AN ELECTRON-RICH, 3-COORDINATED SULFUR ATOM - A CORRELATED ABINITIO STUDY OF 1,6-DIOXA-6A-THIAPENTALENE

Ab initio calculations on 1,6-dioxa-6a-thiapentalene (DOTP) and 1,6,6a-trithiapentalene (TTP) have provided insight into the symmetry and structure of these unusual molecules which contain three-coordinated sulfur atoms. Hartree-Fock geometry optimizations using a 4-21G basis set for carbon and 3-3-21G for sulfur give bond lengths agreeing with results of a gas-phase electron diffraction study within the experimental errors of the latter. They do, however, incorrectly predict a classically bonded, asymmetric ground-state structure rather than the C2v form with extensive electron delocalization, suspected from experimental evidence to be the ground state. In the case of DOTP, at the Hartree-Fock level, the transition barrier at the symmetrical configuration is 0.9 kcal/mol, increasing to 3.8 kcal/mol with the 6-3 IG** basis set and further increasing to 4.8 kcal/mol with the 6-311G** basis set. For TTP, the barrier at the symmetrical transition state is predicted considerably higher with the 4-21G calculation, approximately 6 kcal/mol. Our newly developed technique for doing electron correlation treatments of larger molecules using localized orbitals made it possible to study DOTP with calculations as large as MP4(SDQ) with 6-31 IG** basis set, although without full geometry optimization. With the inclusion of electron correlation, the correct symmetric minimum is predicted with an energy about 1 kcal/mol lower than the classically bonded asymmetric form.