Cycloheptane twist boat

Cycloheptane is found in two main groups of conformers the chair form (including the half-chair) and the boat form (including the twist boat), with the chair form lower in energy by about 7 kcal/mol (ap-... 

Cycloheptane and cyclooctane data on the thermal properties are also given in Table 3.1 They show little change from the cyclopentane and cyclohexane properties. Again, there is no indication of increasing amounts of conformational entropy in the transition entropies. For cyclooctane in solution H and NMR could prove ring-inversions and pseudorotation among the boat-chair conformations through the twist-boat-chair intermediate to very low temperatures (100 K). Only about 6% of the cydooctane could be found at about 300 K in the other three crown-family... 

Semi-quantitative potential-energy curves for pseudorotation within chair and boat forms of 1,1-dimethyl- and 1,1,2,2-, 1,1,3,3-, and 1,1,4,4-tetramethyl-cycloheptanes have been constructed interconversion of the preferred twist-chair forms of the latter is calculated to proceed via the twist-boat form rather than by pseudorotation within the chair family. 

After a short qualitative introduction to the principles of dynamic nuclear magnetic resonance spectroscopy , the proposed interconversion processes for cycloheptanes and cycloheptenes are explained in detail. According to calculations, the most favourable conformation for cycloheptanes seems to be the twist chair cycloheptenes prefer the chair form. Possible conformational processes for chair and boat forms are discussed and illustrated. 

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