Conformation eclipsed position

These results are quite compatible with the preferred conformations of the two-chain carbonyl diacids in aqueous media mentioned above (Porter et al, 1986b, 1988). The meso-compounds preferred collinear conformation, which places the hydrogens at the asymmetric carbons in a nearly eclipsed position relative to each other (Fig. 44), is more stable than that of the ( )-diastereomer by about 1.2kcalmol 1. In this conformation, the two carboxylic acid groups at the ends of the chain can be attached to the water surface side-by-side. The entire molecule can then behave as a good-amphiphile whose structure is similar to a pair of single-chain fatty acid molecules bound side-by-side, each chain mirroring the other about the molecules plane of symmetry. 

A clockwise rotation of 120° of the rear methyl group from the eclipsed position in 2 and 3 gives a second eclipsed conformation identical with... 

Nearly all side-chain prediction methods depend on the concept of side-chain rotamers. From conformational analysis of organic molecules, it was predicted long ago [157, 158] that protein side chains should attain a limited number of conformations because of steric and dihedral strain within each side chain and between the side chain and the backbone (dihedral strain occurs because of Pauli exclusion between bonding molecular orbitals in eclipsed positions) [159]. For sp3-sp3 hybridized bonds, the energy minima for the dihedral are at the staggered positions that minimize dihedral strain at approximately 60°, 180°, and —60°. For sp3-sp2 bonds, the minima are usually narrowly distributed around +90° or —90° for aromatics and widely distributed around 0° or 180° for carboxylates and amides (e.g., Asn/Asp y2 and Glu/Gln /3). 

The conformations of the adducts 9 and 10 are markedly different as illustrated by the Newman projections in Figure 6. As a result of the loss of n bonding in 10, the substituents can apparently rotate about the central silicon-carbon bond to a position of minimal steric hindrance. This is reached when the smallest substituent, the donor atom F, adopts an essentially eclipsed position with respect to a bulky SiMe(Bu-t)2 group. In contrast, the bulkier THF molecule in adduct 9 prefers a position far away from the carbon substituents. However, it should be noted that the energy barrier to rotation about the central silicon-carbon bond will be much larger in 9, for which a pronounced degree of double bond character is retained. 

Thus, it may seem that the probability of ring closure should not be higher than calculated on a theoretical basis, already including the assumption of a lack of ring strain. The theory requires, however, that all possible conformations of a macromolecule be equally probable. If we assume that, due to the interaction between substituents or electron pairs or to the presence of certain bonds in eclipsed position, some conformations are excluded, and if these factors do not operate in the conformations leading to ring closure, then the formation of this particular ring will be favoured. 

The difference in energy between an eclipsed conformation and a staggered conformation of ethane is approximately 12.6 kJ (3.0 kcal)/mol (approximately 4.2 kJ (1.0 kcal)/mol for each eclipsed H/H) and is referred to as torsional strain. Torsional strain (also called eclipsed-interaction strain) arises when nonbonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation. In ethane, for example, torsional strain occurs when pairs of hydrogens H(4)-H(6), H(5)-H(8), and H(3)-H(7) on adjacent carbons are forced into eclipsed positions. The models shown here represent only one of the three different but equivalent eclipsed conformations of ethane. 

Using ab initio LCAO-MO calculations on alkyl formates as a basis, the electronic structures and conformations of polysaccharide esters have been interpreted. Acetate units were calculated to be 30° to the eclipsed position due to non-bonded interactions. ... 

Eclipsed conformation of D erythrose showing C 4 hydroxyl group in position to add to carbonyl group... 

Cyclic monomers with five- and six-membered ring atoms exist in strainless puckered conformations their heats of polymerization are either negative or have small positive values due to the repulsion of eclipsed hydrogen atoms. Because the nthalpy and entropy contributions are comparable, the free energies of polymerization are either positive or may become positive at high temperatures. 

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