Gauche alignment

Experimental structural data (electron diffraction) is available for A-fluoromethyl-A,lV-dimethylamine. The only conformation observed is the anti arrangement of the unshared pair and C-F bond. MP2/6-311G(2 ,p) calculations suggest that the gauche alignment is about 5 kcal/mol less stable. 

CH3NH2, showing the 6° variation in N-C-H angle as the H rotates from gauche to anti alignment with the vicinal nN lone pair. The overall methyl tilt (dotted line) is evidently dominated by the strong outward tilting of the C—H bond anti to the lone pair, as discussed above (cf. Fig. 3.66 and Table 3.24). 

A related stereoelectronic effect with particular relevance to fluoroaliphatic compounds is the gauche effect [38]. Contrary to expectations based solely on the steric repulsion of fluorine, 1,2-difluorocthanc prefers the gauche rather than antiparallel alignment of the fluorine substituents, by 0.5-0.9 kcal mol [39]. The same is observed for related compounds in which one fluorine atom is replaced... 

Gauche Synonomous with a synclinal alignment of groups attached to adjacent atoms (i.e., a torsion angle of near +60° or -60°). See torsion angle. 

When methyl groups are added, as in butane, two additional conformations are possible. There are two staggered conformations, called anti and gauche, and two eclipsed conformations, one with methyl-methyl eclipsing and the other with two hydrogen-methyl alignments. In the methyl-methyl eclipsed conformation, van der Waals repulsions come into play. The barrier for this conformation increases to about 6 kcal/mol, as shown in Figure 1.35. We pursue the conformation of hydrocarbons further in Section 2.2.1. 

Dimethoxymethane prefers a conformation that allows alignment of an unshared pair on each oxygen (donor) with a C-0 a orbital on the other. This condition is met in the conformation labeled gauche, gauche. In contrast, the extended hydrocarbon-like anti, anti conformation does not permit this alignment. 

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