Rotational barriers, definition

In order to see the effect of the peri substituent on the barrier to rotation, 2,3-dichloro-9-(l-cyano-l-methylethyl)triptycene and 2,3-dichloro-9-(l-meth-oxycarbonyl-l-methylethyl)triptycene (103, X = H, Y = Cl) were prepared (147). The data are included in Table 21. The barrier to rotation becomes definitely higher when the peri substituent is removed. Thus the peri substituent in... 

These studies suggested a planar ylidic carbon and definite participation of the P d orbitals in the description of the HOMO. The earliest ab initio study of methylenephosphorane found a very small rotational barrier [0.003 kcal mol-1 (1 kcal = 4.184 kJ)] about the P=C double bond64. These three points, the degree of pyramidalization at the ylidic carbon, the role of the P d orbitals and the P=C rotational barrier, remain the focal points of all theoretical studies of the ylides. A summary of the theoretical structures determined using ab initio techniques is given in Table 7. 

The rotational barrier of the SiH3 group is definitely larger in c-C3H5SiH3 than in MeSiH3, while there is only a little difference between the corresponding barriers for CH3... 

A rationally designed system for the study of the steric effects of flanking substituents on the rotational barriers in biphenyl was introduced by Bott et al. (80JA5618). The barriers were determined by DNMR. These barriers allowed the definition of interference values that can be used to predict the rotational barriers in 2,2 -biphenyls. These parameters were used to estimate the barrier in a 2-iodo-3-(2-methylphenyl)thiophene derivative 222. 

A calculation may concern a rotation around a bond. The simplest way to calculate a rotational barrier is to keep the geometry of the substituent constant, turn the double bond in small steps, calculate new ground state energies, and finally connect them to a continuous curve. To obtain an improved result, the geometry of the substituent should be reoptimized for each rotation angle. This is indeed the definition of a potential surface everything has to be optimized. 

These results clearly indicate that barriers to all isomerisation processes are at least less than about 8kcalmol 1. In /V-benzyloxy-7V-chlorobenzamide 44 the amide isomerisation was not observable but the anomeric overlap resulted in diastereotopic benzylic hydrogens, which at coalescence afforded a barrier for rotation about the N-OBn bond of around 10.3 kcalmol-1.32 Like its /V-chloro analogue, the amide isomerisation barrier in 43 is too low to be observed by 3H NMR and even though there is definitive X-ray and theoretical evidence for anomeric effects in /V-acyloxy-/Y-alkoxyamidcs, the barrier to isomerisation about the N-OBn bond must be lower than 10.3 kcalmol-1. The n0-CN ci anomeric interaction in 44 is predicted to be stronger than the n0-CN OAc interaction in 43 on purturbation arguments.32... 

In acidic medium, the 1° form should be present in equilibrium with the T+ form because hemi-orthoesters are weak bases. Consequently, hemiortho-esters must be allowed to undergo molecular rotation prior to their breakdown in this medium. Also, there is no evidence so far that molecular rotation can compete with the breakdown of an intermediate in the T+ ionic form. What we know is that the barrier for cleavage should be definitely lower in the T+ than in the T° form (24). At pH higher than 11, hemiortho-esters will exist exclusively in the T ionic form, and it will be seen that in some cases, the energy barrier for the cleavage of T is lower than that of molecular rotation. 

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