User`s guide

7. Exercises

Locate the transition state for this reaction by first building the norbornene product. Perform a

PM3 geometry optimization of the product, and verify that the resulting geometry is reasonable.

View the result, choose

New Job Using This Geometry, and open the molecule in the WebMO

Editor. Adjust the two carbon-carbon bond lengths that are formed in the reaction to 2.2

Angstroms, recalling that the first selected atom is the one that is moved. Open the Z-Matrix

Editor with

Tools:Z-Matrix.... If necessary change the z-matrix connection definition so that the

above two carbon-carbon bonds are directly referenced in the z-matrix (in the

Na column) and

then click the

ReConnect button. Fix both bond length to 2.2 by selecting F (fixed) from the

corresponding dropdown menu, and click OK. Perform a PM3 Geometry Optimization

calculation on this geometrically constrained molecule.

View the result, and verify that the two carbon-carbon bond lengths formed in the reaction are 2.2

Angstroms. Choose

New Job Using This Geometry, and perform a PM3 Transition State

Optimization calculation.

View the result, note the bond length of the two carbon-carbon bonds being formed, and note the

near planarity of the cyclopendiene ring. Choose

New Job Using This Geometry, and perform

a PM3 vibrational frequencies calculation. Verify that there is a single negative (imaginary)

frequency that corresponds to the reaction coordinate. Report the value of the negative frequency

and an image of the corresponding vibrational mode.

31. Transition States of Diels-Alder Stereoproducts (Pro)

Substituted reactants undergoing a Diels-Alder reaction can result in various stereoproducts. For

example, the addition of acrylonitrile to 5-fluorocyclopentadiene leads to four possible

stereoproducts.

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syn endo

syn exo anti endo anti exo

Endo products are generally favored over exo products. Anti products are preferred for alkyl

substituted cyclopentadiene, and syn products are preferred for alkoxy or halide substituted

cyclopentadiene.

The preferred product can be predicted by comparing the energies of the transition states.