2-methyl-2-butyl cation, rotation

Step through the sequence of structures depicting rotation about the C i - C+ bond in 2-methyl-2-butyl cation. Plot energy (vertical axis) vs. CCCC dihedral angle (horizontal axis). What is the preferred conformation, with the ethyl group in plane or perpendicular to the plane ... 

The tert-butyl cation structure (7) with Cs symmetry is better suited for hyperconjugation than the C3h form and is thus energetically slightly favored.29 The energy surface for methyl-group rotation is however very flat. 

In a cation such as the (2,4-di-ferf-butyl-6-methyl)benzyl cation 147, a high rotational barrier around the v/r-hybridized atom is observed. The methylene protons are found magnetically nonequivalent in the1H NMR spectrum.356 Recent combined experimental and theoretical studies for the related cation 143 suggest357 that structure 143b is an important resonance contributor. 

Fig. 2. Isomerization of protonated tertiary butanol (1) to a hydrogen bonded complex between the tertiary butyl cation and water (2) is provided by the successive rotations of two methyl groups. Results of MP2/6-31G calculations...

In respect to the cation structure, aUcyl lengthening or fluorination make the salt more viseous. Also the reduction of freedom of rotation (from butyl to isobutyl) inereased the viscosity. Unexpectedly, methylation at the C2(H) of the imidazolium ring inereased viseosity even if it suppressed the position most likely to hydrogen bond with the anion. 

---