Conformation change energy barriers

The main features in the conformational analysis of these compounds, i.e. relative stabilities of syn and anti conformers, rotation energy barriers and eventually out of plane distortions of the anti conformers, result in manyfold weak interactions and it would be tedious and illusive to give their complete qualitative analysis. We can nevertheless display the most important of them and indicate the trends induced by the change of the nature of the substrate (enol or thioenol) or the position and the nature of substituents. Two interactions seems to be effective enough to be taken into account electrostatic repulsions and attractions, and ir orbital effects. 

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. 

If the spin-spin information was being transmitted by the normal through-bond mechanism the upfield three proton signal would be expected to occur as a doublet because these protons are the only ones which can assume the required planar zig-zag conformation 77>78h Preliminary results, using the change in chemical shift method 79>, indicates that the energy barrier to rotation is of the order of 20 k.cal.mole O. As expected the silicon compound (39) shows a nine proton doublet... 

Conformational changes frequently involve free energy barriers small enough to allow structures spread across the whole reaction coordinate to be observed in a series of crystal structures (see Section 2). Rate constants are also readily obtained by suitable NMR experiments, so this is an attractive area for the examination of possible relationships between crystal structure correlations and free energy relationships. In fact not a great deal of work has been done, though more can confidently be expected. 

Figure 9. Potential energy surface for cellobiose at 400 K. The trajectory of conformational changes during a portion of the simulation are shown on the left. Energy contours in the vicinity of minima 1-3 are shown on the right. Barrier heights 5.3 Kcal/mol between minima 1 and 2,1.3 Kcal/mol between 2 and 3. (MM2(85) functions).

The energy barrier to this conformational change is about 22 kJ mol . There is no reason why any one particular carbon should be out of the plane, and at room temperature there is rapid interconversion of all possible variants. Again, a planar form would feature as the energy maximum in tlie interconversions. The conformation with four carbons in plane and one out of plane is termed an envelope conformation. This terminology comes from the similarity to an envelope with the flap open. 

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