AB-INITIO MOLECULAR-ORBITAL STUDY OF ENERGIES AND CONFORMERS OF 3,4-DIHYDRO-1,2-DITHIIN, 3,6-DIHYDRO-1,2-DITHIIN, 4H-1,3-DITHIIN, AND 2,3-DIHYDRO-1,4-DITHIIN

Optimized geometries and energies for 3,4-dihydro-1,2-dithiin (1), 3,6 -dihydro-1,2-dithiin (2), 4H-1,3-dithiin (3), and 2,3-dihydro-1,4-dith iin (4) were calculated using ab initio 6-31G and MP2/6-31G*//6-31G* methods. At the MP2/6-31G//6-31G* level, the half-chair conformer of 4 is more stable than those of 1, 2, and 3 by 2.5, 3.5, and 3.6 kcal/m ol, respectively. The half-chair conformers of 1, 2, 3, and 4 are 2.9, 7.1, 2.0, and 5.6 kcal/mol, respectively, more stable than their boat conformers. The calculated half-chair structures of 1-4 are compared with the calculated chair conformer of cyclohexane and the half-chair structures for cyclohexene, 3,4-dihydro-1,2-dioxin (5), 3,6-dihydro-1, 2-dioxin (6), 4H-1,3-dioxin (7), and 2,3-dihydro-1,4-dioxin (8). (C) 1 998 John Wiley & Sons, Inc.