Conformational analysis of 5-methyl- and 2,2,5-trimethyl-1,3-dithianes

Conformational isomerization of 5-methyl- and 2,2,5-trimethyl-1,3-dithianes was studied using quantum-chemical HF/6-31G(d), HF/pVDZ, and PBE/3z approximations. The potential energy surface of molecules of both compounds is shown to contain the main minimum corresponding to the equatorial chair conformer C-5e. The calculated potential barriers of the conformational isomerization were found. Based on the experimental (¹H NMR) and theoretical vicinal coupling constants the ??G ⁰ values were determined of the methyl group at the ring C⁵ atom. The reasons for the distinctions between the values of this parameter from the ??G ⁰, obtained by energy minimizations of the equatorial and axial chair conformers are discussed.