Hindered Rotation about Formal Single Bonds

Hindered rotation about formal single bonds 

The classical case of dimethylformamide has been re-investigated using the techniques now available. The parameters and AG have been re-determined. The rotational barriers in other amides (and compounds that can be considered as amide derivatives) continue to occupy the attention of organic chemists in some, e.g., 2-methyl-6-t-butyl-AT-methylchloracetanilide, the two isomers have separate stable existence (68A and 68B),i whereas rotamers are usually 

C—N bond in similar compounds has been observed. The existence of a rotational barrier in jp-methoxyphenyl-di-t-butylmethanol (78) follows from the unsymmetrical nature of its n.m.r. spectrum at room temperature.  

The first case of a rotational barrier being studied in the vapour phase by n.m.r. methods has been reported gaseous dimethylinitro-samine has a free energy of activation for internal rotation AG ) of 21 1 kcal./mole as compared with 23-3 kcal./mole for the liquid. 

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E. Hindered rotation about formal single bonds... 

Hindered rotation about (formally) single bonds, which exhibit a double character due to the conjugation effects, is typical of many organic compounds. A classic example of this type is the group of substituted amides [17], in which the double-bond character of the C-N bond may be represented by a contribution of structure b to a resonance hybrid ... 

Hindered rotation occurs on the NMR time scale for numerous other systems with partial double bonds, including carbamates, thioamides, enamines, nitrosamines, alkyl nitrites, diazoketones, aminoboranes, and aromatic aldehydes. Formal double bonds can exhibit free rotation when alternative resonance structures suggest partial single bonding. The calicene 5-5, for example, has a barrier to rotation about the central bond of only 20 kcal mol . ... 

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