Ab initio calculations find 2,2-disilylcyclopentane-1,3-diyl is a singlet diradical with a high barrier to ring closure

Ab initio calculations on the lowest singlet and tripler states of 1,2-disi lylcyclopentane-1,3-diyl find that the singlet lies well below the triplet. The C-2 Singlet diradical is calculated to be a minimum on the potential e nergy surface with an enthalpic barrier to ring closure of Delta H-298(doub le dagger) = 13.5 kcal/mol at the CASPT2/6-31G* level of theory. The energy of the 1,3-divinyl-substituted singlet diradical is calculated to be only 0.8 kcal/mol higher than that of 5,5-disilyl-1,3-divinylbicyclo[2.1.0]penta ne at this level of theory, but the transition state for their equilibratio n is computed to be 12.8 kcal:mol above the diradical in energy.