The following model depicts the transition state of the Diels-Alder reaction between (E)-1,3-pentadiene and acrylonitrile. Carbon atoms are gray, hydrogen is cyan and nitrogen is blue. The partial bonds between the C1 and C4 carbons of the diene and the double bond carbons of the dienophile are colored pink. One of these partial bonds is shorter than the other (2.00 vs 2.28Å), demonstrating unsymmetrical yet concerted bond formation.
The model can be rotated by clicking on the structure and dragging the mouse. Holding the shift key during this operation allows zooming.

The following model depicts the transition state of the Diels-Alder reaction between 2-methyl-1,3-butadiene and acrylonitrile. Carbon atoms are gray, hydrogen is cyan and nitrogen is blue. The partial bonds between the C1 and C4 carbons of the diene and the double bond carbons of the dienophile are colored pink. One of these partial bonds is shorter than the other (2.04 vs 2.22Å), demonstrating unsymmetrical yet concerted bond formation.
The models can be rotated by clicking on the structure and dragging the mouse. Holding the shift key during this operation allows zooming.