Gutowsky and Holm in the earliest days of NMR spectroscopy (31). A number of papers have been published since then. However, due to various difficulties, the barriers... 

As an extension of this work, Atkinson and co-workers (123) prepared l-dibenzylamino-l,2-dihydro-2-quinolone (78) and 1 -(/V-benzy l-N-carboxy-methyl)amino-l, 2-dihydro-2-quinolone (79). The benzylic protons of 78 showed an AB quartet that did not coalesce up to 180°C, and 79 was resolved into optical isomers. The E, for racemization was 26.2 0.4 kcal/mol. Various attempts were made to elucidate the possible pathways for isomerization in these quinolone derivatives (123). Radical dissociation, a sigmatropic shift followed by rotation, and restricted rotation about the S—N bond were excluded. The aforementioned authors (123) also excluded the possibility of nitrogen inversion and preferred restricted rotation about the N—N bond as an explanation for the existence of stereoisomers. They supported this explanation by examining the steric effects... 

Complex molecules which rotate about several bond axes can undergo conformational changes on various surfaces and thereby exhibit different cross-sectional areas. 

Configurational isomers differ from each other only in the arrangement of their atoms in space, and cannot be converted from one into another by rotations about single bonds within the molecules. Before we look into the details of various configurational isomers, we need to understand the concept of chirality. 

The conformations in solution of various acylated l,l-bis(acyl-amido)-l-deoxypentitols have been examined by p.m.r. spectroscopy.793 In line with general behavior observed78 with other acyclic-sugar derivatives, it was found that the arabino and lyxo derivatives adopt extended, zigzag conformations, whereas the ribo and xylo derivatives favor sickle conformations that result by rotation about one carbon-carbon bond of the backbone chain to alleviate a destabilizing 1,3-interaction of acyloxy substituents that would be present in the extended, zigzag conformation. 

Figure 2.4 Rotation about a o-bond leading to various spatial arrangements of the atoms in a molecule.

Disilacyclobutenes, variously substituted, can be readily oxidized, notably with bromine, to yield the isomeric mixture of hindered tetrasilylethylenes through rotation about the carbon-carbon double bond. They also give a variety of derivatives with silylenes, metal carbonyls and alkynes (Schemes 128 to 132) (80AG(E)620, 78JOM(162)C43, 74CC1013, 81IC3456). 

As important to biochemists as configurations, the stable arrangements of bonded atoms, are conformations, the various orientations of groups that are caused by rotation about single bonds.5 8 In many molecules such rotation occurs rapidly and freely at ordinary temperatures. We can think of a -CH3 group as a kind of erratic windmill, turning in one direction, then another. However, even the simplest molecules have preferred conformations, and in more complex structures rotation is usually very restricted. 

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