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 BLYP, MP2, and CCSD(T) levels of theor y predict that tetracyclo[3.1.0.0(1.3).0(3.5)]hexane (2) is an energeticall y high-lying isomer on the C6H6 potential energy surface. 2 is similar to 7 0-80 kcal/mol higher in energy than [3]radialene (3) and similar to 150-160 kcal/mol higher in energy than benzene. The activation barrier for the fir st step of the rearrangemrnt of 2 to 3 is similar to 10 kcal/mol, which mig ht 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. (C) 2000 Elsevier Science B.V. All rights reserved.