Stability and thermal rearrangement of (E,E)-1,3-cycloheptadiene and trans-bicyclo[3.2.0]hept-6-ene.

The highly strained (E,E)-1,3-cycloheptadiene was shown to be a minimum on the potential energy surface; two structural isomers were found at the MP2 level, but multiconfiguration self-consistent field calculations show that only one is a true minimum. The isomerization of (E,E)-1,3-cycloheptadiene was investigated through double bond rotation, and electrocyclic ring closure. The first pathway gives (E,Z)-1,3-cycloheptadiene, with a barrier of 7.2 kcal x mol(-1), and the second pathway gives the trans isomer of bicyclo[3.2.0]hept-6-ene with a barrier of 13.0 kcal x mol(-1). The strain energy of (E,E)-1,3-cycloheptadiene was calculated using homodesmotic reactions and found to be about 96 kcal x mol(-1) whereas that for (E,Z)-1,3-cycloheptadiene was only 38 kcal x mol(-1), implying that the second trans double bond imparts an additional 58 kcal x mol(-1) in strain energy. The trans isomer of bicyclo[3.2.0]hept-6-ene was calculated to have a strain energy of 69 kcal x mol(-1) and a barrier of 27 kcal x mol(-1) for isomerization to (Z,Z)-1,3-cycloheptadiene. Although many of the structures reported here could be described using a single determinant wave function, several could not, making a multireference method necessary for a complete description of the potential energy surface.