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. 

Therefore, in the final analysis, the quadruple bond consists of one a bond, two it bonds, and one S bond. The configuration in which the Cl atoms are in eclipsed positions provides maximum overlap. From the rather intuitive analysis provided here, it is possible to arrive at a qualitative molecular orbital diagram like that shown in Figure 21.23 to describe the Re-Re bond. 

Tetramethyl[2.2]paracyclophane (20a), in which the methyl groups are not in an "eclipsed position, occurs as main product in the dimerization of 2,5-dimethyl-p-xylylene (2/)48> the isomer (20b) was discovered much later 49>. 

Burwell (1) disagreed with Rooney s solutions to both problems. First, he maintained that the ad-diadsorbed species is eclipsed vicinal diadsorbed alkane. Part of his evidence for this view was that only molecules containing 2 or more vicinal hydrogen atoms that are already eclipsed or may easily move into eclipsed positions undergo the a0 process. For example, bicyclo[2,2,1] heptane (Fig. 1) initially exchanges only one set of 2H atoms, those that are eclipsed on C2 and... 

In Mn2(CO)io and Re2(CO)io (and the isostructural Tc2(CO)io) there are no bridging CO groups, the molecule consisting of two —M(CO)j joined by a metal-metal bond. Each metal atom forms six octahedral bonds (to 5 CO and M) and the two octahedra are rotated through about 45° from the eclipsed position. 

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). 

This lack of examples is verified again for the F2 and F 2 polyhedra (see Tables 1 and 2). We note here that F2 can be viewed either as a rectangle opposed to an edge or as two triangles arranged in an eclipsed position. F 2 corresponds to two facing triangles in a star position. 

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. 

---