Eclipsed conformation molecular model

Ethane is the simplest hydrocarbon that can have distinct conformations Two the stag gered conformation and the eclipsed conformation, deserve special mention and are illustrated with molecular models m Figure 3 1... 

Eclipsed conformation, 94 molecular model of, 94 Edman, Pehr Victor, 1031 Ed man degradation, 1031-1032 mechanism of. 1032 Eicosanoid, 1067-1070... 

If A and B are large bulky groups they will bump together, attainment of the eclipsed conformation will be almost impossible, and rotation will be severely restricted. Even if A and B are hydrogen atoms (ethane), there will be a rotational barrier in the eclipsed conformation which amounts to 12 kj (3 kcal) per mole because of the crowding of the hydrogen atoms as they pass each other.5 9 This can be appreciated readily by examination of space-filling molecular models. 

Begin by drawing a perspective view of the molecular model shown in the problem. To view the compound as a Fischer projection, redraw it in an eclipsed conformation. 

Analysis of the low temperature VT NMR (,9F and H) and 19F- H heteronuclear NOE spectra of 65 and 66 in conjunction with molecular modeling has revealed the presence of an equilibrium between two conformers in protic solvent systems. Interpretation of the temperature dependence of the coupling constants between H2 and H3 for 65 shows that one of these conformers (conformer C) has an unusual near-eclipsed arrangement around the H2 -C2 -C3,-H3 dihedral angle (7H2- H3-= 5.2 Hz, corresponding to the H2 -C2 -C3 -H3 torsion angle of 124° based on the MM2 calculation), and is found to be more prevalent at ambient temperatures. The... 

The conformation of cyclobutane. Part (a) shows computer-generated molecular models. Part (c) is a Newman projection along the C1-C2 bond, showing that neighboring C-H bonds are not quite eclipsed. 

Make a molecular model of ethane and confirm that 3 represents an eclipsed conformation. 

Figure 7). Osawa s recent force-field calculation study (21) of cage-shaped molecules with ethano bridges affords an excellent demonstration of this twist deformation at the molecular level. In six achiral cage-shaped molecules so far studied, his calculations showed that each molecule assumed a twisted, chiral conformation to minimize torsional and nonbonding strain. Tricyclo[4.2.2.2.2,s]dodecane (5) was shown to be 1.1 kcal/mol more stable in a twisted D2 than in the eclipsed D2h conformation, and his calculation also suggests that perhydrotriquinacene and C16-hexaquinane should assume C3 rather than conformations, contrary to naive pictures obtained by a casual observation of molecular models. 

Build a 3D model of ethane (using a molecular model kit) and use it as you read this section to model the staggered and eclipsed conformations shown in Figure 2.5. 

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