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 C_2h 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 C_2h symmetric chair form. The calculated strain energy for twist to chair process is 61.1 kJ mol⁻¹.

Keywords:

• Ab initio calculations
• conformational analysis
• cyclic disulfide
• stereochmistry

Notes

^a Relative energy with respect to the most stable conformation from HF/6-31+G∗//HF/ 6-31+G∗ calculations.

^b Relative energy with respect to the most stable conformation from MP2/6-31+G∗// HF/6-31+G∗ calculations.

^c Relative energy with respect to the most stable conformation from B3LYP/6-31+G∗// HF/ 6-31+G∗ calculations.

^{d 1}H NMR data, see Claeson et al. ¹⁵

^a Relative energy with respect to the most stable conformation from HF/6-31+G∗//HF/6-31+G∗ calculations.

^b Relative energy with respect to the most stable conformation from MP2/6-31+G∗//HF/6-31+G∗calculations.

^c Relative energy with respect to the most stable conformation from B3LYP/6-31+G∗//HF/6-31+G∗calculations.

^{d 1}HNMR data, see Claeson et al. ¹⁵

^a Relative energy with respect to the most stable conformation from HF/6-31+G∗//HF/6-31+G∗ calculations.

^b Relative energy with respect to the most stable conformation from MP2/6-31+G∗//HF/6-31+G∗ calculations.

^c Relative energy with respect to the most stable conformation from B3LYP/6-31+G∗//HF/6-31+G∗ calculations.

^d 1H NMR data, see Bushweller et al. ¹⁷

^e X-ray crystallographic data for bis(pentamethylene)-s-tetrathiane, see Bushweller et al. ¹⁶

^a Relative energy with respect to the most stable conformation from HF/6-31+G∗//HF/6-31+G∗ calculations.

^b Relative energy with respect to the most stable conformation from MP2/6-31+G∗//HF/6-31+G∗ calculations.

^c Relative energy with respect to the most stable conformation from B3LYP/6-31+G∗//HF/6-31+G∗ calculations.