Twisting rigidity

Double helical DNA is of course just a polymer, but a very peculiar one -in many respects. One peculiarity is that double helix has twisting rigidity. Usual chemical polymer chains, such as the ones shown in Figure 2.1, or protein chains in Figure 5.4, if we twist one end with respect to the other, can relax the deformation by turning around the single covalent bonds of... 

Very recently Zhou et al. used twisted rigid ligands to synthetize compounds based on Keggin clusters with left- and right-handed hehcal chains [148] (see Fig. 3.24). Helical compounds based on HPAs have attracted attention because of their attractive structural features and potential applications, for instance, in asymmetrical catalysis. The chains are further extended to three-dimensional supramo-lecular structures by hydrogen bonds. The methylene blue dye has been photocatalytically bleached in the presence of this solid. Stacking of the chains leads to small pore-size and narrow pore-size distribution which are not beneficial for the photocatalytic activity. 

Conformation. Neutron diffraction studies of sucrose revealed the presence of two strong intramolecular hydrogen bonds 0-2—HO-1 and 0-5—HO-6 in the crystal form (7,8). These interactions hold the molecule in a weU-ordered and rigid conformation. The two rings are disposed at an angle close to 90°, with the glucopyranosyl and fmctofuranosyl residues adapting chair and T" twist conformations, respectively. 

The shape of the ethylene molecule has been learned by a variety of types of experiments. Ethylene is a planar molecule—the four hydrogen and the two carbon atoms all lie in one plane. The implication of this experimental fact is that there is a rigidity of the double bond which prevents a twisting movement of one of the CHj groups relative to the other. Rotation of one CHt group relative to the other—with the C—C bond as an axis—must be energetically restricted or the molecule would not retain this flat form. 

For cyclohexane, there are two extreme conformations in which all the angles are tetrahedral. These are called the boat and the chair conformations and in each the ring is said to be puckered. The chair conformation is a rigid structure, but the boat form is flexible and can easily pass over to a somewhat more stable form known as the twist conformation. The twist form is 1.5 kcal mol (6.3 kJ mol )... 

There is a considerable presumption that steric twisting from coplanarity, which diminishes effective transmittal of pi-electron effects, has appreciable effect on X. For example, the X = 1.10 for the phthalide saponification rate (no. 28 of Table III) compared with X=. 89 for the corresponding benzoate saponification rate is probably a reflection of the rigid coplanarity conditions imposed in the former structure. However, the reactions were carried out in differing compositions of H2 0-Et0H solvent, so that a solvent effect may also be involved. 

Pt and 119Sn NMR data show Pt(SnCl3)5 to be non-rigid (on the NMR timescale) down to 183 K, owing to an intramolecular process, possibly a Berry twist mechanism [132],... 

Triplet photoaddition of simple non-cyclic monoolefins is unknown. The sensitized dimerization of ethyl vinyl ether gives exclusively head-to-head adducts, Eq. 21, and probably should not be classed as an example of simple acyclic olefin. Usually the triplets have high energies and are severly twisted. 55> Some cyclic rigid molecules, Eq. 20, that do dimerize 63> do not incorporate substituents that allow regioselectivity to be determined. Butadiene gives principally head-to-head dimerization, Eq. 19, concordant with the PMO prediction, and so does indene, Eq. 22. The anti dimer that is formed would not be expected from a singlet excimer reaction. 

One important aspect of the photochemistry of aikenes 301,302) is the E—Z isomerization around the C—C double bond 303). This is also valid for cycloalkenes with the obvious exception of cyclopropenes304a,b) which exhibit a distinct photochemical behaviour, and cyclobutenes and cyclo-pentenes where the ring is to rigid to allow sufficient twisting of the double bond. 

Kirby s configurationally rigid l-azaadamantan-2-one 1 is the extreme of this class.21 23,25 The nitrogen properties of this lactam are clearly amine-like while the carbonyl is ketonic in all respects. Twisted amides undergo rapid hydrolysis or reduction and exhibit enhanced reactivity. 

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