The Naphthylcarbene Potential Energy Hypersurface

The naphthylcarbene potential energy surface (PES) was examined ab initio, employing self-consistent field (SCF), second-order perturbation theory (MP2), and density functional (Becke3LYP) methods in conjunction with 6-31G*, DZ, DZP, and 6-311+G* basis sets. All stationary structures were characterized by vibrational frequency analyses at the Becke3LYP/6-31G* level; final energies were evaluated at the Becke3LYP/6-311+G*//Becke3LYP/6-31G* + ZPVE level. Cyclobuta[ de ]naphthalene is the global minimum on this part of the C 11 H 8 PES. Generally, seven-membered benzocarbenes are no minima as they converge to their corresponding allenes. Both 1- and 2-naphthylcarbene have triplet ground states, but the small S−T gaps (ca. 5 kcal mol -1 ) allow facile rearrangements in the singlet manifold to take place. The triplet rotational barrier for the exo -methylene in 2-naphthylcarbene is relatively small (3.5 kcal mol -1 ) due to weak π-bonding. At low temperatures, singlet 2-naphthylcarbene equilibrates with 2,3-benzobicyclo[4.1.0]hepta-2,4,6-triene and bicycloheptatetra-1,3,5,7-ene, but not with 4,5-benzocycloheptatrienylidene which is not a minimum; rearrangement to singlet 1-naphthylcarbene occurs only at higher temperatures via bicycloheptatetra-1,2,4,6-ene, the second lowest minimum. As the rearrangement barriers from 1- and 2-naphthylcarbene to bicycloheptatetra-1,2,4,6-ene are of similar magnitude (ΔΔ E ⧧ = 1.9 kcal mol -1 ), the latter species may be observed in small quantities only. The allenes bicycloheptatetra-1,2,4,6-ene, bicycloheptatetra-1,3,5,7-ene, and bicycloheptatetra-2,3,5,7-ene are thermodynamically remarkably stable and should be observable at low temperatures.