AB INITIO MOLECULAR ORBITAL STUDY OF 1,2-DITHIANE AND 1,2,4,5-TETRATHIANE

Abstract

Ab initio calculations at HF/6-31+G* level of theory for geometry optimization, and MP2/6-31+G*//HF/6-31+G* and B3LYP/6-31+G*//HF/6-31+G* levels for a single-point total energy calculation, are reported for the chair and twist conformations of 1,2-dithiane (1), 3,3,6,6-tetramethyl-1,2-dithiane (2), 1,2,4,5-tetrathiane (3), and 3,3,6,6-tetramethyl-1,2,4,5-tetrathiane (4). The C2 symmetric chair conformations of 1 and 2 are calculated to be 21.9 and 8.6 kJ mol−1 more stable than the corresponding twist forms. The calculated energy barriers for chair-to-twist processes in 1 and 2 are 56.3 and 72.8 kJ mol−1, respectively. The C2h symmetric chair conformation of 3 is 10.7 kJ mol−1 more stable than the twist form. Interconversion of these forms takes place via a C2 symmetric transition state, which is 67.5 kJ mol−1 less stable than 3-Chair. The D2 symmetric twist-boat conformation of 4 is calculated to be 4.0 kJ mol−1 more stable than the C2h symmetric chair form. The calculated strain energy for twist to chair process is 61.1 kJ mol−1.