Tetracyclo[3.1.0.0(1.3).0(3.5)]hexane: A new C₆H₆ isomer with an unusual bonding situation

Quantum chemical calculations at the B3LYP, MP2 and CCSD(T) levels of theory predict that tetracyclo[3.1.0.0(1.3).0(3.5)]hexane (2) is an energetically high-lying isomer on the C₆H₆ potential energy surface. 2 is ∼70-80 kcal/mol higher in energy than [3]radialene (3) and ∼150-160 kcal/mol higher in energy than benzene. The activation barrier for the first step of the rearrangement of 2 to 3 is ∼10 kcal/mol, which might be high enough to isolate 2 in a low-temperature matrix. The analysis of the bonding situation shows that 2 is a cyclic carbene which is stabilized by charge donation from a C=C double bond to the carbene carbon atom.