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 silyl substituents at C(2) of the (0,0)-trimethylene diradical ( 2c ) should strongly stabilize the lowest singlet state by hyperconjugative electron donation to the in-phase combination of p-π AOs at C(1) and C(3). After correction for zero point energy differences, singlet 2c is found to be an energy minimum, with a barrier to closure to 1,1-disilylcyclopropane ( 1c ) of 6.4 kcal/mol, and to lie below triplet 2c by 11.1 kcal/mol. The conrotatory transition state that connects 1c to 2c is computed to be 8.5 kcal/mol lower in energy than the transition state for cleavage of the C(2)−C(3) ring bond in 1c by rotation of just one of these methylene groups and 6−7 kcal/mol lower in energy than the transition 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 stereomutation of 1c .