Energy barrier for rotation

The small optical rotations of the alditols arise from the low energy barrier for rotation about C—C bonds, permitting easy iaterconversion and the existence of mixtures of rotational isomers (rotamers) ia solution (12). 

The free-energy barriers for rotation about the N-CO bond in the N-acetyl and N-benzoyl derivatives of 2 and 124 have been calculated from the coalescence temperature for the NMR signals for the bridge-... 

The thiocarbene complex [(CO)5WC(SMe)Me] has been prepared by the reaction of [(CO)sWC(OMe)Me] with HSMe at room temperature. The position of the v(C )) stretching frequencies, the energy barrier for rotation about the C—S bond, the ionization potential, and the dipole moment of the... 

Up to six nitric oxides readily add to ketoenolates when they are treated with strong base in methanol. This sequential addition is stereospecific, generating Z-configured products. The tris(diazene-iV-oxide-iV -hydroxylates) are an intriguing type of molecular propeller (Figure 21) with relatively low energy barriers for rotation of the three N2O2 blades around the C—N bond. 

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. 

Due to a partial 77-character, aromatic carbonyl compounds have an activation energy barrier for rotation around the phenyl-carbonyl bond, the value of which is substantially increased upon protonation.44 In para-anisaldehyde a second protonation of the methoxy group will drastically decrease their barrier. The temperature-dependent NMR spectrum will reflect both exchange processes, intra- and intermolecular, as shown in Scheme 1.1. 

The activation energy for a torsional process around a covalent bond depends, among other factors, on the 71-electron density associated with this bond. It is of interest to investigate whether the energy barrier for rotation around a carbon-carbon double bond can be sufficiently reduced to allow the establishment of the dynamic equilibrium between the isomers 26 and 28 in the ground state of the system. The complete equilibrium must also include the transformations 26 29 and 28 30, which are associated with restricted rotation around the C—N bond (Scheme 1). 

Another geometric factor affecting Tg is cis-trms configuration. Double bonds in the cis form reduce the energy barrier for rotation of adjacent bonds, soften the chain, and hence reduce Tg (Table 4.5). 

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