Polymethylene, conformational

Like the polymethylene hydrocarbons, acyclic carbohydrates and alditols tend to adopt the planar, zigzag conformation wherein all of the carbon atoms lie in the same plane. " However, non-bonded interactions between parallel hydroxyl groups in the 1,3 positions (sy/i-axial interaction) are the most highly destabilizing, and, in xylitol and ribitol, this... 

As a simplified initial approach to the problem, we chose a polymethylene chain with a single spin residing at every other carbon center, to allow sufficient flexibility between spins. The calculational procedure is based on that of Flory and Mark (16), with conformer parameters taken from Abe, et al. (17) and Yoon, et al. (18, 19) The elongation and field directions were col I inear. 

The simple pictorial interpretation of the gem-dimethyl effect presented by Dale (1963, 1966) was based on the idea that the ease of cyclisation depends more on the conformation of the open-chain reactant than on the tension in a resulting strained ring. According to Dale (1963), a polymethylene chain tends to be straight zig-zag but, when substituted by a methyl group, it does not matter for the neighbouring C—C bond whether the chain itself or the methyl group continues in the trans-position hence the probability of a bent... 

Other bridges, and the conformational interconversion must be more or less concerted (125). For additional examples of correlated intramolecular movements in molecules having polymethylene bridges, see ref. 127. 

It is a familiar fact that the skeletal factor s defined in Eq. (2) can very often be decomposed into two independent parts, depending respectively on the restriction of bond angles and on the hindrance to internal rotations. In the simplest case of the polymethylene chain with an internal rotational potential that is symmetric about the irons conformation, we have... 

Photoisomerization of an azobenzene function located in a complex molecule is often accompanied by conformational changes. This approach has again been employed in the construction of photoresponsive crown ethers, a topic which has been the subject of a recent review. Cylindrical ionophores in which two diaza-crown ethers are linked by two photoresponsive azobenzene groups change their ability to bind polymethylene-diammonium salts on irradi-... 

Barker, Bourne and Whiffen have shown that, if the preferred conformation of glycitols is that with the planar, zig-zag arrangement of carbon atoms characteristic of long-chain polymethylene compounds, it is possible to explain the main features of the observed pattern of cyclic-acetal formation. 

Figure 27. Polymethylene-linked dyads. For longer members of the series, added to ensure the chain was kept in the extended conformation [86].

Figure 1. Second nearest neighbor conformational energy map for polymethylene ...

Figure 6-16. Polymethylene backbone in the fully extended conformation.

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