Single bonds free rotation about

Protein chains are not the sprawling, ill-defined structures that might be expected from a single polypeptide chain. Most proteins are compact molecules, and the relative positions of atoms in the molecule contribute significantly to its biological role. A particularly important contributor to the shape of proteins is provided by the peptide bond itself. Drawn in its simplest form, one might expect free rotation about single bonds, with a variety of conformations possible (see Section 3.3.1). However,... 

The principle of free rotation about single bonds was a necessary simplification during the formative years of organic chemistry, when estimation of the number of isomers played a major role in the elucidation of structure1. By the middle of the present century it had... 

Just like diastereotopic signals in an NMR spectrum, diastereotopic faces are always different in principle, but sometimes not so in practice. The very first reaction of Chapter 33 is a case in point this C=0 group has two diastereotopic faces, which, due to free rotation about single bonds, average out to about the same reactivity, so we cannot expect any reasonable level of diastereo-selectivity. 

Describe the following spin systems as AB, etc. CH2—CHF PF3 cubane CH3CHOHCH3 H2 chlorobenzene n-propane. Assume free rotation about single bonds. 

Unvulcanized rubber consists of a large number of flexible long molecules with a structure that permits free rotation about single bonds in the primary chain. On deformation the molecules are straightened, with a decrease in entropy. This results in a retractive force on the ends of the polymer molecules. The molecular structure of the flexible rubber molecules makes it relatively easy for them to take up statistieally random conformations under thermal motion. This property is a result of the weak intermolecular attractive forces in elastomers and distinguishes them chemically from other polymers which are more suitable for use as plastics or fibers. 

The NMR spectroscopic examples discussed so far have assumed that, with the exception of free rotation about single bonds, the molecule or ion is static in solution. For the majority of organic and inorganic species, this assumption is valid, but the possibility of stereochemical non-rigidity... 

The other extreme of the conformation spectrum that may be assumed by the polymer chain is the completely random coil. Polymers that are in solution, in melt, or amorphous in the solid state assume this conformation. Between these two extremes (planar zigzag and random coil conformation) the number of conformation shapes that a polymer chain can assume is virtually limitless. This, of course, assumes that there is free rotation about single bonds. In practice, however, there is no such thing as completely free rotation. All bonds have to overcome certain rotational energy barriers whose magnitude depends on such factors as steric hindrance, dipole forces, etc. (Figure 3.7). 

Note that for some properties, such as melting temperature, both the chemical structure and the long chain nature of polymers play important roles. As will be seen later, the stiffness and flexibility characteristics of polymers are largely determined by the degree of hindrance to free rotation about single bonds in the backbone chain. 

However, definite valence angles occur in real macromolecular coils. The relationship between the mean end-to-end distance Lo/of this valence angle chain (with implicit assumption of free rotation about single bonds) and the number Nof bonds of length b in the chain is, for an infinitely large number N of constant angles r,... 

The NMR spectroscopic examples discussed so far have assumed that, with the exception of free rotation about single bonds, the molecule or ion is static in solution. For the majority of organic and inorganic species, this assumption is valid, but the possibUily of stereochemical non-rigidity (fluxionality) on the NMR spectroscopic timescale must be considered. Five-coordinate species such as Fe(CO)5, 4.10, PF5,4.11, and BrF5,4.12, constitute one group of compounds for which the activation barrier for dynamic behaviour in solution is relatively low, and exchange of substituent positions is facile. 

Conformations Free rotation about single bonds staggered 2.9 kcal moF more stable (lower energy) than eclipsed (2-8, 9)... 

---