PM3-MO CALCULATIONS OF MONOLITHIATED SULFONES, SULFOXIDES, AND 1,3-DITHIANES - COMPARISON WITH AB-INITIO OR X-RAY RESULTS

Geometries of dimethyl sulfone, the methylsulfonyl anion, and its monolithiated isomers 3-6, CH3-SO2-CH2Li, were calculated by ab initio (MP2(full)/6-31+G*//MP2-(full)/6-31+G*) and semiempirical (MNDO and PM3) methods. d-Orbitals are not involved in the stabilization of the anion (NBO analysis). From calculation of the energy differences (MP4(sdtq)/6-31+G*//MP2(full)/6-31+G*, with inclusion of the ZPE correction) between the lithiated isomers, structure 3 (C-1, the Li+ in contact with the C-a carbon, and one oxygen) turns out to be most stable. The second minimum, 4 (C-s, the Li+ in contact with the two oxygens), is 6.5 kcal/mol less stable. The transition states 5 (23.5 kcal/mol above 3) and 6 (4.9 kcal/mol above 3) are associated with two different modes for the interconversion of 3 into its mirror image. The PM3 (but not the MNDO) results are in acceptable agreement with the energetic ordering and the structural data obtained from ab initio calculations. PM3 and MNDO were applied to a variety of known X-ray structures in which the sulfur atom is found in different oxidation states (8, [Ph-SO2-CHPhLi.TMEDA](2), 9, [Ph-SO-CPH(Me)- Li.TMEDA](2), and 10, PhC(S-(CH2)(3)-S-)Li.TMEDA.THF). These complexes are described much better by PM3 than by MNDO.