Application of J(C,H) coupling constants in conformational analysis

Conformational equilibria for a number of methyl substituted 1,3-dioxanes 1, 1,3-oxathianes 2 and 1,3-dithianes 3 were calculated at the HF and DFT levels of theory. In addition to the chair conformers also the energetically adjacent twist conformers were considered and the positions of the corresponding conformational equilibria estimated. On the basis of the global energy minima of conformers, participating in the conformational equilibria, the ¹J_C,Hax,equ coupling constants were calculated using the GIAO method and compared with the experimental values obtained from ¹³C,¹H coupled ¹³C NMR spectra. The Perlin effect, the influence of the solvent and the suitability of this NMR parameter for assigning the conformational equilibria present are critically discussed.