Benzooxirene. Ab initio calculations

Ab initio calculations have been performed on benzooxirene, the corresponding oxo carbene (''ketocarbene''), and the transition state linking the two. At the highest level used, QCISD(T)/6-31G*//MP2(FULL)/6-31G* with MP2(FULL)/6-31G* zero point energy corrections, the relative energies of the oxirene, the transition state and the carbene are 0, 24.6, and -17.8 kJ mol(-1). Correlation energy effects are very important in this system: at the QCISD(T) level the oxirene lies above the carbene, as at the MP4 and HF levels, but at the MP2 level the ordering is reversed. Benzooxirene is probably slightly nonplanar: the HF/6-31G* geometry is C-2v but the MP2(Fermi contact)/6-31G* geometry is C-s with a 6-/3-ring coplanarity deviation of about 6.9 degrees, although in the MP2(FULL)/6-31G* geometry this is reduced to about 3.1 degrees.