Twists, simple/double

Fig. 3 Comparison of simple-twist and double-twist arrangements...

FIGURE 12.9 (a) Double-stranded DNA as an imaginary ladderlike structure, (b) A simple right-handed twist converts the ladder to a helix. 

It is easy to see how simple MO arguments lead one to expect that two non-bonding orbitals will result if a single bond is stretched sufficiently (Fig. 6), as a double bond is twisted by 90 degrees (Fig. 5), or as atoms are rearranged to make a carbene. Return through such biradicaloid... 

It is possible in principle to deduce from the position of the first absorption band the twisting angle between the double bonds in cyclic conjugated polyenes, e.g. on the basis of a simple LCBO model50. This has been attempted for cycloocta-l,3,5-triene 184257 and cyclooctatetraene 239260. For the former it was deduced that the --system becomes... 

The most favourable relative configuration of identical chiral molecules is that where all neighbouring molecules are twisted relative to each other. This is achieved by a "double-twist" stacking, illustrated in Fig. 4.32. In three-dimensional euclidean space, this double-twist caimot be realised throughout space some "disclination" singularities must occur [61]. How then can this double twist be most closely approached A simple model, involving nothing more than potatoes, and oven and matches, is useful. The lower-... 

Syn-anti isomerization about a C=N double bond is intrinsically more complicated than cis-trans isomerization of a C=C double bond. This is due to the fact that n—excitations have to be discussed in addition to excitations, and because syn-anti isomerization can be effected by either of two linearly independent kinds of motion or their linear combination, namely twisting and in-plane inversion at the nitrogen atom. (See Figure 7.7.) It is believed that in simple azomethines thermal isomerizations occur through inversion, while photochemical isomerizations proceed along a twisting path (Paetzold et al., 1981). 

The least common of the simple pi bond sources are allenes, R2C=C=CR2, whose two perpendicular double bonds must again be considered separately. Allene reactions have a bit of a strange twist to them, literally. There are two possible sites for the electrophile to attack on the allene above. It would seem to be no contest between a resonance-stabilized ally lie carbocation and the less stable vinyl carbocation. But a look at the orbital arrangement below for the reaction reveals that all is not right with the allylic system in fact, it is twisted 90° out of alignment, so no resonance stabilization can occur at the transition state for addition. 

Fig. 2 Frustration in twisted arrangements, a In the case of simple twists, arrangements from Points A and B, which are separated, do not result in inconsistency, b Double twists result in inconsistency...

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