Total dihedral groups

Next come the dihedral angles (or torsions), and the contribution that each makes to the total intramolecular potential energy depends on the local symmetry. We distinguish between torsion where full internal rotation is chemically possible, and torsion where we would not normally expect full rotation. Full rotation about the C-C bond in ethane is normal behaviour at room temperature (although 1 have yet to tell you why), and the two CH3 groups would clearly need a threefold potential, such as... 

Optimization of the valence and dihedral angles yields planar cyclic structures for the 3- to 5-ring intermediates in contrast to a chair conformation for that of the 6-ring. In the cases of n = 4, 5, 6 the oxygen atom is placed almost in the plane of the three C-atoms directly bonded to it. Therefore, an intramolecular solvation of the cationic chain end by methoxy groups which are bonded to the polymer backbone is preferred in the gas phase. The calculations show that for a non-polar solvent such as CH2C12 a decrease in stability of the cyclic intermediates exists. But this decrease does not result in a total break of the intramolecular solvation (Table 13). An equilibrium between open chain and cyclic intermediates must only be taken into account in more polar solvents, due to the competition of intra- and intermolecular solvation. 

Dimethyl- and 2,4,4,6-tetramethyl-heptyl cations The total isotope effect of a CD3-group is independent of conformation (Sunko et al., 1977), and the CDj-substituted cations [155] and [156] can therefore serve as a model for a dihedral angle of 0° whereas the cations [157] and [158] deuteriated in the methylene group provide information on the conformational dependence of the equilibrium isotope effect. 

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