Restricted rotation about a bonds

Geometrical Isomerism. Rotation about a carbon-carbon double bond is restricted because of interaction between the p orbitals which make up the pi bond. Isomerism due to such restricted rotation about a bond is known as geometric isomerism. Parallel overlap of the p orbitals of each carbon atom of the double bond forms the molecular orbital of the pi bond. The relatively large barrier to rotation about the pi bond is estimated to be nearly 63 kcal mol (263 kJ mol-i). 

Cis-trans (or syn-anti) isomerism occurs when there is restricted rotation about a bond between two atoms (e.g. a double bond or a bond in a ring). Groups attached... 

Figure B2.4.1. Proton NMR spectra of the -dimethyl groups in 3-dimethylamino-7-methyl-l,2,4-benzotriazine, as a fiinction of temperature. Because of partial double-bond character, there is restricted rotation about the bond between the dunethylammo group and the ring. As the temperature is raised, the rate of rotation around the bond increases and the NMR signals of the two methyl groups broaden and coalesce.

The experimentally observable phenomenon of optical activity is usually considered in the context of variation of molecular chirality arising from a particular stereochemical configuration at a particular atom such that the molecule has no improper rotation S axis. Molecules with opposite chirality configurations are enantiomers and show oppositely signed optical activity. Molecules differing only in conformation are called conformers or rotational isomers. In most cases, the difference in energy between rotational isomeric states is very small, such that at ambient temperature all are populated and no optical activity results. However, if one particular conformer is stabilized, for example, by restriction of rotation about a bond, the molecule can become chiral, and thus optically active. 

Compounds with restricted rotation about a sigma bond. 

On account of this overlap there is considerable resistance to rotation about a double bond and it produces a rigid molecule, hi other words the disposition of groups attached to the carbon atom can be shown in different ways in space, giving rise to isomers. Therefore geometrical isomerism is a consequence of restricted rotation about double bonds. 

Gossypol is a biphenyl system with ortho substituents, so there is restricted rotation about the bond joining the aromatic rings, creating torsional asymmetry and the existence of enantiomers without a chiral centre. We are only asked about the shape, and this will have the planar aromatic systems at right angles to minimize interaction between the ortho substituents. 

The restricted rotation about amide bonds often occurs at a rate that gives rise to observable broadening in NMR spectra. 

The progress made in the field of modern structural chemistry during the past year has consisted in the main in the determination of the structures of a number of especially interesting molecules and crystals. I have been glad to have the opportunity provided by the exhaustion of the first edition of this book to revise it by the inclusion of references to these researches and of discussion of the new structures. A few corrections have been made, and the argument ip some places has been expanded in an effort to improve the clarity of its presentation. Two new sections have been added, dealing with restricted rotation about single bonds (Sec. 14d) and the conditions for equivalence or nonequivalence of bonds (Sec. 22a). 

The chirality of biphenyls results from restricted rotation about a single bond imposed by the bulky nature of ortho substituents. Models will help you. visualize the degree of difficulty of having the substituents pass by one another. If X = H and Y = F (Table 13-3), the enantiomers are not stable at room temperature if X = H and Y = Br, they are marginally stable if X = H and... 

A polymer chain has an enormous number of chemical bonds. For this, in the solution and amorphous states the NMR chemical shifts of polymers are often the averaged values for all of the possible conformations because of rapid interconversion by rotation about chemical bonds. In solids, however, chemical shifts are often characteristic of specific conformations because of strongly restricted rotation about the bonds. The NMR chemical shift is affected by a change of the electronic structure through the structural change(2). Solid state NMR chemical... 

The other systems, esters 2.71, enamines 2.72, and enol ethers 2.73, similarly have restricted rotation about the bond drawn as a single bond, but the barrier is... 

Amino acids are combined (linked together) through peptide bonds (-C-N-) (Figure 8.1) the peptide bond formed is planar (flat), due to the delocalisation of electrons that form the partial double bond, restricting rotation about the bond. The rigid peptide dihedral angle, co (the bond between C and N), is always close to 180°. The dihedral angles phi (the bond between N and Ca) and psi (the bond between Ca and C) can only have a number of possible values, and so effectively control the protein s three-dimensional structure. 

Here the C-C bond retains some ji-character which would restrict rotation about the bond, and favour a planar transition state. 

In a few cases, single-bond rotation is so slowed that cis and trans isomers can be isolated even where no double bond exists (see also p. 230). One example is N-methyl-A -benzylthiomesitylide (69 and 70), the isomers of which are stable in the crystalline state but interconvert with a half-life of 25 h in CDCI3 at 50°C. This type of isomerism is rare it is found chiefly in certain amides and thioamides, because resonance gives the single bond some double-bond character and slows rota-tion. (For other examples of restricted rotation about single bonds, see pp. 230-233). 

W) Restricted rotation about the bond between the surface intermediate and the active site (the co-ordination bond). This is also a steric effect, free... 

Atropisomers Stereoisomers that may be separated, because they do not readily interconvert by reason of restricted rotation about a single bond. 

Variable-temperature, EXSY and NOESY NMR spectra of (C-N)PdCl(X), where H(C-N)+ = (199) (Ar = 2,6-1Pr2C6H3) provided a detailed analysis of the molecular dynamics, e.g. for X = Cl, there was evidence for a flip of the boat conformation.959 The dynamic behaviour of [(ri3-allyl)Pd(L)]+, where L = (200) (R = Ph, Cy) was followed by variable-temperature experiments.960 Variable-temperature and 2-D 111 and 31P NMR spectra for PdC2(L), where L = (201), reveal restricted rotation about a number of P-QpSO aryl bonds.961... 

S)-Metolachlor is the active ingredient of the herbicides Dual and Bicep, which are widely applied in the United States and were developed by the Swiss company Ciba-Geigy (now Novartis). The steric hindrance near the aryl-N bond restricts rotation about that bond. This restricted rotation results in a type of stereoisomerism called atropisomerism, resulting in aR and a,S configurations about the chiral aryl-N bond axis.113 There is also a stereogenic center (position /) on a carbon next to the N-atom in the amino side chain. All four possible stereoisomers of metolachlor are shown in Figure 9-3. 

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