On the nature of bonds between carbon atoms

The combined effect of conjugation and van der Waals' interaction has been calculated for carbon sp²-sp² single bonds of different lengths and for different molecular conformations. The bond shortening effect of conjugation is found to be comparable to that of hybridization. The attractive van der Waals' interaction, on the other hand, is always found to be small. It seems thus most improbable that this interaction should be the cause of the planarity of butadiene.