Amides rotational energy barrier

The more lipophilic thioamide unit is also characterized by the poor H-bond acceptor nature of the sulfur and by its larger covalent radius (1.04 vs 0.74 A for O). The thioamide unit mainly adopts a Z-planar conformation, with a rotation energy barrier averaging about 23 vs 18 kcal-mol-1 for amides. 11 A recent ab initio computational study 12 suggests that conformational perturbation in linear thioamides is more likely effected in the C-terminal side. 

This is also reflected in quite high rotational energy barriers, which surprisingly are greater for thioamide than for the amide analogues. 

RNA, mRNA, rRNA, and tRNA. See Ribonucleic acid Roberts, John D., 928 Robinson, Sir Robert, 4, 402, 724 Robinson annulation, 724, 728 Rotamer, 90. See also Conformation Rotational energy barrier alkenes, 172—173 amides, 779 butane, 94—95 conjugated dienes, 31(r-371 ethane, 93—94... 

Rotation about single bonds and conformational changes can be studied. Amides constitute a classic example. Because of the partial double bond character of the carbon-nitrogen bond as a consequence of the contribution of 2 to the electronic structure, there is an energy barrier to rotation about this bond. 

Another group that frequently - and perhaps surprisingly, in view of secondary amide characteristics -exhibits rotameric behaviour is the secondary carbamate (R-COO-NHR1), though the energy barrier to rotation tends to be a little lower than in the amide case. 

Anomeric effects are evident from dynamic NMR studies on at least one substrate, N-benzyloxy-Af-chlorobenzamide (2c) ". In acetone-de the benzyl aromatic signal (S 7.85) de-coalesced into two signals (ratio 2 1) close to 200 K, corresponding to a free energy barrier of ca 10-11 kcalmoH Amide isomerization appeared to be faster than N—0 rotation since benzoyl resonances were largely unaffected. 

Conformational results on proline analogs have also been reported. N-Acetyl-2,3-dehydroproline (91) shows an equilibrium of the two conformers in almost equimolecular amounts (82MI3). The equilibrium is shifted by different acidic conditions, while the energy barrier for rotation around the amide bond ( 63 kJ mol" ) is lower than in proline, owing to the presence of the unsaturated bond. Only the s-trans isomer was observed (82M13) in the case of N-acetyl-5-oxo-L-proline (92), and apparently no effect is found on changing the pH of the solution. 

Furancarboxamides and furanthiocarboxamides somewhat resemble the oximes in that there are two sources of isomerism. The rotational isomerism about the C(2)—C(O) bond has not been observed, however — only restricted rotation in the amide link. This is easier than in the benzamides since the furan ring is electron-releasing thus reducing the N—C(O) interaction and giving an energy barrier of about 58kJmol 1. There is little difference... 

We saw in Chapter 7 that rotation about the C-N bond in an amide is relatively slow at room temperature—the NMR spectrum of DMF clearly shows two methyl signals (p. 165). In Chapter 13 you learned that the rate of a chemical process is associated with an energy barrier (this holds both for reactions and simple bond rotations) the lower the rate, the higher the barrier. The energy barrier to the rotation about the C-N bond in an amide is usually about 80 kj mol-1, translating into a rate of about 0,1 s-1 at 20 °C. Rotation about single bonds is much faster than this at room temperature, but there is nonetheless a barrier to rotation in ethane, for example, of about 12 kj mol-1. 

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