Rearrangements of C7H6 isomers: Computational studies of the interconversions of bicyclo[3.2.0]hepta-1,3,6-triene, bicyclo[3.2.0]hepta-3,6-diene-2-ylidene, bicyclo[3.2.0]hepta-2,3,6-triene, and cyclohepta-1,2,4,6-tetraene

The structures of several bicyclo[3.2.0] isomers on the C7H6 potential energy surface, and the transition states which interconnect them, have been fully optimize using density functional theory (BLYP/6-31G*). Relative energies were determined using single-point calculations at higher levels of theory (CCSD(T), CASPT2N). These calculations show that bicyclo[3.2.0]hepta-3,6-diene-2-ylidene (2) readily undergoes ring opening to cycloheptatetraene (4) with a barrier of 5 kcal/mol. The rearrangement of of 2 to 4 occurs without intervention of bicyclo[3.2.0]hepta-1,3,6-triene (1). Triene 1 is much more stable than carbene 2, but faces a much higher barrier (35 kcal/mol) to rearrangement to cycloheptatetraene (4). The singlet and triplet states of carbene 2, along with the triplet state of triene 1 and the singlet state of bicyclo[3.2.0]hepta-2,3,6-triene (3), lie very close in energy, ca. 55 kcal/mol higher than the singlet state of 4. The computed transition states for the electrocyclic ring closure of cyclohetatetraene (4) to bicyclo[3.2.0]hepta-1,3,6-triene (1) and for the degenerate 1,5-sigmatropic H-shift in triene 1 occur at very similar energy. This finding provides a rationalization for the experimental observation of C-13-label scrambling upon high-temperature pyrolysis of various C7H6 isomers. The calculated IR frequencies and intensities for 1, (1)2, (3)2, anti 3 may aid future identification of these species.