STEREOELECTRONIC INTERPRETATION FOR THE ANOMALOUS H-1-NMR CHEMICAL-SHIFTS AND ONE-BOND C-H COUPLING-CONSTANTS (PERLIN EFFECTS) IN 1,3-DIOXANES, 1,3-OXATHIANES, AND 1,3-DITHIANES - SPECTROSCOPIC AND THEORETICAL OBSERVATIONS

The reverse Perlin effects observed for all C-H one bond coupling constants in 1,3-dithianes, i.e., (1)J(C-Hax) > (1)J(C-Heq), seem to support the existence of a dominant sigma C-S --> sigma*(C-Heq) stereoelectronic interaction, which leads to a weakening of the equatorial C-H bonds in these heterocycles. Further analysis of the (1)J(C-H) patterns in 1,3-dioxanes and 1,3-oxathianes confirm the apparent manifestation of n(O) --> sigma*C-H-ax as well as the novel beta-oxygen effect involving n(O/C-Heq) orbital interactions in a W array, as proposed very recently in the literature. Furthermore, the above mentioned stereoelectronic interactions may be at least partly responsible for the anomalous chemical shift behavior at C(2) in 1,3-dithianes and at C(5) in 1,3-dioxanes. Theoretical support for the proposed stereoelectronic orbital interactions was gained from ab initio Hartree-Fock 6-31G** and Kohn-Sham /LSD + BP + NLSCF/ DZVP2 calculations on the heterocycles of interest.