Addition of C₂H₄ to C₄H₆ (Diels-Alder addition)

The method used is AM1, although other methods could, of course, be used.  The calculations described here do not depend on the method used.

Description of Reactants:

 Cis-butadiene and ethylene form distinct species separated by a large distance. DH_f C₂H₄: +16.45 Kcal/mol; DH_f C₄H₆: +30.64 Kcal/mol; DH_f C₂H₄ + C₄H₆: +47.17 Kcal/mol
 

Description of Transition State:

At the transition state, the two C -- C bonds that are forming both have distances of  2.119Angstroms, that is, they are symmetric, and the geometry of the transition state has Cs symmetry. DH_f: +70.08 Kcal/mol.

Description of Products:

In cyclohexene there is one double bond, the rest of the C-C bonds being normal, single, bonds. DH_f C₆H₁₀: -6.88 Kcal/mol.
 

Use of a GRID to generate the transition state:

Reaction occurs when the C -- C distances decrease to those in single bonds.  There are two of these, between the carbon atoms of ethylene and the C₁ and C₄ atoms of butadiene.  Inspection of the Potential Energy Surface shows a smooth increase in energy in going from the reactants to the products.  The transition state is likely to be located somewhere on the rounded ridge between reactants and products, but simple inspection does not indicate whether the transition state is symmetry or not. However, the transition state must be either symmetric or unsymmetric.  By selecting a point on the ridge, but not the symmetric point, a transition state refinement can be run.  If the transition state is indeed symmetric, then the refinement would produce that.  This is, in fact, what happened.

The files used in these calculation can be found in the ZIP file.