Dihedral angles changes

Q Use Newman projections to compare conformational energies and predict the most stable conformation. Show how the torsional energy varies as the dihedral angle changes. Problems 3-38,42, 46, and 47... 

Dihedral angles Range of dihedral angle changes (in degrees) 8-Cl TIBO 9-Cl TIBO ... 

If the above conclusions are applied to address the abrupt thermochromism of poly(dihexylsilane) not only in the solid state but also in solution, the conclusion is that the increase in UV absorption intensity does indeed result from a coil-to-rod transition but that the red shift arises from a helix-to-helix transition of the main chain. Recent calculations have indicated that the absorption maximum gradually red shifts as the helical dihedral angle changes from 60° to... 

TABLE 3. Dihedral angle changes in some rotational and librational processes... 

It is a general rule for a carbon-carbon bond that the greater the dihedral angle change between conformational minima, the higher the barrier is likely to be, so a onefold or twofold rotational barrier should be higher than a sixfold. The exceptions tend to be the most memorable, but generally the very factors which produce minima at only one or two positions in the rotational cycle are likely to increase the potential between the two minima. 

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