Rotational barriers electronic effects

The rotational barriers obtained from pseudo-potential and all-electron calculations generally agree within ca 0.15 kcal mol-1, even when X, Y = Pb. Hence, relativistic effects do not appear to influence the barriers. 

Having obtained stable rotamers of compound 6, Staab and Lauer (45) extended the work to see whether rotamers of amides that normally have lower barriers as a result of a disfavored canonical structure 2 due to electronic effects are also isolable. They found that the rotamers of 2,4,6-trwm-butylbenzoben-zimidazolide (7) were isolable, but those of the corresponding imidazolide (8) were not. The barrier to rotation of the former in hexachlorobutadiene solution was 28.7 kcal/mol for the E Z process at 80°C. The barrier to rotation of the latter was estimated at less than 23 kcal/mol. It is possible to attribute this result to electronic effects that raise the ground state energy, because the aromatic... 

The second chapter, by Jan Sandstrom, deals with stereochemical features of push-pull ethylenes. The focus is on rotational barriers, which span a large range of values. The ease of twisting is partly a matter of electron delocalization and partly a matter of steric and solvent effects. Electronic structure and such related items as dipole moments and photoelectron spectra for these systems are discussed. The chapter also deals with the structure and chiroptical properties of twisted ethylenes that do not have push-pull effects, such as frans-cyclooctene. 

As a rule, if the unpaired electron density in the anion-radical is redistributed, the rotation barrier decreases. Thus, the barrier of the phenyl rotation in the benzaldehyde anion-radical is equal to 92 kJ mol", whereas in the 4-nitrobenzaldehyde anion-radical, the barrier decreases to 35 kJ mor (Branca and Gamba 1983). Ion-pair formation enforces the reflux of the unpaired electron from the carbonyl center to the nitro group. Being enriched with spin density, the nitro group coordinates the alkali metal cation and fixes the unpaired electron to a greater degree. The electron moves away from the rotation center. The rotation barrier decreases. The effect was revealed for the anion-radical of 4-nitrobenzophenone and its ionic pairs with lithium, sodium, potassium, and cesium (Branca and Gamba 1983 Scheme 6.19). 

Delocalization of the odd electron into extended n systems results in considerable radical stabilization. The C—H BDE at C3 of propene is reduced by 13 kcal/mol relative to that of ethane. That the stabilization effect in the allyl radical is due primarily to delocalization in the n system is shown by the fact that the rotational barrier for allyl is 9 kcal/mol greater than that for ethyl. Extending the conjugated system has a nearly additive effect, and the C—H BDE at C3 of 1,4-pentadiene is 10 kcal/mol smaller than that of propene. Delocalization of the odd electron in the benzyl radical results in about one-half of the electron density residing at the benzylic carbon, and the C—H BDE of the methyl group in toluene is the same as that in propene. 

Further experimental and theoretical studies on the rotational barriers of the metal fragment in (cycloheptatriene)Cr(CO)3 complexes195 suggest that (cycloheptatriene)-Cr(CO)3 complexes in general are in equilibrium with their norcaradiene valence isomers and their ground state conformation is controlled by the same electronic factors which effect the cycloheptatriene-norcaradiene equilibrium195. 

It is interesting to compare tert-butylbenzyl methyl amine (35) with the tetrahedral intermediate 36 derived from N-benzyl-N-methylacetamide which has a similar degree of substitution. The rotation barrier for the (CH3)3C — N bond and the nitrogen inversion barrier in 35 have been found identical and estimated at 6.2 kcal/mol (30). The higher value of 8.0 kcal/mol for the intermediate 36 must be a consequence of the double-bond character of the C —N bond (nitrogen atom has one secondary electronic effect (n-o )). 

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