AB-INITIO CALCULATIONS OF THE EFFECTS OF GEMINAL SILYL SUBSTITUENTS ON THE STEREOMUTATION OF CYCLOPROPANE AND ON THE SINGLET-TRIPLET SPLITTING IN TRIMETHYLENE

(10/10)CASSCF and CASPT2N/6-31G calculations predict that geminal sil yl substituents at C(2) of the (0,0)-trimethylene diradical (2c) shoul d strongly stabilize the lowest singlet state by hyperconjugative elec tron donation to the in-phase combination of p-pi AOs at C(1) and C(3) . After correction for zero point energy differences, singlet 2c is fo und to be an energy minimum, with a barrier to closure to 1,1-disilylc yclopropane (1c) of 6.4 kcal/mol, and to lie below triplet 2c by 11.1 kcal/mol. The conrotatory transition state that connects Ic to 2c is c omputed to be 8.5 kcal/mol lower in energy than the transition state f or cleavage of the C(2)-C(3) ring bond in Ic by rotation of just one o f these methylene groups and 6-7 kcal/mol lower in energy than the tra nsition states for breaking one of the ring bonds to C(1) by methylene rotation. Coupled conrotation of C(2) and C(3) is therefore predicted to dominate rotation of just one of these methylene groups in the ste reomutation of 1c.