DFT molecular orbital calculations and natural bond orbital analysis of 1,2,7-thiadiazepane conformers

Density functional theory (B3LYP/cc-pVDZ//B3LYP/cc-pVDZ) is used to optimize the geometries of 1,2,7-thiadiazepane and natural bond orbital (NBO) analyses have been carried out employing the HF/6-31G(d,p) level using B3LYP/cc-pVDZ geometries to study the stereoelectronic effects on the stability of the stereoisomers (axial-axial, equatorial-equatorial, and axial-equatorial). The results of NBO calculations showed that the axial-axial or axial-equatorial stereoisomers are the most stable conformers, where not only the stereoelectronic effect, but also the steric repulsion significantly affects their stability.