Gauche conformation donor-acceptor molecules

Second-order perturbative estimates indicate that each trans-like donor-acceptor interaction (Fig. 3.55(a)) stabilizes the molecule by 2.58 kcal mol-1, compared with only 0.89 kcal mol-1 for the riv-likc interaction (Fig. 3.55(b)). The smaller gauche-like stabilizations (0.20 kcal mol-1 at 60° in the staggered conformer, 0.70 kcal mol-1 at 120° in the eclipsed conformer) diminish the difference somewhat, but still preserve a significant hyperconjugative advantage for the staggered conformer. 

We have already seen in Section 2.2.3.3 how conformation can be affected by anomeric interactions, which can lead electronegative substituents to be axial at the 2-position in tetrahydropyranyl rings, and sometimes cause a chain of atoms to adopt a seemingly more hindered gauche conformation 2.81-2.83, 2.85 and 2.86 rather than the more usual zigzag arrangement. Similar hyperconjugative interactions in neutral molecules between two a bonds, one a a donor and the other a a acceptor, can lead them to adopt conformations in which the stereoelectronic effect overrides the purely steric effect. 

Because of their common physical origin, the various donor-acceptor effects discussed here have been collectively called the gauche effect. The best conformation of a molecule has the maximum number of gauche interactions between adjacent lone pairs and/or polar... 

Replacement of a methyl hydrogen by fluorine leads to a situation where the dominant interaction is no— Ocf, since in this molecule the C—H bond acquires a donor ability comparable to the oxygen lone pair and the C—F bond is a better acceptor than an 0—H bond. The molecule CH2F—OH can exist in the syn, gauche and anti conformations shown below. 

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