Eclipsed, definition

Another set of terms considered by Eyring, et al. consisted of structures having a double bond between carbons. These should be expected to be involved to some small extent—small because they have one less real bond. This is essentially invoking hyperconjugation. These authors did not reach a definite conclusion regarding the importance of these structures and decided that they probably would stabilize the eclipsed configuration, which was not known at that time to be the incorrect one. It is still not simple to make a reliable estimate of the importance of this contribution. 

As in the definition of rotations for the backbone, the sense of rotation is such that a right-handed rotation about any side-chain bond, looking in the direction of numbering, is taken to be positive. The choice of zero angle of rotation is such that the planar eclipsed (cis) conformation is denoted by Xj — 0 (Fig. 4). For the C —bond, the N—C and the —Cy bonds are used to define the angle xi In branched chains, Branch 1 is always used to define the angle of rotation. 

An alternative way of analyzing molecular shape, besides drawing a Newman projection, is to analyze the amount of twist about the bond that is necessary to cause some or all of the bonds from the near (upper) and far (lower) atoms to be totally eclipsed. This leads us to a definition of torsion angle. 

A torsion angle is determined by how much a bond has to be twisted to cause two substituents on the atoms it connects to be eclipsed (Fig. 28). By definition, a torsion angle is positive if a clockwise twist about the central of three connected bonds that is needed to make the near (upper) bond [A-B in a series of four-bonded atoms A-B-C-D) eclipse the far (lower) bond (C-D) a negative torsion angle is a counterclockwise twist. The direction of view along the bond [that is, which is nearer to you (A-B) or (D-C)] is immaterial reversing the sequence of... 

Fig. 9.18. Definition of the torsion angle for a phenyl ring on triphenylphosphine oxide. The figure shows a projection of the molecule down a P-C bond onto the corresponding phenyl group. Zero torsion angle means eclipsing of the O - P bond with the phenyl ring. The smaller of the two torsion angles is taken...

Unfortunately, there is more than one definition for threo- and erythro. Winstein and co-workers gave the following definition of these diastereomers In a compound with two asymmetric carbons that has two common ligands and a third that differs, the isomers that would be meso if the third ligand were identical are erythro diastereomers.An alternative definition is If two asymmetric carbons have only one ligand in common, then the other four ligands are paired in the same commonsense way and isomers that would have equal pairs eclipsed in any conformation are erythro. Eliel, Mislow and their co-workers defined erythro and threo in terms of Fischer projections. The aldol products (see sec. 9.4.A) 68 and 69, and the ester-aldehyde condensation products 70 and 71.30 shown with the erythro and threo notation. 

Figure 3-5 shows that isotactic polymers in certain projections definitely do not have their substituents always on the same side as is frequently stated. Such an assumption is only correct for the two types of projections in the eclipsed conformation. In projections of trans-staggered conformations differences exist according to the monomeric unit. 

Figure Projection of the ethylene-S02 complex in the ac plane showing definitions of 0i and 02- The line of sight is along the C=C bond, with the O atoms eclipsed. 0i and O2 are the tilt angles or deviation of the C2H4 and SO2 planes from being perpendicular to i cm- Angle directions as shown are defined as positive.

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