Rotation to maximum bond alignment

Molecules look better when their bonds are aligned with principal axes or multiples of 30° and 45°. As 15° subsumes the other two, that number is 

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Let us now consider the contribution of diffusion to the maximum rates of homogeneous reactions involving associations as a first step in the interactions of proteins with both small and large substrates. In the present context association to form a specific complex does not include any covalent bond formation. Three events are involved in the approach to equilibrium collision, alignment to correct mutual orientation and for some processes separation after the event. These events depend on translational diffusion for approach and separation and on rotational diffusion for alignment. Earlier in this section the diffusion constant was defined using the principle of random Brownian motion and this will now be used in equations which have been derived to evaluate maximum collision rates. 

FIGURE 10.6 Conformations and electron delocalization in 1,3-butadiene. The s-cis and the s-trans conformations permit the 2p orbitals to be aligned parallel to one another for maximum TT electron delocalization. The s-trans conformation is more stable than the s-cis. Stabilization resulting from tt electron de-localization is least in the perpendicular conformation, which is a transition state for rotation about the C-2—C-3 single bond. The green and yellow colors are meant to differentiate the orbitals and do not indicate their phases. 

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