Gauche repulsive

It is quite clear from the data that there is substantial "excess attractive interaction in compound 4 i.e. the slight experimental preference for the axial conformation contrasts with a substantial calculated preference for the equatorial one the difference has been ascribed to the "gauche attractive effect" between oxygens (14,17). In contrast, for compounds 1 and 2 there is a substantial "excess" repulsion, i.e. the equatorial conformation is preferred more than calculated. The effect, which has been called "gauche repulsive effect" (14) is especially marked for gauche interactions between sulfur atoms, as in 2. Only in the case of compound 3 is the difference between calculated and observed preference for the equatorial position sufficiently small to make any conclusion uncertain, especially in view of the fact that, in the absence of energy minimization, the calculated energy difference (1.50 kcal/mol) may be somewhat overestimated. 

Both this hypothesis and the finding of the gauche repulsive effect tend to throw new light on the Msteric requirements of unshared electrons on atoms below the second row of the Periodic Table. 

Further substitution can introduce additional factors, especially nonbonded repulsions, which influence conformational equilibria. For example, methyl substitution at C—2, as in 2-methyl-l-butene, introduces a methyl-methyl gauche interaction in the conformation analogous to B, with the result that in 2-methyl-l-butene the two eclipsed conformations are of approximately equal energy. Increasing the si2e of the group at... 

Which conformation is more stable, anti or gauchel What evidence is there for steric repulsion between methyl groups in the gauche conformation (Hint Look for distortions in the CCC bond angles as a function of conformation.)... 

The gauche conformations (III and V) the two methyl groups are close enough to each other => the van der Waals forces between them are repulsive => the torsional strain is 3.8 kJ mol-1 (0.91 kcal moE1 2 3). 

When there are chances for the presence of both anti and gauche forms in an equilibrium, the anti isomer is preferred, because the steric and dipole repulsion come into play in the latter, as is illustrated from the example of ethylene glycol. The two forms of this glycol are ... 

X-ray diffraction has settled the conformation of stilbene dibromide. The meso form is that when both phenyl groups and bromine atoms are anti so that steric and dipolar repulsions are at a minimum. The dl form crystallizes in conformation in which the bromines are approximately gauche, and thus the steric repulsion of the bulky phenyl groups is at a minimum, even though there is steric and dipolar repulsions of the bromine atoms. 

It should be emphasized that this analysis is based on the assumption of some appreciable 2—5 overlap in all-c/s and cis-gauche 1,3,5-hexatriene. Furthermore, it should be pointed out that our analysis of the conformational preference of 1,3,5-hexatriene is aiming at revealing electronic patterns. In reality, the all-c/s conformation of 1,3,5-hexatriene is unfavorable due to repulsive interactions between the two methylene groups, i.e. conformational preference varies in the order all-frans > cis-gauche. 

Experimental data for 1,2-dicyanoethane are available. In the gas phase, the anti conformer is more stable while in the liquid phase, where dipole-dipole repulsive effects are deemphasized, the gauche is more stable67. ... 

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