DFT Studies of the Molecular Structures and Conformational Processes of 1,2‐, 1,3‐ and 1,4‐Dithiepane

In this study density functional theory (DFT) calculations at B3LYP/6‐31G(d), B3LYP/6‐31+G(d) and B3LYP/6‐311+G(2df,2p) levels for geometry optimization and total energy calculation were applied for investigation of the important energy‐minimum conformations and transition‐state of 1,2‐, 1,3‐, and 1,4‐dithiepanes. Moreover, ab initio calculations at HF/6‐31G(d) level of theory for geometry optimization and MP2/6‐311G(d)//HF/ 6‐31G(d) level for a single‐point total energy calculation were reported for different conformers. The obtained results reveal that, the twist‐chair conformer is a global minimum for all of these compounds. Also, two local minimum were found in each case, which are twisted‐chair and twisted‐boat conformers. The boat and chair geometries are transition states. The minimum energy conformation of 1,2‐dithiepane is more stable than the lowest energy forms of 1,3‐dithiepane and 1,4‐dithiepane. Furthermore, the anomeric effect was investigated for 1,3‐dithiepane by the natural bond orbital method. The computational results of this study shows that all conformers of 1,3‐dithiepane have a hypercojugation system. Finally, the ¹³C NMR chemical shifts for the conformers of 1,4‐dithiepane were calculated, which have good correlation with their experimental values.